Radio Experimenter Theory Notes

VHF Transmitter

The VHF transmitter usually consists of a VFO running at around 6MHz. This is because it is not usual to have an oscillator running at high frequencies such as 144 MHz. The VFO is multiplied by three multiplier stages. The buffer amplifier is used following the multiplier stages to avoid the possibility of the PA stage interfering with the previous stages. The PA delivers the final output to the antenna.

Click [ HERE ] for a diagram of the VHF transmitter. (Requires Adobe Acrobat Reader).

MEMORY TIP:
The multipliers are 4x3x2, the same as the 432 MHz band. For UHF - add an additional multiplier before the buffer stage multiplying by three.

HF Transmitter

The basis of a typical HF transmitter is a VFO running at 1.75 MHz - 2 MHz. This is because the HF bands with the exception of the WARC bands are all harmonically related, i.e., 21 MHz band = 1.750 - 1.782 x 12.

This VFO will be followed by one or more stages of frequency multiplicatioin which will be selected by a front panel switch when you change bend. This combination of the VFO and the frequency multiplier stages is known as the exciter. This is used to dribe the power amplifier which in turn delivers the final output to the antenna. NOTE: Most modern transmitters use frequency synthesisers instead of the VFO.

Oscillators

Basically, there are two types of oscillators, the bariable frequency oscillator and the crystal oscillator. The crystal oscillator is the most stable but the disadvantage is that it usually operates on a single frequency or in a VXO in a very narrow band of frequencies, typically around 50 kHz whereas the VFO operates in a much wider band and thus is the more useful of the two.

The most critical requirement in an oscillator is the stability. If an oscillator is not stable, the final output frequency will bary (drift) and this may be quite severe, particularly if frequency multiplication is being used. For example, on the 28 MHz band the typical VFO is being multiplied by 16 times, so any instability is also multiplied by a factor of 16.

PRECAUTIONS IN CONSTRUCTION:
Oscillator circuits should be constructed from components which do not change value with temprature. Power supply must be requlated. The oscillator must be mechanically stable. The oscillator should only be lightly loaded by the following stages as excess loading may cause a change in frequency or may stop oscillation. The oscillator should never be keyed (chirp).

Frequency Multipliers

These are usually low power stages running in class C in order to produce a signal rich in harmonics. The multiplication is usually between 2 and 4. The output is coupled to the following sytage by a small capacitor. It is most important that unwanted frequencies produced in the multiplication stages do not reach the PA stage as they may be radiated and cause interference.

Click [ HERE ] for a diagram of the HF exciter. (Requires Adobe Acrobat Reader).