Learn/ Extra course/Module 8 of 10
Extra Course, Module 8: Measurement and the Honest Receiver
Covers NCVEC 2024-2028 Extra Pool, Subelement E4 (Amateur Practices). Five questions on your exam come from this material.
Ray uncovers the corner of the bench you were never allowed to touch: the spectrum analyzer, the vector network analyzer, the good scope. "Extra measurement is about knowing what your instruments cannot tell you as well as what they can. And receiver performance is where amateurs are most often fooled by marketing. Five sections, and at the end of them, nobody sells you anything again."
8.1The Serious Instruments
Each instrument, with its blind spot:
- The digital oscilloscope's ceiling is its analog-to-digital converter's sampling rate: sample too slowly and Module 5's Nyquist law bites back as aliasing, a false, jittery low-frequency version of the waveform displayed as if real. The scope lies confidently when undersampled.
- Probe discipline: compensate the probe by displaying a square wave and adjusting until the tops are flat (Fourier in service of calibration: flat tops mean all harmonics arriving equally), and minimize the probe's ground lead length, because Module 3's lead inductance rings measurements at RF.
- For a linear power supply's ripple, trigger the scope on line: the ripple is locked to the AC mains, so line triggering freezes it.
- The spectrum analyzer displays amplitude versus frequency, and it is the instrument for viewing spurious signals and intermodulation products from an SSB transmitter. The scope shows the wave; the analyzer shows the sins.
- A frequency counter reaching above its range uses a prescaler to divide the signal down to countable frequencies.
- The antenna analyzer's advantage over a simple SWR bridge is that it computes SWR and impedance automatically, and its reach extends to velocity factor, cable length, and the resonant frequency of tuned circuits. All of these. SWR itself is measured by directional wattmeter, antenna analyzer, or vector network analyzer: all of these too.
Knowledge Check 8.1
Tap an answer. In your own words first, then check yourself.
1. What limits the highest frequency a digital oscilloscope can display accurately?
2. How is an oscilloscope probe compensated?
3. Which instrument displays an SSB transmitter's intermodulation products?
E4A 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.
E4A01 Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?
Answer Sampling rate of the analog-to-digital converter
Tip A digital scope can only capture as fast as it samples, so the ADC sampling rate sets the highest frequency it can show.
E4A02 Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?
Answer Signal amplitude and frequency
Tip A spectrum analyzer plots amplitude up the side and frequency across the bottom, so you see how much signal sits at each frequency.
E4A03 Which of the following test instruments is used to display spurious signals and/or intermodulation distortion products generated by an SSB transmitter?
Answer Spectrum analyzer
Tip To catch spurs and intermod from an SSB rig you need a frequency-domain view, so reach for the spectrum analyzer.
E4A04 How is compensation of an oscilloscope probe performed?
Answer A square wave is displayed, and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible
Tip Feed the probe a square wave and tweak its trimmer until the flat tops and bottoms sit truly flat, no overshoot or droop.
E4A05 What is the purpose of using a prescaler with a frequency counter?
Answer Reduce the signal frequency to within the counter's operating range
Tip A prescaler divides a too-fast signal down until it lands inside the counter's working range.
E4A06 What is the effect of aliasing on a digital oscilloscope when displaying a waveform?
Answer A false, jittery low-frequency version of the waveform is displayed
Tip Sample too slowly and the scope folds the signal down into a phantom, jittery low-frequency wave that is not really there.
E4A07 Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge?
Answer Antenna analyzers compute SWR and impedance automatically
Tip An antenna analyzer does the math for you, reading out SWR and impedance directly instead of just nulling a bridge.
E4A08 Which of the following is used to measure SWR?
Answer All these choices are correct
Tip A directional wattmeter, an antenna analyzer, and an SWR bridge all get you there, so every choice measures SWR.
E4A09 Which of the following is good practice when using an oscilloscope probe?
