Application Note 02:

Transmitter Crystals

Receiver crystals have to meet only one criterion: when plugged into the receiver, they must operate at the [design frequency] plus-or-minus tolerance. The total allowed frequency "error" (tolerance) is determined by the design of the receiver if filters, tolerances in oscillator components, frequency [in]accuracy of the crystal itself, frequency drift of the crystal with temperature variations, and aging of the receiver crystal -- plus errors in the center frequency of the transmitter (also determined by a crystal with its own tolerances as for the receiver crystal) -- and in the setup (tuning) of the transmitter's shift points.

Now, the transmitter crystal has an added requirement to that of the receiver crystal. First, it is of much lower frequency than the channel frequency, its signal being multiplied a number of times. But (very important!) the transmitter crystal must also meet certain "pullability" requirements; i.e., the frequency change per change in load capacitance. This is how the frequency shift or FM is achieved in the transmitter. In the transmitter, the shift points are individully set by means of two potentiometers or variable capacitors or a combination thereof.

It should be noted that these adjustments will be different for every crystal unless this "pullability" spec is tightly controlled by the manufacturer, which costs money.

From the above, it should be clear that you should always use the manufacturer's crystal so that you will stay within your radio's design limits. Don't forget: all these tolerances build up.

It is amazing that our radios work this well with all these tolerances, but the precision of the quartz crystal helps us here -- you can typically expect a max error in the order of between 3 and 10 ppm (parts per million), depending on brand, which means that your frequency error will be between 3 and 10 times "one millionth times the frequency." For 72 MHz, this would be a total error of 360 Hz (for each crystal) when the tolerance is 5 ppm for each; total frequency error can be as much as 720 Hz in that case.

It will be easily seen that, since tolerance is expressed in ppm, the real (absolute) error at 72 MHz is about twice as great as the absolute error one sees at 35 MHz, with the same crystal specs.