Learn/ Extra course/Module 5 of 10
Extra Course, Module 5: Circuits, End to End
Covers NCVEC 2024-2028 Extra Pool, Subelement E7 (Practical Circuits). Eight questions on your exam come from this material, the largest share of any subelement.
Ray clears the whole bench. "Eight questions means eight circuit families, and together they are a complete transceiver: logic to run it, amplifiers to give it muscle, filters to give it manners, supplies to feed it, modulators to give it a voice, DSP where the magic moved, op-amps for the glue, and oscillators for the heartbeat. Big module. Bring the kit you started, because everything here is on its board somewhere."
5.1Logic in Motion
Module 4 taught gates at rest; this section sets them running.
Gate behavior, stated as truth tables in prose: an OR gate outputs 1 if any input is 1. A NAND outputs 0 only if all inputs are 1, the AND with its answer inverted. An exclusive NOR outputs 0 if one and only one input is 1, the difference detector inverted. A truth table itself is the list of inputs and corresponding outputs for a digital device, and positive logic means high voltage represents 1, low represents 0.
Circuits with memory and rhythm:
- The flip-flop is the bistable circuit: two stable states, one bit of memory. Its bonus talent: a flip-flop divides a pulse train's frequency by 2, because it toggles once per input pulse. Chain them and division compounds: four flip-flops divide by 16 (2 to the 4th).
- A decade counter produces one output pulse for every 10 input pulses, the digit wheel of electronics.
- The multivibrator family sorts by stability: an astable multivibrator continuously alternates between two states with no external clock, the free-running square-wave source; a monostable switches to its alternate state temporarily, for a set time, then returns, the one-shot timer behind every "key for 3 seconds" function.
Knowledge Check 5.1
Tap an answer. In your own words first, then check yourself.
1. How many flip-flops divide a frequency by 16?
2. Which circuit continuously alternates states without an external clock?
3. A NAND gate outputs 0 when:
E7A the easy way: all 11 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7A01 Which circuit is bistable?
Answer A flip-flop
Tip Bistable means two stable states, so a flip-flop holds whichever one it is set to until the next clock.
E7A02 What is the function of a decade counter?
Answer It produces one output pulse for every 10 input pulses
Tip A decade counter counts to ten, so it gives one output pulse for every 10 that come in.
E7A03 Which of the following can divide the frequency of a pulse train by 2?
Answer A flip-flop
Tip A flip-flop toggles once per two input pulses, which is exactly a divide by 2.
E7A04 How many flip-flops are required to divide a signal frequency by 16?
Answer 4
Tip Each flip-flop halves the frequency, and 2 to the 4th is 16, so you need 4 of them.
E7A05 Which of the following circuits continuously alternates between two states without an external clock signal?
Answer Astable multivibrator
Tip Astable means no stable state, so it free-runs and flips back and forth with no clock.
E7A06 What is a characteristic of a monostable multivibrator?
Answer It switches temporarily to an alternate state for a set time
Tip Monostable has one resting state and visits the other briefly, so it fires a timed pulse then returns.
E7A07 What logical operation does a NAND gate perform?
Answer It produces a 0 at its output only if all inputs are 1
Tip NAND is AND then inverted, so the output drops to 0 only when every input is 1.
E7A08 What logical operation does an OR gate perform?
Answer It produces a 1 at its output if any input is 1
Tip OR is the any gate, so one or more inputs at 1 drives the output to 1.
E7A09 What logical operation is performed by a two-input exclusive NOR gate?
Answer It produces a 0 at its output if one and only one of its inputs is 1
Tip XNOR outputs 1 when inputs match, so a single 1 among the two inputs forces a 0.
E7A10 What is a truth table?
Answer A list of inputs and corresponding outputs for a digital device
Tip A truth table just lists every input combination next to the output it produces.
E7A11 What does “positive logic” mean in reference to logic devices?
Answer High voltage represents a 1, low voltage a 0
Tip Positive logic reads the higher voltage as 1 and the lower voltage as 0.
5.2Amplifiers, the Full Taxonomy
The General course's class system, completed:
- Class A: biased halfway between saturation and cutoff, conducting the full cycle. Cleanest, least efficient.
- Class AB: each device in a push-pull pair conducts more than 180 but less than 360 degrees, the linearity-efficiency compromise inside nearly every SSB rig.
- Class C: the General warning restated as consequence: amplifying SSB with Class C yields signal distortion and excessive bandwidth. Constant-envelope modes only.
- Class D: the newcomer, an amplifier using switching technology for high efficiency. Switching amplifiers win because the device sits at saturation or cutoff most of the time, Module 4's transistor-switch operating points, where the transistor drops nearly no voltage while conducting and passes no current while off, so it barely dissipates. The price: a square-switched output is rich in harmonics, so a switching amplifier's output requires a filter to remove harmonic content.