Answer Minimize the length of the probe's ground connection
Tip Keep the probe ground lead short so its stray inductance cannot ring and distort fast edges.
E4A10 Which trigger mode is most effective when using an oscilloscope to measure a linear power supply’s output ripple?
Answer Line
Tip Ripple from a linear supply rides on the AC mains, so the Line trigger locks it rock steady on screen.
E4A11 Which of the following can be measured with an antenna analyzer?
Answer All these choices are correct
Tip SWR, impedance, and resonant frequency all fall out of an antenna analyzer, so every choice is fair game.
8.2Measuring Without Fooling Yourself
Measurement technique is mostly self-defense:
- A frequency counter's accuracy rests on its time base: the counter is only as honest as the oscillator it counts against, which is why Module 5's GPS-disciplined references matter on the bench.
- An analog voltmeter's ohms-per-volt rating decodes as loading: full-scale reading times ohms-per-volt equals the meter's input impedance, the General course's loading rule made computable.
- A directional wattmeter reading 100 watts forward and 25 watts reflected is delivering 75 watts to the load: absorbed power is forward minus reflected.
- A tuned circuit's Q is measured from the bandwidth of its frequency response, Module 3's f/Q relationship run backward: sweep it, find the half-power points, divide.
The vector network analyzer deserves its own paragraph, because the hundred-dollar NanoVNA has made this a mainstream amateur instrument. Its language is S parameters, whose subscripts name the ports measured: S11 is the input-port reflection (return loss, the VSWR-equivalent), S21 is forward gain through the device. A two-port VNA therefore measures a filter's frequency response end to end, along with input and output impedance and reflection coefficient. All of these. Its honesty ritual is calibration against three standards: a short, an open, and a 50-ohm load, teaching the instrument what perfection looks like before it judges anything else.
The formal transmitter linearity test carries over from General with its instrumentation specified: two non-harmonically related audio tones in, spectrum analyzer on the output, intermodulation products read directly.
Knowledge Check 8.2
Tap an answer. In your own words first, then check yourself.
1. Which S parameter is forward gain?
2. What three standards calibrate a VNA?
3. Your wattmeter shows 100 watts forward, 25 reflected. Power absorbed by the load?
E4B 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.
E4B01 Which of the following factors most affects the accuracy of a frequency counter?
Answer Time base accuracy
Tip A counter is only as good as its clock, so the time base reference sets its accuracy.
E4B02 What is the significance of voltmeter sensitivity expressed in ohms per volt?
Answer The full scale reading of the voltmeter multiplied by its ohms per volt rating is the input impedance of the voltmeter
Tip Multiply the full-scale volts by the ohms-per-volt rating to get the meter's input impedance on that range.
E4B03 Which S parameter is equivalent to forward gain?
Answer S21
Tip S21 is signal out of port 2 for drive into port 1, which is forward gain.
E4B04 Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?
Answer S11
Tip S11 is what bounces back out of the input port, so it maps to return loss, reflection, and VSWR.
E4B05 What three test loads are used to calibrate an RF vector network analyzer?
Answer Short circuit, open circuit, and 50 ohms
Tip Calibrate a VNA with the SOL set, a short, an open, and a 50 ohm load.
E4B06 How much power is being absorbed by the load when a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power?
Answer 75 watts
Tip Power into the load is forward minus reflected, so 100 minus 25 is 75 watts absorbed.
E4B07 What do the subscripts of S parameters represent?
Answer The port or ports at which measurements are made
Tip The two subscripts name the output and input ports, so they tell you where each measurement was taken.
E4B08 Which of the following can be used to determine the Q of a series-tuned circuit?
Answer The bandwidth of the circuit's frequency response
Tip Q equals center frequency over bandwidth, so measure the width of the response to back out Q.
E4B09 Which of the following can be measured by a two-port vector network analyzer?
Answer Filter frequency response
Tip A two-port VNA drives one end and reads the other, which is exactly what tracing a filter's frequency response needs.
