Syllabus Sections:-

Frequency synthesis

4c.1 Recall the block diagram of a frequency synthesiser and the functions of the stages (i.e. oscillator, fixed divider, phase detector, LPF, voltage controlled oscillator and programmable divider).

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Where Fout = the final output frequency

fcrystal = the frequency of the crystal

N = the programmable divide by number

A = the fixed divide by number

The frequency out is given by the frequency of the crystal times the divide by N divided by the fixed divider A.

The equation above is a useful one to fully understand as it is difficult to work out from the diagram above or the text below !!!

Another equation associated with the above is

fstep = the size of the frequency step from one frequency to the next

fcrystal = the frequency of the crystal

A = the fixed divide by number

Explanation

The diagram above show a modern Frequency Synthesiser. This is a PLL or Phase locked Loop.

The crystal reference oscillator

The crystal reference oscillator is made to a high standard so that it can provide a good stable output frequency that acts as the heart of the synthesiser.

the fixed divider

The "reference frequency" is then divided down by the fixed divider. This is a solid state digital chip which divides down the original 6MHz to 1kHz and this has the same accuracy as the crystal oscillator because it is derived directly from the crystal oscillator.

Note: The frequencies given, in the diagram above, are examples and many other designs exist.

Voltage controlled oscillator

The VCO is a voltage controlled oscillator whose frequency is controlled by changes in applied voltage. When it starts running it is not stable and said to be "Out of lock" and the output would need to be inhibited so that transmission did not occur. Even when out of lock the pulses are sent to the Programmable divider for processing.

The programmable divider

The programmable divider is set to divide the VCO frequency down so that the output of the programmable divider is also 1kHz.

Phase detector

A comparison can then be made between the two 1kHz pulses. If the pulse from the programmable divider is slightly low then the pulses will be out of phase with each other and this is detected in the Phase comparator.

Low pass filter

This error is then passed as a voltage change to the low pass filter to clean it up and then to the VCO. The VCO frequency then changes as a result of the voltage change, and becomes very close to the desired frequency (in effect it is now "LOCKED").

This process continues to make small adjustment to the VCO as required, hence keeping the transceiver on frequency.

When it is desired that a large change of frequency is made, this is achieved by setting the programmable divider by the control lines. Immediately, the synthesiser is "unlocked" or "out of lock" and a large error voltage results. This causes the VCO to change frequency rapidly and "lock" to the new setting. It is very desirable that there is no output from the transmitter whilst the synthesiser is "unlocked" or "out of lock", and it is usual to take a signal from the synthesiser which inhibits the transmitter until "lock" has occurred.

Recall how sine waves may be produced by direct digital synthesis and the block diagram of a simple synthesiser. Recall that increasing the number of bits in the synthesiser will increase the purity of the signal.

The frequency control knob changes the rate clock pulses which are used by the "Sinewave lookup table" to an output on the data lines. This output is then used by the "Digital to analogue converter" to generate the frequency sinewave which is passed to the low pass filter and there on to its output.

Greater the amount of data the greater the purity

The greater the number of Data lines the greater the number of bits of data (or bits resolution) that can be passed and the purity of the signal increased.


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