The Frequency Stability of a Transmitter is a measure of how close the actual frequency transmitted is to the wanted.
Frequency Stability is directly determined by the Crystal Oscillator, as all frequencies produced in the Synthesizer are locked to this reference. Frequency Stability is generally quoted in Parts per Million (ppm) but can sometimes be quoted as a discrete number of Hertz (Hz). The difference between the wanted frequency and that actually produced is known as the Frequency Error or Frequency Drift.
To illustrate this, consider the diagram below:
If the Crystal Oscillator were now to drift +2ppm away from the nominal 12.8MHz, it’s actual frequency would be: 12.8MHz + (12.8MHz * 2 *10-6) = 12.8000256MHz
This error or drift would manifest in all the other oscillators within the Synthesizer that are locked to this reference, and the actual frequencies produced would be:
As can be seen, the actual Tx Drive frequency is 800Hz higher than the desired 400MHz. This may not sound much, but when we consider that the bandwidth of the Receiver picking up this signal may only be 7.5kHz for a NB signal, we can see that the frequency error is significant.
Note: A similar error may exist in the Receiver, so if you happened to be unlucky, you could end up with double this error in practice.
The main cause of Frequency Drift is the ambient temperature inside the radio, which can depart from the outside temperature rapidly when the radio is transmitting. Most Tait radios are designed to operate within the ambient temperature range -30 to +60degC. However at +60degC ambient, the radio’s internal temperature may be much higher (eg: +85degC) and it is important the Crystal Oscillator is designed to cope with this.
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