E4B10 Which of the following methods measures intermodulation distortion in an SSB transmitter?
Answer Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
Tip Feed two non-harmonic audio tones in and watch the spectrum analyzer for the mixing products that expose intermod.
E4B11 Which of the following can be measured with a vector network analyzer?
Answer All these choices are correct
Tip Impedance, return loss, and gain all live in the S-parameters, so a VNA measures every one.
8.3The Receiver's Noise Ledger
Receiver performance begins at the bottom, with noise:
- The universe sets the floor: -174 dBm is the thermal noise in a 1 Hz bandwidth at room temperature, the quietest any input can theoretically be. A receiver's noise figure is how far, in dB, its own added noise sits above that theoretical minimum.
- Bandwidth buys noise: widen from 50 Hz to 1000 Hz and the noise floor rises 13 dB (10 log of 20). This is the arithmetic underneath three courses of "match the bandwidth to the mode." MDS, the minimum discernible signal, names the weakest signal detectable above that floor.
- The low-band paradox, resolved: input attenuation on 160 or 80 meters barely hurts signal-to-noise, because atmospheric noise exceeds receiver noise even after attenuation. The band's own roar drowns the receiver's whisper; attenuate both and the ratio survives. This is also Module 7's receiving-antenna license, restated from the receiver's side.
Then the modern failure modes, all children of the strong-signal environment:
- Reciprocal mixing: the local oscillator's phase noise mixes with strong adjacent signals, painting noise onto your quiet frequency. In an SDR, the same disease lives in the master clock oscillator's phase noise. Oscillator purity is receiver performance.
- The architectural defenses: a high IF makes image rejection easier for the front-end filtering (the image sits farther away), a narrow roofing filter improves blocking dynamic range by discarding strong near-frequency signals before they reach vulnerable stages, and the front-end preselector remains the cure for strong out-of-band signals. IF shift handles the tactical case, sliding the passband away from an adjacent interferer.
- One FM-specific behavior gets named: capture effect, the suppression of the weaker of two same-frequency FM signals by the stronger, the Technician course's "FM winner takes all" with its proper title.
- And the SDR's hard ceiling: overload occurs when input signals exceed the ADC's reference voltage, Module 5's converter running out of numbers.
Knowledge Check 8.3
Tap an answer. In your own words first, then check yourself.
1. What does -174 dBm represent?
2. Why can you attenuate an 80-meter receiver's input almost for free?
3. What is reciprocal mixing?
E4C 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.
E4C01 What is an effect of excessive phase noise in an SDR receiver’s master clock oscillator?
Answer It can combine with strong signals on nearby frequencies to generate interference
Tip A dirty master clock smears strong neighbors across your passband, so phase noise turns nearby signals into interference.
E4C02 Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?
Answer A front-end filter or preselector
Tip A front-end filter or preselector blocks the out-of-band offenders before they can overload the receiver.
E4C03 What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency?
Answer Capture effect
Tip In FM the louder signal wins and mutes the weaker one on the same frequency, and that winner-take-all is the capture effect.
E4C04 What is the noise figure of a receiver?
Answer The ratio in dB of the noise generated by the receiver to the theoretical minimum noise
Tip Noise figure is how many dB worse the receiver is than a perfect, noiseless one.
E4C05 What does a receiver noise floor of -174 dBm represent?
Answer The theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature
Tip At room temperature the thermal noise in a 1 Hz slice is -174 dBm, the floor no real receiver beats.
E4C06 How much does increasing a receiver’s bandwidth from 50 Hz to 1,000 Hz increase the receiver’s noise floor?
Answer 13 dB
Tip Noise rises as 10 log of the bandwidth ratio, and 10 log(1000/50) is 10 log 20, about 13 dB.
E4C07 What does the MDS of a receiver represent?