Stability and topology facts:
- Unwanted oscillations in an RF power amplifier are prevented by installing parasitic suppressors and/or neutralizing the stage, the General course's neutralization plus Module 4's ferrite beads, deployed together.
- The grounded-grid tube amplifier's signature is low input impedance, which is why classic HF amplifiers load your exciter comfortably without fancy input matching.
- The emitter follower (common collector) offers near-unity voltage gain with high input impedance and low output impedance: the impedance transformer of the transistor world, buffering delicate stages from hungry loads.
- An amplifier's honesty check remains the two-tone test and its intermodulation products, now traced to their circuit cause from Module 8 of the General course: nonlinearity. Linearity is not a virtue amplifiers have; it is a bias point they are held to.
Knowledge Check 5.2
Tap an answer. In your own words first, then check yourself.
1. Why are switching amplifiers so efficient?
2. What must follow an RF switching amplifier's output?
3. What is characteristic of a grounded-grid amplifier?
E7B the easy way: all 12 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7B01 For what portion of the signal cycle does each active element in a push-pull, Class AB amplifier conduct?
Answer More than 180 degrees but less than 360 degrees
Tip Class AB sits between B and A, so each device conducts more than 180 but less than 360 degrees.
E7B02 What is a Class D amplifier?
Answer An amplifier that uses switching technology to achieve high efficiency
Tip Class D switches fully on or off rather than acting linearly, which is why it runs so efficiently.
E7B03 What circuit is required at the output of an RF switching amplifier?
Answer A filter to remove harmonic content
Tip A switching amp output is full of harmonics, so a filter must clean them off before the antenna.
E7B04 What is the operating point of a Class A common emitter amplifier?
Answer Approximately halfway between saturation and cutoff
Tip Class A is biased right in the middle, roughly halfway between saturation and cutoff, for full-cycle conduction.
E7B05 What can be done to prevent unwanted oscillations in an RF power amplifier?
Answer Install parasitic suppressors and/or neutralize the stage
Tip Tame a runaway RF amp with parasitic suppressors and neutralization to kill the unwanted feedback.
E7B06 What is a characteristic of a grounded-grid amplifier?
Answer Low input impedance
Tip The grounded-grid amp drives the cathode, which presents a low input impedance.
E7B07 Which of the following is the likely result of using a Class C amplifier to amplify a single-sideband phone signal?
Answer Signal distortion and excessive bandwidth
Tip Class C is nonlinear, so feeding it SSB wrecks the signal with distortion and extra bandwidth.
E7B08 Why are switching amplifiers more efficient than linear amplifiers?
Answer The switching device is at saturation or cutoff most of the time
Tip A switching device spends its time at saturation or cutoff, where little power is wasted, so efficiency is high.
E7B09 What is characteristic of an emitter follower (or common collector) amplifier?
Answer Input and output signals in-phase
Tip The emitter follower just follows the input, so output and input stay in phase.
E7B10 In Figure E7-1, what is the purpose of R1 and R2?
Answer Voltage divider bias
Tip A pair of resistors from the supply sets the base voltage, which is voltage divider bias.
E7B11 In Figure E7-1, what is the purpose of R3?
Answer Self bias
Tip The emitter resistor develops its own bias from emitter current, so R3 gives self bias.
E7B12 What type of amplifier circuit is shown in Figure E7-1?
Answer Common emitter
Tip Signal in at the base and out at the collector with the emitter common marks a common emitter stage.
5.3Filters and Matching Networks
Filters have family names, and each name is a shape:
- Chebyshev: accepts ripple in the passband in exchange for a sharper cutoff.
- Elliptical: the sharpest cutoff of all, achieved with notches (nulls) in the stop band. Park a null on the specific frequency you hate.
- Crystal lattice: Module 4's quartz crystals arranged into the narrow, precise filter that carves SSB and CW bandwidths at IF.
- The universal high-Q tax: sharp filters ring, smearing fast transitions. CW through too crisp a filter acquires ghost dits. Selectivity and fidelity remain a negotiated settlement.
- An absorptive filter dissipates unwanted frequencies in a resistive load instead of reflecting them back at the source, kinder to amplifiers than a pure reflective stopband.
- A receiving filter's steepness gets a number: shape factor, the ratio of a filter's bandwidth at two attenuation levels, describing its ability to reject signals in the adjacent channel. Two more specialists complete the drawer: the cavity filter, a resonant metal chamber whose extreme Q makes it the filter of choice inside a repeater duplexer, separating transmit from receive on one antenna a few hundred kilohertz apart; and in the DSP world, the Hilbert-transform filter, the 90-degree phase-shift network that generates SSB digitally, the phasing method reborn in software inside every SDR from Section 5.6.
