2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing

2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing

ACMA Exam Focus: Syllabus items 5.17, 5.18 and 5.21 - recognise that the mixer and local oscillator together form a frequency converter, state the important characteristics of an IF amplifier, and explain how a transceiver shares oscillators and IF stages between transmit and receive, including the function and use of the receiver incremental tune (RIT) control.

Knowing the names of the superhet stages is only half the syllabus. The Standard Theory paper also asks why the architecture is arranged that way: what the mixer and local oscillator do as a combined unit, what makes a good IF amplifier, and how a single box manages to transmit and receive on the same frequency without two separate sets of oscillators.

The frequency converter

The mixer and the local oscillator (LO) are rarely treated as separate ideas in practice. Taken together they form a frequency converter (also called a converter stage or first converter): a functional block whose whole job is to move a signal from one frequency to another without altering the modulation riding on it. Feed 14.200 MHz single sideband into a converter with a 9.000 MHz output and you get 14.200 MHz single sideband translated to 9.000 MHz - same audio, same sideband, same bandwidth, different carrier frequency.

Two consequences follow, and both are examinable:

• The converter preserves the information. It shifts the whole signal envelope; it does not demodulate anything.
• The converter is where tuning happens. Because the output frequency is pinned by the following filter, changing the LO frequency changes which input frequency is accepted. Everything downstream stays put.

Converters are used well beyond the receiver front end. A transverter for the VK 6 m or 2 m band is nothing more than a frequency converter bolted onto an HF transceiver: it takes a 28-30 MHz signal and converts it up to 50-52 MHz or 144-146 MHz, and converts received signals back down again.

Why a fixed IF is easier to filter and amplify

The gain of the superhet lives in the intermediate frequency (IF) amplifier, and the reason is a piece of arithmetic every candidate should be able to reproduce. The bandwidth of a single tuned circuit is set by its resonant frequency divided by its quality factor:

• Bandwidth = f / Q

Take a practical tuned circuit with a Q of 100 and see what bandwidth it gives at three different frequencies:

At 455 kHz that circuit is already close to a usable communications bandwidth. At 28.5 MHz it is more than a hundred times too wide to separate one SSB signal from its neighbours. A tunable RF amplifier therefore gets worse as you tune higher, and its gain, bandwidth and alignment all change as the dial moves. Convert everything to one fixed frequency instead and the filter can be built, aligned and left alone: it never has to track, and its shape is identical on every band the radio covers.

Important characteristics of an IF amplifier

For the exam, an IF amplifier is defined by four properties:

1. It operates at a single fixed frequency. Common choices are 455 kHz, 9 MHz, 10.7 MHz and 45 MHz. Nothing in the stage is retuned during normal operation.
2. It supplies high, stable gain. Typically 60 dB to 100 dB of the receiver's total gain comes from the IF strip, and because the frequency never changes, that gain is the same on 80 m as it is on 10 m.
3. It sets the receiver bandwidth. Selectivity comes from a crystal, ceramic or mechanical filter, not from ordinary LC tuned circuits, because those filters achieve effective Q values in the tens of thousands. Typical amateur bandwidths are 500 Hz for CW, 1.8-2.4 kHz for SSB, 6 kHz for AM and 12-15 kHz for narrowband FM.
4. It is where automatic gain control is applied. AGC works by rectifying a sample of the IF output to produce a DC control voltage, which is fed back to reduce the gain of the IF amplifier (and often the RF amplifier as well). This holds the audio output roughly constant as signals fade or as you tune from an S3 signal to an S9 + 40 dB signal. The same AGC voltage drives the S-meter, which is why the meter reads nothing when AGC is switched off.

Transceivers: sharing between transmit and receive

A transceiver is not simply a receiver and a transmitter in one cabinet. It is a design in which the expensive, frequency-determining sections are shared by both functions and switched between them. In a typical HF rig:

• The VFO or synthesiser that acts as the receive local oscillator becomes the transmit mixing oscillator.
• The carrier/beat frequency oscillator used by the product detector on receive becomes the carrier oscillator feeding the balanced modulator on transmit.
• The crystal filter in the IF strip shapes the transmitted sideband as well as the received one, so transmit and receive bandwidths match automatically.
• Transmit/receive switching is done by relays or PIN diodes, muting the receiver and routing the shared blocks in the opposite direction.

The advantages are fewer parts, lower cost, smaller size, and - most importantly on air - you always transmit exactly where you are listening. There is no need to "net" a separate transmitter onto the other station's frequency.

Receiver incremental tune (RIT)

That guaranteed transmit/receive equality becomes a nuisance when the other station is slightly off frequency, or drifting. Retuning the VFO to make them sound right would drag your transmit frequency off with it. The cure is receiver incremental tune (RIT), sometimes labelled clarifier on older sets.

RIT adds a small, adjustable offset - commonly plus or minus 1 kHz, up to plus or minus 9.99 kHz on some rigs - to the receive frequency only. The transmit frequency stays exactly where the main dial says it is. Turn the RIT until the other operator's voice sounds natural or their CW note is at your preferred pitch, and answer them on your original frequency.

Practical points worth remembering:

• XIT (transmitter incremental tune) is the mirror image: it offsets the transmit frequency while the receiver stays put.
• Always check the RIT indicator before calling CQ. Leaving RIT switched on is a classic cause of "why can't anyone hear my reply on frequency?"
• RIT is for small corrections. Deliberate operation on two different frequencies, such as a DXpedition listening 5 kHz up, is split operation using a second VFO, not RIT.