Answer The minimum discernible signal
Tip MDS just spells out minimum discernible signal, the faintest signal you can pull out of the noise.
E4C08 An SDR receiver is overloaded when input signals exceed what level?
Answer The reference voltage of the analog-to-digital converter
Tip An SDR clips once the signal tops the ADC's reference voltage, the ceiling of what it can digitize.
E4C09 Which of the following choices is a good reason for selecting a high IF for a superheterodyne HF or VHF communications receiver?
Answer Easier for front-end circuitry to eliminate image responses
Tip A high IF pushes the image far from the wanted signal, so the front-end filter rejects it easily.
E4C10 What is an advantage of having a variety of receiver bandwidths from which to select?
Answer Receive bandwidth can be set to match the modulation bandwidth, maximizing signal-to-noise ratio and minimizing interference
Tip Matching the bandwidth to the signal lets in all the signal and little else, boosting SNR and cutting interference.
E4C11 Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?
Answer Atmospheric noise is generally greater than internally generated noise even after attenuation
Tip On the low HF bands the band noise dwarfs the receiver's own, so padding the input cuts overload without hurting SNR.
E4C12 How does a narrow-band roofing filter affect receiver performance?
Answer It improves blocking dynamic range by attenuating strong signals near the receive frequency
Tip A roofing filter knocks down strong close-in signals early, so it widens blocking dynamic range.
E4C13 What is reciprocal mixing?
Answer Local oscillator phase noise mixing with adjacent strong signals to create interference to desired signals
Tip A noisy local oscillator mixes with a strong neighbor and smears its phase noise onto your frequency, that is reciprocal mixing.
E4C14 What is the purpose of the receiver IF Shift control?
Answer To reduce interference from stations transmitting on adjacent frequencies
Tip IF Shift slides the passband off an adjacent interferer while keeping your wanted signal in the audio.
8.4Dynamic Range and the Numbers That Matter
The specification sheet, decoded:
- Blocking dynamic range: the difference in dB between the noise floor and the signal level causing 1 dB of gain compression. It is the receiver's headroom, and poor dynamic range shows up as cross-modulation and desensitization from strong adjacent signals: the loud neighbor audible everywhere, the band mysteriously deaf.
- Desensitization itself: reduced sensitivity caused by a strong signal near the received frequency, and its field cure is Section 8.3's logic inverted: insert attenuation before the first RF stage, sacrificing gain to restore linearity.
- Intermodulation keeps its cause (nonlinear circuits) and gains its ugliest habitat: two nearby repeaters interfering because their output signals mix in the final amplifier of one or both transmitters. The cure is a properly terminated circulator at the transmitter output, a ferrite one-way valve (Module 4's ferrite physics, promoted) that lets power out but not in. And the reason odd-order products obsess receiver designers stands from General: odd-order products of in-band signals land in-band.
- The benchmark number: a third-order intercept of 40 dBm means a pair of 40 dBm input signals would theoretically produce third-order products equal in amplitude to the inputs themselves. The intercept is a mathematical extrapolation, never an operating condition, but higher means more linear, and it is the one number on the specification sheet that predicts crowded-band behavior.
The engineering payoff is the link budget arithmetic, General Module 6's concept now computed in dBm: received level = transmit power + gains - losses. The pool's pair: 10 W (+40 dBm) + 6 dBi + 3 dBi - 100 dB path = -51 dBm received. And link margin = received level minus (MDS + required SNR): +40 +10 -3 -136 = -89 dBm received, against -103 dBm MDS plus 6 dB required = -97 dBm needed, margin +8 dB. One last unit check the pool loves: -100 dBm is 0.1 picowatt. Ten orders of magnitude below your transmit watt, and routinely copied: that number is the whole miracle of radio, stated in dB.
Knowledge Check 8.4
Tap an answer. In your own words first, then check yourself.