Matching networks are filters moonlighting as transformers, and the job description is precise: an impedance-matching circuit transforms a complex impedance to resistive by cancelling the reactive part and transforming the resistive part to the desired value.
- The Pi-network (shunt C, series L, shunt C) is the classic tuner and tube-amplifier output circuit, matching impedance while low-pass filtering. Its advantage over a simple L-network: the Q of a Pi-network can be varied, letting the designer choose the bandwidth-versus-harmonic-suppression tradeoff, where the L-network's Q is dictated by the impedances being matched.
- Rearrange the parts and the character flips: a T-network with series capacitors and a shunt inductor is a high-pass filter, the same three components as the Pi, wired into the opposite personality. Topology, not parts, sets the filter type.
- The Pi-L adds a series inductor after the Pi, buying additional harmonic suppression, the network of choice where cleanliness outranks simplicity.
- The whole toolbox (L, Pi, T networks, transformers, transmission-line sections) does one job with different tradeoffs in Q, and Module 3 already told you what Q costs: bandwidth.
Knowledge Check 5.3
Tap an answer. In your own words first, then check yourself.
1. Which filter family features notches in its stop band for the sharpest cutoff?
2. What is the cost of very high filter Q?
3. Why choose a Pi-L network over a plain Pi?
E7C the easy way: all 11 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7C01 How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network’s input and output?
Answer A capacitor is connected between the input and ground, another capacitor is connected between the output and ground, and an inductor is connected between the input and output
Tip A low-pass Pi has a shunt capacitor at each end to ground and the inductor in series between them.
E7C02 What is the frequency response of a T-network with series capacitors and a shunt inductor?
Answer High-pass
Tip Series capacitors pass highs and the shunt inductor shorts lows, so this T-network is high-pass.
E7C03 What is the purpose of adding an inductor to a Pi-network to create a Pi-L-network?
Answer Greater harmonic suppression
Tip Adding the series inductor to make a Pi-L buys you greater harmonic suppression.
E7C04 How does an impedance-matching circuit transform a complex impedance to a resistive impedance?
Answer It cancels the reactive part of the impedance and changes the resistive part to the desired value
Tip Matching cancels the reactive part to leave pure resistance, then transforms that resistance to the value you want.
E7C05 Which filter type has ripple in the passband and a sharp cutoff?
Answer A Chebyshev filter
Tip A Chebyshev trades ripple in the passband for a steeper, sharper cutoff.
E7C06 What are the characteristics of an elliptical filter?
Answer Extremely sharp cutoff with one or more notches in the stop band
Tip An elliptical filter cuts off extremely sharply and drops in notches down in the stop band.
E7C07 Which describes a Pi-L network?
Answer A Pi-network with an additional output series inductor
Tip A Pi-L is simply a Pi-network with one extra series inductor added at the output.
E7C08 Which of the following is most frequently used as a band-pass or notch filter in VHF and UHF transceivers?
Answer A helical filter
Tip Helical filters are the go-to band-pass or notch for VHF and UHF front ends.
E7C09 What is a crystal lattice filter?
Answer A filter for low-level signals made using quartz crystals
Tip A crystal lattice filter uses quartz crystals to shape low-level signals with a very tight response.
E7C10 Which of the following filters is used in a 2-meter band repeater duplexer?
Answer A cavity filter
Tip A 2-meter duplexer leans on cavity filters, which give sharp selectivity with low loss.
E7C11 Which of the following measures a filter’s ability to reject signals in adjacent channels?
Answer Shape factor
Tip Shape factor compares bandwidths at two attenuation points to rate how well a filter rejects adjacent channels.
5.4Power Supplies, Regulated
The General course built the supply; the Extra course regulates it.
- A linear regulator conducts continuously, dropping excess voltage across itself as heat: quiet, simple, inefficient. Its key specification is dropout voltage, the minimum input-to-output difference below which regulation fails.
- A switching (switchmode) regulator chops instead of drops, and wins on efficiency for exactly the Class D reason from Section 5.2: its pass device lives at saturation or cutoff. High-frequency switching also shrinks the magnetics, the General course's three-pound 30-amp supply explained a second time from the regulator's side. The tax, also familiar: switching noise, the birdies-every-few-kilohertz signature your receiver already knows.
- Filter architecture matters: a capacitor-input filter yields higher output voltage than a choke-input filter, at the cost of higher peak rectifier currents.
- The bleeder resistor returns for its final appearance with its full portfolio: discharging the filter capacitors at power-off and improving voltage regulation by providing a minimum load.
Off-grid, the General course's solar rules gain their reasons: the charge controller required for lithium chemistry exists to prevent overcharging, tapering and terminating charge where lead-acid would merely gas and lithium would fail dangerously. The series blocking diode still guards the night, and the controller now manages the day.
