General Course, Module 6: Signals, Sidebands, and Digital

6.1The Ways Information Rides a Wave

There are only three properties of a carrier you can vary, so there are only three great families of modulation:

• Amplitude modulation varies the instantaneous power level of the RF signal. Its visual definition on a scope: the modulation envelope, the waveform created by connecting the peak values of the modulated signal. AM's refined child is single sideband, still the narrowest-bandwidth phone emission, which is why it owns HF voice.
• Frequency modulation changes the instantaneous frequency of the wave to convey information.
• Phase modulation changes the phase angle. FM and PM are close cousins, and one construction detail proves it: a reactance modulator connected to a transmitter's RF amplifier stage produces phase modulation.

Digital modes just automate these levers. Direct binary FSK is generated by changing an oscillator's frequency directly with a digital control signal, machine fingers on the frequency lever. FT8 uses 8-tone frequency shift keying, eight distinct tones instead of two. And QPSK works the phase lever with precision: digital data transmitted as 0-, 90-, 180-, and 270-degree phase shifts, each shift representing a pair of bits. The exam adds that QPSK31 is sideband sensitive, includes error correction, and occupies about the same bandwidth as BPSK31: all of these.

Now the ugly signal on Ray's waterfall. Overmodulation (too much audio drive) causes excessive bandwidth: your 3 kHz conversation smeared across your neighbors' frequencies. On a scope it appears as flat-topping, distortion from excessive drive or speech levels where the envelope peaks slam into the amplifier's ceiling and shear off flat. Loud is not the goal; clean is the goal. Module 8's ALC discussion will show you the knob that prevents it.

Two engineering terms close the group, borrowed from the microwave world and increasingly on HF digital lips:

• A link budget: the sum of transmit power and antenna gains, minus system losses, as seen at the receiver. It is the accounting sheet for whether a path can work.
• Link margin: the difference between the received power level and the minimum required signal level at the receiver's input. Budget is the plan; margin is the surplus.

6.2Mixers, Bandwidth, and the Signals You Did Not Mean to Make

Module 5 introduced the mixer; now the numbers. Feed a mixer two signals and its output contains their sum and difference frequencies. That act of mixing has a proper name, heterodyning, and it is how a receiver converts any signal to its intermediate frequency: the local oscillator is the mixer input that gets tuned, steering whichever incoming frequency you want onto the fixed IF.

The design has one famous blind spot: a signal at twice the IF away from your desired signal also mixes to the same IF. That unwanted guest is the image response, and rejecting images is a chief job of the filtering ahead of the mixer.

FM transmitters for VHF often generate their signal at a low frequency and use a multiplier stage, which produces a harmonic of the lower-frequency signal, to reach the operating frequency. Multiplication scales everything, including deviation, which sets up the exam's arithmetic: a 146.52 MHz transmitter with 5 kHz deviation built on a 12.21 MHz oscillator (a 12x multiplication) needs oscillator deviation of 5000/12 = 416.7 Hz.

Bandwidth rules to carry:

• FM bandwidth is approximately twice the sum of deviation and modulating frequency: 5 kHz deviation with 3 kHz audio occupies 2 x (5 + 3) = 16 kHz.
• For digital modes, higher symbol rates require wider bandwidth. Speed costs spectrum, always.
• On receive, matching receiver bandwidth to the mode's bandwidth yields the best signal-to-noise ratio, the reason behind Module 8 of the Technician course telling you to pick 2400 Hz for SSB.
• And know your mode's duty cycle: some modes (FT8, RTTY, anything key-down continuous) have duty cycles high enough to exceed a transmitter's average power rating. A rig rated 100 watts SSB is rated for voice's peaks-and-valleys; run it at 100 watts of continuous FT8 and you are cooking the finals. Turn it down.

Finally, the signals you did not mean to make. Intermodulation is the process where two signals combine in a non-linear circuit to produce unwanted spurious outputs. The products that land closest to the original frequencies, and therefore cause the most trouble, are the odd-order products, with 2F1 - F2 as the classic third-order example. When two strong signals visit one overloaded circuit, their intermod children move in next door to everyone.

6.3Reading the Waterfall

Back to the big monitor, and the vocabulary to read it.

First, the display itself: on a waterfall, frequency runs horizontally, signal strength is shown by intensity (color or brightness), and time flows vertically. Every mode paints a recognizable texture on it, and one diagnostic pattern the exam checks: vertical lines appearing on either side of a data or RTTY signal mean overmodulation, Section 6.1's sin rendered visible.

The digital modes each contribute their exam facts:

• RTTY uses Baudot code, a 5-bit code with additional start and stop bits, and FSK's two frequencies are named mark and space. If RTTY will not decode despite looking right, the causes are reversed mark and space, wrong baud rate, or wrong sideband. All of these.
• PSK31 sends characters in Varicode, where common letters get short codes. The catch: uppercase letters use longer Varicode sequences and slow the transmission. PSK31 etiquette is lowercase for a reason.
• FT8 is the narrow-band mode that decodes at very low signal-to-noise ratios, and its signal reports are literal: a report of +3 means a signal-to-noise ratio of +3 dB referenced to a 2.5 kHz bandwidth. Machine-honest numbers instead of the flattering S9.
• WSPR is the specialist: a low-power beacon mode for assessing HF propagation, the automated descendant of Module 1's beacon network.

Error handling comes in two philosophies:

• ARQ, from your Technician course, now with protocol detail: a NAK response means the receiver is requesting retransmission of a packet, and if excessive transmission attempts still fail, the connection is dropped.
• FEC, forward error correction, skips the conversation: the transmitter sends redundant information along with the data, and the receiver repairs errors on its own. FT8's resilience lives here.

Packet structure repeats one Technician fact with sharper aim: the header carries the routing and handling information. And mesh networks earn their resilience honestly: if one node fails, a packet can still reach its target via alternate nodes.

Digital voice closes the module with a list to memorize as a set: DMR, D-STAR, and System Fusion are the digital voice modes. You programmed two of them at Technician; now you can name the family.