1. What is blocking dynamic range?
2. Two co-sited repeaters generate intermod. What is the standard cure?
3. What is the received signal level with +40 dBm transmit, 6 dBi and 3 dBi antenna gains, and 100 dB path loss?
E4D 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.
E4D01 What is meant by the blocking dynamic range of a receiver?
Answer The difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression
Tip Blocking dynamic range runs from the noise floor up to the signal level that squashes gain by 1 dB.
E4D02 Which of the following describes problems caused by poor dynamic range in a receiver?
Answer Spurious signals caused by cross modulation and desensitization from strong adjacent signals
Tip Weak dynamic range shows up as cross-modulation spurs and as desensitization when a strong neighbor hits.
E4D03 What creates intermodulation interference between two repeaters in close proximity?
Answer The output signals mix in the final amplifier of one or both transmitters
Tip One transmitter's output sneaks into the other's final amplifier and mixes there, breeding the intermod.
E4D04 Which of the following is used to reduce or eliminate intermodulation interference in a repeater caused by a nearby transmitter?
Answer A properly terminated circulator at the output of the repeater’s transmitter
Tip A terminated circulator lets your power out but dumps the intruding signal into its load, so it cannot mix in the final.
E4D06 What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
Answer Desensitization
Tip A strong nearby signal drives the front end toward compression and deafens it, which is desensitization.
E4D07 Which of the following reduces the likelihood of receiver desensitization?
Answer Insert attenuation before the first RF stage
Tip Pad the input ahead of the first RF stage so the strong signal no longer overdrives the front end.
E4D08 What causes intermodulation in an electronic circuit?
Answer Nonlinear circuits or devices
Tip Intermod is born only where a device is nonlinear and lets signals multiply instead of simply add.
E4D09 What is the purpose of the preselector in a communications receiver?
Answer To increase the rejection of signals outside the band being received
Tip A preselector is a tunable bandpass up front that sharpens rejection of signals outside your band.
E4D10 What does a third-order intercept level of 40 dBm mean with respect to receiver performance?
Answer A pair of 40 dBm input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals
Tip At the 40 dBm intercept the extrapolated third-order product rises to match the test tones themselves, and higher IP3 means a cleaner front end.
E4D11 Why are odd-order intermodulation products, created within a receiver, of particular interest compared to other products?
Answer Odd-order products of two signals in the band being received are also likely to be within the band
Tip Odd-order products land close to the originals, so two in-band signals breed interference right inside the band.
E4D12 What is the link margin in a system with a transmit power level of 10 W (+40 dBm), a system antenna gain of 10 dBi, a cable loss of 3 dB, a path loss of 136 dB, a receiver minimum discernable signal of -103 dBm, and a required signal-to-noise ratio of 6 dB?
Answer +8dB
Tip Received is 40 +10 -3 -136 = -89 dBm, and margin is that minus the -103 MDS plus 6 dB SNR, so -89 -(-97) = +8 dB.
E4D13 What is the received signal level with a transmit power of 10 W (+40 dBm), a transmit antenna gain of 6 dBi, a receive antenna gain of 3 dBi, and a path loss of 100 dB?
Answer -51 dBm
Tip Add the gains and subtract the loss, 40 +6 +3 -100 = -51 dBm.
E4D14 What power level does a receiver minimum discernible signal of -100 dBm represent?
Answer 0.1 picowatts
Tip Minus 100 dBm is ten decades below a milliwatt, 10^-13 watts, which is 0.1 picowatt.
8.5The Noise Around You
The final measurement discipline is the noisiest one: everything radiating in and around your own station.
Identification by fingerprint:
- Switch-mode power supplies: carriers at regular intervals across a wide frequency range, the harmonics of the switching frequency marching up the band.
- Computer network equipment: unstable modulated or unmodulated signals at specific frequencies.
- Arcing in its loud AC form: intermittent roaring or buzzing from arcing thermostats, defective doorbell transformers, or failing illuminated signs. All of these.