Knowledge Check 5.4
Tap an answer. In your own words first, then check yourself.
1. What is a linear regulator's dropout voltage?
2. Why are switching supplies more efficient than linear ones?
3. What is a solar charge controller's core job?
E7D the easy way: all 15 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7D01 How does a linear electronic voltage regulator work?
Answer The conduction of a control element is varied to maintain a constant output voltage
Tip A linear regulator varies how hard its pass element conducts to hold the output voltage steady.
E7D02 How does a switchmode voltage regulator work?
Answer By varying the duty cycle of pulses input to a filter
Tip A switchmode regulator adjusts the duty cycle of its pulses and the filter averages them to the right voltage.
E7D03 What device is used as a stable voltage reference?
Answer A Zener diode
Tip A Zener holds a fixed voltage across it, which makes it the classic stable reference.
E7D04 Which of the following describes a three-terminal voltage regulator?
Answer A series regulator
Tip A three-terminal regulator passes current in line with the load, so it is a series regulator.
E7D05 Which of the following types of linear voltage regulator operates by loading the unregulated voltage source?
Answer A shunt regulator
Tip A shunt regulator sits across the output and draws extra current, loading the source to hold the voltage.
E7D06 What is the purpose of Q1 in the circuit shown in Figure E7-2?
Answer It controls the current to keep the output voltage constant
Tip Q1 is the pass transistor, adjusting current flow to keep the output voltage constant.
E7D07 What is the purpose of C2 in the circuit shown in Figure E7-2?
Answer It bypasses rectifier output ripple around D1
Tip C2 shunts the leftover rectifier ripple around the zener so the reference stays quiet.
E7D08 What type of circuit is shown in Figure E7-2?
Answer Linear voltage regulator
Tip A series pass element holding the output steady marks this as a linear voltage regulator.
E7D09 How is battery operating time calculated?
Answer Capacity in amp-hours divided by average current
Tip Battery runtime is capacity in amp-hours divided by the average current you draw.
E7D10 Why is a switching type power supply less expensive and lighter than an equivalent linear power supply?
Answer The high frequency inverter design uses much smaller transformers and filter components for an equivalent power output
Tip Running the inverter at high frequency shrinks the transformer and filter parts, so the supply is lighter and cheaper.
E7D11 What is the purpose of an inverter connected to a solar panel output?
Answer Convert the panel’s output from DC to AC
Tip A solar panel puts out DC, so an inverter turns it into AC.
E7D12 What is the dropout voltage of a linear voltage regulator?
Answer Minimum input-to-output voltage required to maintain regulation
Tip Dropout is the smallest input-to-output gap a linear regulator needs to keep regulating.
E7D13 Which of the following calculates power dissipated by a series linear voltage regulator?
Answer Voltage difference from input to output multiplied by output current
Tip A series regulator dissipates the voltage it drops times the current it passes, (Vin minus Vout) times Iout.
E7D14 What is the purpose of connecting equal-value resistors across power supply filter capacitors connected in series?
Answer All these choices are correct
Tip Equal resistors across series filter caps share the voltage evenly and bleed the charge, so all the reasons apply.
E7D15 What is the purpose of a step-start circuit in a high-voltage power supply?
Answer To allow the filter capacitors to charge gradually
Tip A step-start eases the inrush so the big filter capacitors charge gradually instead of slamming full on.
5.5Making and Unmaking Modulation
The modulator and detector circuits behind Module 6 of every course so far:
- A reactance modulator on an oscillator produces direct FM, pushing the frequency itself; the same circuit on an RF amplifier stage produces phase modulation, the General fact now paired with its sibling.
- The balanced modulator still emits double-sideband suppressed carrier, and the filter method still carves SSB from it: the General course's transmit chain, now recognized as a specific engineering choice among alternatives.
- On receive, the product detector recovers SSB and CW by remixing with a local carrier, while AM surrenders to the simplest detector of all: the diode envelope detector, which recovers audio by rectification and filtering of the modulated envelope. Rectify, smooth, listen: the crystal set from Module 4's point-contact diode, formalized.
- Baseband names what all these circuits move around: the frequency range occupied by the message signal before modulation, your 300-to-3000 Hz voice as it exists before any carrier meets it.
- FM systems play one more trick: pre-emphasis boosts higher audio frequencies at the transmitter, and de-emphasis at the receiver restores the balance, the pair together riding treble above FM's rising noise floor. It is why FM voice sounds crisp and why music through a flat-response link sounds wrong.
Knowledge Check 5.5
Tap an answer. In your own words first, then check yourself.
