4. Transmitters and Receivers
4a.1 Understand the block diagram of an s.s.b transmitter employing mixers to generate the final frequency.
A general over view
When considering a transmitter it is probably best to think what it is trying to do. So what is it trying to do ?
Understand the block diagram of an FM transmitter employing either frequency multipliers or mixers to generate the final frequency.
This FM version is using frequency multipliers. The Audio signal is acting directly onto the frequency oscillator which then passes the signal to a buffer amplifier to make it suitable to go into the frequency multiplier stage. When is has been multiplied up sufficiently to the output frequency it is filtered and the driver prepares the correct level of signal for the RF amplifier, which is followed by filtering to present the signal to the aerial.
However this arrangement has for the most part been overtaken by the general transmitter diagram fig 1 which can also be used for FM - when suitable modulators are employed.
So what else is there to know ?
This general over view, above, has shown you what happens to the signal from mic to aerial. The other sections, which follow below will give a more detailed account as to the activity of individual parts.
The start of this process is with the crystal oscillators. These have to create :-
frequency suitable for the part of the circuit in which they are employed.
The reason for the stability is so that they do not "drift" off frequency and possibly put the transmissions outside the amateur bands but almost certainly a drift in frequency would take you into another QSO!! (Drift is a slight and gradual, yet unwanted change or frequency.)
The reason for the accuracy is that you must know for absolute certainty that you are inside the amateur bands. It is desirable to ensure that you use the appropriate frequency for the mode you are using - SSB, FM, CW or data ETC.
There are several ways to achieve the above:-
The old way was to use a crystal oscillator and to change the crystal to change frequency, very stable and the frequencies are accurately known but limited to the number of crystals you have and they were and still are expensive!
VARIABLE Frequency Oscillator
Another old way was to use a VARIABLE Frequency Oscillator. This gave you an infinite number of frequencies in a section of the amateur band.
Changing bands involved using a harmonic (multiple) of the same oscillator and retuning later stages of the transmitter to suit. The draw back was that such units were affected by heat, impact, when, say, you tapped your rig (microphonics) and thus did drift off frequency, and could give additional modulation to the wanted mode.
The modern way is to use a crystal oscillator which gives a stable frequency and link this to a frequency synthesiser which then gives you an accurate range of frequencies.
In all the methods mentioned above a stable voltage is needed which is in addition to the voltage power source for the power amplifier -again this is to ensure stability.
The synthesiser has two variants the PLL or Phase Locked Loop and the DDS or Direct Digital Synthesis (the DDS is not covered here as not part of the syllabus).
HOWEVER there is one draw back in using a synthesiser rather than a crystal oscillator or VFO and that is what is called "noise level" - this is the back ground level of noise present in higher quantities than crystal or VFO oscillators.
WHY 2 Crystal oscillators?
These are in two different parts of the circuits and thus are performing quite separate operations (Note: some designs can use more than two crystal oscillators).
So what happens from microphone to aerial ?
The audio amplifier is linked to the modulator. Once the appropriate modulation has been applied, we now have a modulated signal which may be appropriately filtered but it is not on the correct frequency. This signal is then mixed with the output from the synthesiser to produce the desired output frequency, this then passes into the RF power amplifier and then is filtered again before it goes into the aerial.
So the transmitter has achieved