- The strangest one: corroded metal connections mixing and re-radiating local AM broadcast signals, a rusty downspout as an unlicensed mixer, generating Section 8.4's intermodulation with no electronics at all.
Suppression by mechanism:
- Vehicle charging-system noise: ferrite chokes on the charging system leads. AC motor noise: a brute-force AC line filter in the power leads. Each cure blocks the conducted path.
- Common-mode current, the recurring villain since the Technician course, gets its formal definition: current flowing equally on all conductors of a cable (or on a shield), and it is exactly what makes shielded cables radiate or receive interference despite the shield. The shield only works when the noise is differential; common-mode rides the outside.
- The receiver-side tools carry their known limits: the noise blanker removes impulse noise but can make strong signals appear distorted and spurious; digital noise reduction helps against most noise types; and the automatic notch filter must be off for CW, because it will efficiently remove the CW signal itself, which is, after all, a carrier.
The panel that ties three courses together: lightning protection concludes with the single point ground panel, the grounded entry plate from your General station now named and specified. Its purpose: ensuring all lightning protectors activate at the same time, no potential differences between protected lines for a surge to exploit. And the station's AC surge protector belongs on that same panel, unifying every conductor's protection at one point. The panel is the physical answer to a question all three safety modules have been building toward: where does everything meet? Here.
Knowledge Check 8.5
Tap an answer. In your own words first, then check yourself.
1. Interference appearing as evenly spaced carriers across a wide range usually indicates:
2. Why can a shielded cable still radiate interference?
3. Where does the station's AC surge protector belong?
E4E 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.
E4E01 What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals?
Answer Removal of the CW signal as well as the interfering carrier
Tip An auto notch sees steady CW as just another carrier, so it can null your Morse along with the interference.
E4E02 Which of the following types of noise can often be reduced by a digital noise reduction?
Answer All these choices are correct
Tip DSP noise reduction chews on random, atmospheric, and man-made hiss, so every listed noise type qualifies.
E4E03 Which of the following types of noise are removed by a noise blanker?
Answer Impulse noise
Tip A noise blanker gates out short spikes, so it targets impulse noise like ignition pulses.
E4E04 How can conducted noise from an automobile battery charging system be suppressed?
Answer By installing ferrite chokes on the charging system leads
Tip Clamp ferrite chokes on the charging system leads to choke off the conducted alternator hash.
E4E05 What is used to suppress radio frequency interference from a line-driven AC motor?
Answer A brute-force AC-line filter in series with the motor’s power leads
Tip A brute-force line filter in series with the motor's power leads blocks the noise from riding the AC wiring.
E4E06 What type of electrical interference can be caused by computer network equipment?
Answer The appearance of unstable modulated or unmodulated signals at specific frequencies
Tip Network gear leaks birdies, unstable carriers at set frequencies that drift and wobble across the dial.
E4E07 Which of the following can cause shielded cables to radiate or receive interference?
Answer Common-mode currents on the shield and conductors
Tip When shield and conductors all carry current the same way, that common-mode flow turns the cable into an antenna.
E4E08 What current flows equally on all conductors of an unshielded multiconductor cable?
Answer Common-mode current
Tip Current flowing the same direction on every conductor at once is common-mode current.
E4E09 What undesirable effect can occur when using a noise blanker?
Answer Strong signals may be distorted and appear to cause spurious emissions
Tip An aggressive blanker can chop strong signals too, distorting them so they look like spurious emissions.
E4E10 Which of the following can create intermittent loud roaring or buzzing AC line interference?
Answer All these choices are correct
Tip Arcing insulators, failing transformers, and loose hardware can all roar on the line, so every choice fits.
E4E11 What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?
Answer Nearby corroded metal connections are mixing and reradiating the broadcast signals
Tip Corroded joints act like diodes, mixing strong AM stations and reradiating the products, the classic rusty-bolt effect.
E4E12 What causes interference received as a series of carriers at regular intervals across a wide frequency range?