1. How does a diode envelope detector recover AM audio?
2. A reactance modulator connected to the transmitter's oscillator produces:
3. What is baseband?
E7E the easy way: all 11 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7E01 Which of the following can be used to generate FM phone signals?
Answer Reactance modulation of a local oscillator
Tip Vary a capacitance or inductance in the oscillator tank with audio and you get FM, which is reactance modulation.
E7E02 What is the function of a reactance modulator?
Answer Produce PM or FM signals by varying a capacitance
Tip A reactance modulator swings an effective capacitance with the audio to produce PM or FM.
E7E03 What is a frequency discriminator?
Answer A circuit for detecting FM signals
Tip A discriminator turns frequency swings into voltage, so it is an FM detector.
E7E04 What is one way to produce a single-sideband phone signal?
Answer Use a balanced modulator followed by a filter
Tip Make SSB by stripping the carrier in a balanced modulator, then a filter removes the unwanted sideband.
E7E05 What is added to an FM speech channel to boost the higher audio frequencies?
Answer A pre-emphasis network
Tip Pre-emphasis lifts the higher audio tones going into an FM transmitter.
E7E06 Why is de-emphasis used in FM communications receivers?
Answer For compatibility with transmitters using phase modulation
Tip De-emphasis rolls the highs back down to match transmitters that use phase modulation.
E7E07 What is meant by the term “baseband” in radio communications?
Answer The frequency range occupied by a message signal prior to modulation
Tip Baseband is the raw message signal in its own frequency range before any modulation moves it up.
E7E08 What are the principal frequencies that appear at the output of a mixer?
Answer The two input frequencies along with their sum and difference frequencies
Tip A mixer puts out both input frequencies plus their sum and their difference.
E7E09 What occurs when the input signal levels to a mixer are too high?
Answer Spurious mixer products are generated
Tip Overdrive a mixer and it stops behaving linearly, throwing off spurious products.
E7E10 How does a diode envelope detector function?
Answer By rectification and filtering of RF signals
Tip A diode envelope detector rectifies the RF and filters it to recover the AM audio.
E7E11 Which type of detector is used for demodulating SSB signals?
Answer Product detector
Tip SSB has no carrier of its own, so a product detector mixes in a reinserted carrier to demodulate it.
5.6The Radio That Is Software
The General course promised the SDR; this section opens it.
The purest architecture is direct sampling: the RF signal is digitized directly by the analog-to-digital converter, no mixers, no analog IF, the entire band captured as numbers. Two laws govern the numbers:
- Nyquist: the sample rate must be at least twice the highest frequency in the signal, or aliases appear, false signals folded down from above, the digital cousin of the superheterodyne's image.
- Bits buy dynamic range: the minimum number of ADC bits is set by the dynamic range required, roughly 6 dB per bit. A band containing S1 whispers beside 40-over locals demands a deep converter.
Once digitized, the workload is arithmetic:
- Decimation reduces the sample rate by discarding samples once the band of interest is narrowed, and it demands an anti-aliasing filter first, removing content above the new, lower Nyquist limit before the discard.
- Digital filters split into two clans: FIR filters can achieve perfectly linear phase (no dispersion, clean transitions) at the price of more computation; IIR filters use feedback to do more with less arithmetic at the price of phase behavior. An FIR filter's taps are its coefficients, and more taps buy sharper response, the digital analog of adding filter poles.
Everything the General course attributed to DSP (arbitrary bandwidths, brick-wall shapes, I/Q universality) is these mechanisms. The filter knob on a modern rig is adjusting tap coefficients, and now you know it.
Knowledge Check 5.6
Tap an answer. In your own words first, then check yourself.
1. What is the minimum sample rate for a signal containing frequencies up to 30 MHz?
2. What must precede decimation in a DSP chain?
3. Which digital filter type can achieve exactly linear phase?
E7F the easy way: all 14 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7F01 What is meant by “direct sampling” in software defined radios?
Answer Incoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal
Tip Direct sampling feeds RF straight into the ADC with no mixer or local oscillator in front.
E7F02 What kind of digital signal processing audio filter is used to remove unwanted noise from a received SSB signal?
Answer An adaptive filter
Tip An adaptive filter learns the noise and adjusts itself to pull the SSB signal out of it.
E7F03 What type of digital signal processing filter is used to generate an SSB signal?
Answer A Hilbert-transform filter
Tip Generating SSB in DSP uses a Hilbert transform to make the needed 90 degree phase shift.
E7F04 Which method generates an SSB signal using digital signal processing?
Answer Signals are combined in quadrature phase relationship
Tip The DSP phasing method combines two signals in quadrature, 90 degrees apart, to form SSB.
E7F05 How frequently must an analog signal be sampled to be accurately reproduced?
Answer At least twice the rate of the highest frequency component of the signal
Tip Nyquist says sample at least twice the highest frequency present to reproduce it faithfully.
