2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers
Key Takeaways
- Every transmitter stage serves one of three jobs: generate a clean signal, impress the intelligence on it, and raise the power - with an output filter to keep harmonics off the air.
- SSB filter method: microphone, audio amplifier, balanced modulator, sideband filter, mixer with the VFO, driver, linear PA, low-pass filter. A 9.000 MHz carrier oscillator mixed with a 5.000-5.350 MHz VFO covers the whole 14.000-14.350 MHz VK 20 m band.
- FM transmitters modulate the oscillator with a reactance or varactor stage and may use an efficient Class C PA; a 12.000 MHz crystal through x3, x2 and x2 gives 144.000 MHz and multiplies deviation by 12 as well.
- In a PLL synthesiser f_out = N x f_reference, so N = 11720 with a 12.5 kHz reference produces 146.500 MHz, and changing channel means loading a new value of N.
- A buffer amplifier isolates the oscillator from the stages that follow so keying and load changes cannot pull the transmitted frequency.
2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers
ACMA Exam Focus: Syllabus item 5.1 - identify the stages of simple AM, SSB and FM transmitters from a block diagram and state the function of each: power supply, audio input stage, carrier oscillator, variable frequency oscillator (VFO), mixer, frequency multipliers, modulator, output amplifier and output filter.
A transmitter has three jobs, and every block on the diagram serves one of them: generate a clean signal on the wanted frequency, impress the intelligence (speech, Morse or data) onto it, and raise the power to the level permitted by your licence - up to 100 W peak envelope power for a VK Standard licensee on most HF bands - without putting anything on the air outside the intended channel.
The stages and what each one does
| Stage | Function |
|---|---|
| Power supply | Converts 230 V 50 Hz mains, or a 13.8 V battery, into the regulated DC rails every other stage needs. A 100 W HF transceiver typically requires 13.8 V at about 20-22 A on speech peaks. |
| Audio input stage | Amplifies the millivolt-level output of the microphone, limits the audio bandwidth to roughly 300-3000 Hz, and may apply speech compression. Its gain is set by the microphone gain control. |
| Carrier oscillator | Produces the fixed-frequency carrier that is fed to the modulator. In an SSB rig this is a crystal oscillator at the IF, commonly 9 MHz. |
| Variable frequency oscillator (VFO) | Produces the tunable signal that determines the final operating frequency. In modern rigs it is a PLL or DDS synthesiser rather than an LC circuit. |
| Mixer | Combines the fixed modulated IF signal with the VFO to translate the whole signal onto the chosen amateur band. |
| Frequency multiplier | A stage biased into Class C so it is deliberately non-linear, with its output tuned to the second or third harmonic of its input. Used to reach VHF and UHF from a lower-frequency oscillator. |
| Modulator | Adds the intelligence to the carrier - amplitude modulator for AM, balanced modulator for SSB, reactance/varactor modulator for FM. |
| Output amplifier (PA) | Raises the low-level signal to the final output power. Must be linear (Class AB) for SSB and AM; may be efficient Class C for CW and FM. |
| Output filter | A low-pass or band-pass filter between the PA and the antenna socket that attenuates harmonics and other spurious products so only the wanted signal reaches the antenna. |
Simple AM transmitter
The classic high-level AM chain runs: carrier oscillator - buffer - driver - Class C output amplifier, with the audio path running microphone - audio amplifier - modulator into the output amplifier's supply rail. The audio power amplifier varies the DC voltage feeding the PA, so the RF envelope follows the speech waveform. High-level modulation requires an audio amplifier delivering roughly half the DC input power of the PA, which is why valve AM transmitters had such large modulation transformers.
Simple SSB transmitter (filter method)
This is the architecture inside virtually every VK HF transceiver:
Microphone - audio amplifier - balanced modulator - sideband filter - mixer (with VFO) - driver - linear PA - low-pass filter - antenna
The balanced modulator cancels the carrier, leaving a double-sideband suppressed-carrier signal. The crystal sideband filter then removes one sideband, leaving single sideband. Worked example: with a carrier oscillator on 9.000 MHz and a VFO tuning 5.000-5.350 MHz, the mixer's sum product covers 14.000 to 14.350 MHz - exactly the VK 20 m band. The unwanted difference product (3.650-4.000 MHz) is removed by the band-pass filter following the mixer. Note that the PA must be linear, because an SSB envelope carries the speech waveform; a Class C amplifier would destroy it.
Simple FM transmitter
Microphone - audio amplifier - pre-emphasis and limiter - reactance (varactor) modulator on the oscillator - frequency multipliers - Class C PA - filter - antenna
Because the information is in the frequency, not the amplitude, an efficient non-linear Class C PA can be used. Worked example: a 12.000 MHz crystal oscillator followed by multipliers of x3, x2 and x2 gives an overall multiplication of 12 and an output of 12.000 x 12 = 144.000 MHz in the VK 2 m band. Remember that multiplication also multiplies the deviation: about 417 Hz of deviation at the oscillator becomes 417 x 12 = approximately 5 kHz at 144 MHz, the standard deviation for narrowband amateur FM.
Oscillators and frequency synthesisers
An oscillator is an amplifier with positive feedback through a frequency-determining network. Three LC topologies appear in the syllabus:
- Hartley - feedback taken from a tapped inductor.
- Colpitts - feedback taken from a capacitive divider across the coil.
- Clapp - a Colpitts with a small capacitor in series with the coil, so that large fixed capacitors swamp the transistor's own junction capacitance and improve stability.
A quartz crystal oscillator replaces the LC tank with a piezoelectric crystal whose Q may exceed 100,000, compared with perhaps 200 for an LC circuit. It is fixed in frequency but far more stable, which is why crystals control the carrier oscillator and the reference in a synthesiser.
Modern rigs derive every frequency from one crystal reference using a phase-locked loop (PLL) synthesiser. A voltage-controlled oscillator (VCO) output is divided by a programmable integer N and compared in a phase detector with the reference; the resulting error voltage, smoothed by a loop filter, steers the VCO until the loop locks. The output is then:
- f_out = N x f_reference
Worked example: with a 12.5 kHz reference and N set to 11720, f_out = 11720 x 12.5 kHz = 146,500 kHz = 146.500 MHz, a standard VK 2 m channel. Changing channel simply means loading a new value of N.
Direct digital synthesis (DDS) takes a different approach: a phase accumulator clocked by the reference addresses a sine lookup table, a digital-to-analogue converter produces the waveform, and a reconstruction filter cleans it up. DDS offers tuning steps below 1 Hz and instant frequency changes.
Finally, a buffer amplifier is placed between the oscillator and the following stage. It presents a constant, light load to the oscillator so that keying or load changes further down the chain cannot pull the frequency.
In a filter-method SSB transmitter, which stage suppresses the carrier and produces a double-sideband suppressed-carrier signal?
A 2 m FM transmitter uses a 12.000 MHz crystal oscillator followed by a multiplier chain of x3, x2 and x2. What is the output frequency?
What is the purpose of the output filter fitted between the power amplifier and the antenna socket of an HF transmitter?