Answer Switch-mode power supplies
Tip A switcher's sharp clock throws harmonics every so many kHz, a comb of carriers marching across the bands.
E4E13 Where should a station AC surge protector be installed?
Answer On the single point ground panel
Tip Mount the AC surge protector right on the single point ground panel so everything clamps to one common reference.
E4E14 What is the purpose of a single point ground panel?
Answer Ensure all lightning protectors activate at the same time
Tip Tying every protector to one panel keeps them at the same potential so they all fire together during a strike.
Review: every Exam Anchor from this module
E4A Digital scope limit: ADC sampling rate; undersampling causes aliasing (false jittery low-frequency waveform). Compensate probes with a square wave until flat-topped; keep probe ground leads short. Line trigger for supply ripple. Spectrum analyzer: amplitude vs. frequency; shows spurs and intermod. Prescaler: divides frequency into the counter's range. Antenna analyzer: automatic SWR and impedance; also velocity factor, cable length, resonance (all). SWR measurement: wattmeter, analyzer, or VNA (all).
E4B Frequency counter accuracy: the time base. Voltmeter input impedance: full scale times ohms-per-volt. 100 W forward, 25 W reflected: 75 W absorbed. Q: from the frequency-response bandwidth. S-parameter subscripts: the ports; S11 input return loss, S21 forward gain. Two-port VNA: filter response, impedances, reflection coefficient (all). VNA calibration: short, open, 50 ohms. IMD test: two non-harmonic audio tones plus a spectrum analyzer.
E4C -174 dBm: thermal noise, 1 Hz, room temperature. Noise figure: dB above the theoretical minimum. 50 to 1000 Hz bandwidth: +13 dB noise floor. MDS: minimum discernible signal. Low-band attenuation is nearly free: atmospheric noise dominates. Reciprocal mixing: LO (or SDR master clock) phase noise mixing with strong adjacent signals. High IF: easier image rejection. Roofing filter: better blocking dynamic range. Preselector: rejects out-of-band signals. IF shift: dodges adjacent interference. Capture effect: stronger FM signal suppresses the weaker. SDR overload: signals exceeding the ADC reference voltage.
E4D Blocking dynamic range: noise floor to 1 dB compression. Poor dynamic range: cross-modulation and desense. Desensitization: strong nearby signal; cure: attenuation ahead of the first RF stage. Repeater intermod: outputs mixing in a final amplifier; cure: terminated circulator. Odd-order products fall in-band. Third-order intercept 40 dBm: theoretical equal-amplitude intermod at 40 dBm inputs. Link math: +40 dBm +6 +3 -100 = -51 dBm; the margin example computes to +8 dB. -100 dBm = 0.1 picowatt.
E4E Switch-mode supplies: regular carriers across a wide spectrum. Network gear: unstable signals at specific frequencies. Roaring/buzzing line noise: arcing thermostats, doorbells, signs (all). Corroded connections re-radiate mixed broadcast signals. Vehicle charging noise: ferrite chokes on the leads. AC motors: brute-force line filter. Common-mode current: equal on all conductors; makes shielded cable radiate or receive. Noise blanker: distorts strong signals. Digital NR: broadly effective. Auto-notch: removes CW signals; disable for CW. Single point ground panel: all protectors fire together; the AC surge protector mounts on it.
Before you move on
The forbidden corner of the bench is yours now: you can compensate a probe, calibrate a VNA against its three truths, read S parameters, audit a receiver's noise ledger from -174 dBm up, decode a specification sheet's dynamic-range claims, compute a link budget in dBm, and identify the neighborhood's noise sources by their fingerprints. Nobody sells you anything again, as promised.
Module 9 takes the engineered station to the frontier: satellites in full, amateur television, contest and DX craft, and the digital procedures of moonbounce and meteor scatter.
© 2026 Anthony Pelfanio, K4DIA. All rights reserved.