E7F06 What is the minimum number of bits required to sample a signal with a range of 1 volt at a resolution of 1 millivolt?
Answer 10 bits
Tip A 1 mV step across 1 volt needs 1000 levels, and 2 to the 10th is 1024, so 10 bits.
E7F07 What function is performed by a Fast Fourier Transform?
Answer Converting signals from the time domain to the frequency domain
Tip An FFT takes a time-domain waveform and shows its frequency content, like a spectrum.
E7F08 What is the function of decimation?
Answer Reducing the effective sample rate by removing samples
Tip Decimation throws away samples to lower the effective sample rate.
E7F09 Why is an anti-aliasing filter required in a decimator?
Answer It removes high-frequency signal components that would otherwise be reproduced as lower frequency components
Tip Before decimating, an anti-alias filter strips the highs that would otherwise fold down and masquerade as low frequencies.
E7F10 What aspect of receiver analog-to-digital conversion determines the maximum receive bandwidth of a direct-sampling software defined radio (SDR)?
Answer Sample rate
Tip In a direct-sampling SDR the ADC sample rate sets how wide a band you can receive.
E7F11 What sets the minimum detectable signal level for a direct-sampling software defined receiver in the absence of atmospheric or thermal noise?
Answer Reference voltage level and sample width in bits
Tip With no outside noise, the floor is set by the reference voltage and how many bits wide each sample is.
E7F12 Which of the following is generally true of Finite Impulse Response (FIR) filters?
Answer FIR filters can delay all frequency components of the signal by the same amount
Tip FIR filters have linear phase, so they delay every frequency component by the same amount.
E7F13 What is the function of taps in a digital signal processing filter?
Answer Provide incremental signal delays for filter algorithms
Tip Taps are the delay stages in a DSP filter, handing the algorithm its stepped-back samples.
E7F14 Which of the following would allow a digital signal processing filter to create a sharper filter response?
Answer More taps
Tip More taps give the DSP filter more to work with, so the response gets sharper.
5.7The Op-Amp: Gain in a Can
The operational amplifier is Module 4's analog IC, idealized into three properties: very high (ideally infinite) open-loop gain, very high input impedance, and very low output impedance. Infinite gain sounds useless until you cage it with feedback, and then it becomes anything:
- The inverting amplifier's gain is set entirely by two resistors: gain = feedback resistance divided by input resistance (with the output inverted). Want a gain of 10? Rf ten times Rin. The op-amp's own gain barely matters, which is the entire point: feedback trades raw gain for precision.
- The non-inverting configuration gives 1 + Rf/Rin, the same bargain without the inversion.
- An op-amp's frequency response and behavior in circuit are set by the external components, not the chip: the same part is an audio filter, an integrator, or a comparator depending on what surrounds it.
- The imperfection worth naming: input offset voltage, the small differential input voltage required to zero the output, born of internal imbalances. Precision circuits null it; casual circuits ignore it; op-amp datasheets lead with it.
Your station is quietly full of these: active audio filters, ALC loops, AGC amplifiers, and the analog housekeeping around every ADC from Section 5.6.
Knowledge Check 5.7
Tap an answer. In your own words first, then check yourself.
1. What sets an inverting op-amp circuit's gain?
2. Which are ideal op-amp characteristics?
3. What is input offset voltage?
E7G the easy way: all 12 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7G01 What is the typical output impedance of an op-amp?
Answer Very low
Tip An op-amp drives its output hard, so its output impedance is very low.
E7G02 What is the frequency response of the circuit in E7-3 if a capacitor is added across the feedback resistor?
Answer Low-pass filter
Tip A capacitor across the feedback resistor rolls off the highs, turning the stage into a low-pass filter.
E7G03 What is the typical input impedance of an op-amp?
Answer Very high
Tip An op-amp draws almost no input current, so its input impedance is very high.
E7G04 What is meant by the term “op-amp input offset voltage”?
Answer The differential input voltage needed to bring the open loop output voltage to zero
Tip Input offset voltage is the tiny differential input you must apply to force the open-loop output back to zero.
E7G05 How can unwanted ringing and audio instability be prevented in an op-amp audio filter?
Answer Restrict both gain and Q
Tip Keep an op-amp audio filter from ringing by holding both the gain and the Q in check.
E7G06 What is the gain-bandwidth of an operational amplifier?
Answer The frequency at which the open-loop gain of the amplifier equals one
Tip Gain-bandwidth is the frequency where open-loop gain has fallen all the way to one.
E7G07 What voltage gain can be expected from the circuit in Figure E7-3 when R1 is 10 ohms and RF is 470 ohms?
Answer 47
Tip Inverting gain is RF over R1, so 470 over 10 is 47.
E7G08 How does the gain of an ideal operational amplifier vary with frequency?
Answer It does not vary with frequency
Tip An ideal op-amp has the same gain at every frequency, so gain does not vary with frequency.
E7G09 What will be the output voltage of the circuit shown in Figure E7-3 if R1 is 1,000 ohms, RF is 10,000 ohms, and 0.23 volts DC is applied to the input?
Answer -2.3 volts
Tip Gain is RF over R1, 10000 over 1000 is 10, inverted, so 0.23 volts in gives -2.3 volts out.
E7G10 What absolute voltage gain can be expected from the circuit in Figure E7-3 when R1 is 1,800 ohms and RF is 68 kilohms?
Answer 38
Tip Gain is RF over R1, 68000 over 1800 is about 38.
E7G11 What absolute voltage gain can be expected from the circuit in Figure E7-3 when R1 is 3,300 ohms and RF is 47 kilohms?
Answer 14
Tip Gain is RF over R1, 47000 over 3300 is about 14.
E7G12 What is an operational amplifier?
Answer A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance
Tip An op-amp is a high-gain, direct-coupled differential amp with very high input and very low output impedance.
5.8Heartbeats: Oscillators and Synthesizers
Every oscillator is Module 5 of the General course's feedback loop; the Extra pool names the feedback plumbing:
- Colpitts: feedback through a capacitive voltage divider.
- Hartley: feedback from a tapped inductor. (C for capacitor, Colpitts; the mnemonic writes itself.)
- Pierce: feedback through a quartz crystal, Module 4's piezoelectric resonator holding the frequency with crystal Q.
Modern radios rarely run these free; they discipline them:
- The phase-locked loop (PLL) closes a loop of phase detector, loop filter, and voltage-controlled oscillator, dragging the VCO into lock with a reference. A PLL performs frequency synthesis and FM demodulation among its talents, one architecture, many jobs.
- The direct digital synthesizer (DDS) computes its output: a phase accumulator stepping through a lookup table feeding a DAC. Its impurities are characteristic: spurious signals at discrete frequencies, quantization's fingerprints, filtered but never fully erased.
Real oscillators fight the physical world:
- Thermal drift is tamed with temperature-stable components, notably NP0 (C0G) capacitors, and beyond that with ovenized (OCXO) or temperature-compensated (TCXO) crystal oscillators; the modern summit is the GPS-disciplined oscillator, a local standard steered by atomic clocks in orbit, and the reason your FT8 clock requirement is trivially satisfiable.
- Microphonics, frequency wobble from mechanical vibration reaching the oscillator, are minimized by mechanically isolating the oscillator circuitry. An oscillator is a Section 4.4 piezoelectric sensor whether you wanted one or not; mounting it softly is the cure.
Knowledge Check 5.8
Tap an answer. In your own words first, then check yourself.
1. Which oscillator uses a tapped inductor for feedback?
2. What are a PLL's essential blocks?
3. What causes microphonics, and what is the cure?
E7H the easy way: all 13 questions, with the right answer
Every pool question in this group. Read the answer, let the tip make it stick.
E7H01 What are three common oscillator circuits?
Answer Colpitts, Hartley, and Pierce
Tip The three classic oscillators to remember are Colpitts, Hartley, and Pierce.
E7H02 What is a microphonic?
Answer Changes in oscillator frequency caused by mechanical vibration
Tip A microphonic is an oscillator whose frequency wobbles when the circuit is physically shaken.
E7H03 What is a phase-locked loop?
Answer An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator
Tip A PLL is a servo loop built from a phase detector, a low-pass filter, a VCO, and a stable reference.
E7H04 How is positive feedback supplied in a Colpitts oscillator?
Answer Through a capacitive divider
Tip Colpitts splits its feedback across a pair of capacitors, a capacitive divider in the tank.
E7H05 How is positive feedback supplied in a Pierce oscillator?
Answer Through a quartz crystal
Tip A Pierce oscillator feeds its feedback through a quartz crystal, which sets the frequency.
E7H06 Which of these functions can be performed by a phase-locked loop?
Answer Frequency synthesis and FM demodulation
Tip A PLL can both synthesize frequencies and demodulate FM by tracking the input phase.
E7H07 How can an oscillator’s microphonic responses be reduced?
Answer Mechanically isolate the oscillator circuitry from its enclosure
Tip Cut microphonics by mechanically isolating the oscillator from its case so vibration cannot reach it.
E7H08 Which of the following components can be used to reduce thermal drift in crystal oscillators?
Answer NP0 capacitors
Tip NP0 capacitors barely change with temperature, so they fight thermal drift in crystal oscillators.
E7H09 What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter?
Answer A direct digital synthesizer
Tip A phase accumulator, lookup table, DAC, and anti-alias low-pass filter together make a direct digital synthesizer.
E7H10 What information is contained in the lookup table of a direct digital synthesizer (DDS)?
Answer Amplitude values that represent the desired waveform
Tip A DDS lookup table stores the amplitude values that trace out the desired waveform.
E7H11 What are the major spectral impurity components of direct digital synthesizers?
Answer Spurious signals at discrete frequencies
Tip DDS cleanup is never perfect, so its main impurities are spurs at discrete frequencies.
E7H12 Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?
Answer Provide the crystal with a specified parallel capacitance
Tip A crystal lands on its marked frequency only when it sees its specified parallel load capacitance.
E7H13 Which of the following is a technique for providing highly accurate and stable oscillators needed for microwave transmission and reception?
Answer All these choices are correct
Tip GPS disciplining, oven control, and rubidium references all help, so all these choices are correct.
Review: every Exam Anchor from this module
E7A OR: 1 if any input is 1. NAND: 0 only when all inputs are 1. XNOR: 0 when exactly one input is 1. Truth table: inputs and their outputs. Positive logic: high = 1. Flip-flop: bistable, divides frequency by 2; four flip-flops divide by 16. Decade counter: one output per 10 inputs. Astable multivibrator: free-runs between states; monostable: temporary state for a set time.
E7B Class A: midway between saturation and cutoff, 100 percent conduction. Class AB push-pull: more than 180, less than 360 degrees per device. Class C on SSB: distortion and excessive bandwidth. Class D: switching for high efficiency; efficiency comes from the device living at saturation or cutoff; output needs a harmonic filter. Oscillation prevention: parasitic suppressors and/or neutralization. Grounded grid: low input impedance. Emitter follower: unity-ish voltage gain, high input and low output impedance.
E7C Chebyshev: passband ripple, sharp cutoff. Elliptical: sharpest cutoff, stop-band notches. Crystal lattice: narrow IF filtering for SSB/CW. High-Q filters ring. Absorptive filter: dissipates rather than reflects unwanted signals. Shape factor: adjacent-channel rejection measure. Cavity filter: repeater duplexers. Hilbert-transform filter: generates SSB in DSP. Matching: cancel the reactance, transform the resistance. Pi-network: two shunt capacitors and a series inductor, matching plus low-pass action; its Q is variable, unlike an L-network. T-network with series capacitors and shunt inductor: high-pass. Pi-L: added output inductor for greater harmonic suppression.
E7D Linear regulator: continuous conduction, excess voltage burned as heat; dropout voltage is its minimum input-output differential. Switching regulator: high efficiency from saturation/cutoff operation; high frequency permits small magnetics; generates switching noise. Capacitor-input filter: higher output voltage than choke input. Bleeder: discharges capacitors and stabilizes regulation. Solar charge controller: prevents battery overcharging.
E7E Reactance modulator: on the oscillator, FM; on an amplifier stage, PM. Balanced modulator: DSB suppressed carrier; filter selects the sideband. Product detector: SSB/CW reception. Diode envelope detector: recovers AM by rectification and filtering. Baseband: the message signal's frequency range before modulation. Pre-emphasis at the FM transmitter, de-emphasis at the receiver, jointly flattening the audio while defeating high-frequency noise.
E7F Direct sampling: the ADC digitizes RF directly. Nyquist: sample at no less than twice the highest frequency. ADC bit depth: set by required dynamic range. Decimation: sample-rate reduction by discarding samples, preceded by an anti-aliasing filter. FIR: linear phase available, computation heavy; taps are the coefficients. IIR: computationally efficient via feedback.
E7G Ideal op-amp: infinite gain, very high input impedance, very low output impedance. Inverting gain: Rf/Rin, inverted. Behavior and frequency response: determined by external feedback components. Input offset voltage: the input differential needed to zero the output, caused by internal imbalance.
E7H Colpitts: capacitive divider feedback. Hartley: tapped inductor. Pierce: crystal feedback. PLL: phase detector, loop filter, VCO; performs frequency synthesis and FM demodulation. DDS: phase accumulator plus lookup table; spurs at discrete frequencies. Thermal drift: NP0/C0G capacitors, TCXO/OCXO, GPS discipline. Microphonics: cured by mechanical isolation.
Before you move on
Eight families, one radio: you can now follow a signal from the oscillator that births it, through the modulator that shapes it, the amplifier classes that scale it, the filters and networks that groom it, into the DSP that increasingly is it, all fed by supplies you can specify and glued by op-amps you can configure. This module is the densest in the program; let the kit build spread it across your hands.
Module 6 turns from circuits to the signals themselves: Fourier's great idea, modulation indexes, the geometry of digital constellations, and spread spectrum.
© 2026 Anthony Pelfanio, K4DIA. All rights reserved.