Crystal

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The precise frequency that a receiver or transmitter uses is determined either by electronic frequency synthesis, or by crystals. Different manufacturers use different specifications for how the crystal's frequency is converted to the transmitted or received frequency, so crystals can often not be mixed between brands. Some countries have regulations prohibiting the changing of crystals by unqualified people.

Transmitter crystals

As it is difficult to create high frequency crystals accurately, transmitter crystals are manufactured to produce a fraction of the broadcast frequency; the radio stage then uses a harmonic of this (usually 5th or 7th?) to create the base frequency to use.

Receiver crystals

Receiver crystals also use a harmonic to determine the frequency it decodes, but unlike a transmitter where the harmonic determines the frequency directly, the receiver decodes the signal to an 'Intermediate Frequency' (usually 455KHz), from which it is easier to design electronics to decode the servo commands. The crystal is therefore tuned a fraction to the difference between the received frequency and the intermediate frequency.

A variation of this scheme is the dual conversion receiver, where the receiver first transforms the signal to a higher intermediate frequency (usually around 10 - 11MHz), before transforming it to the normal intermediate frequency. These receivers require special 'dual conversion' crystals that are tuned to a fraction of the difference between the received frequency and the higher intermediate frequency.

Receiver crystal cross-compatability

This chart shows crystal/ receiver compatiability, specifically for UK 35MHz equipment.

While this chart shows experimental results, when using any crystal (but particularly one different from the make of receiver) you should satisfy yourself of its suitability by performing a thorugh range check.

v RX/ XTAL > Futaba Futaba DC Hitec Hitec DC JR
Futaba Y n Y N y
Futaba DC N Y N N n
Hitec Y N Y N y
Hitec DC N N N Y n
JR y n y n Y

Key: Y/N confirmed, y/n believed.

To summarize, in the UK, there are single conversion crystals, which appear to be generally intercompatiable between brands; and dual conversion crystals, which are not



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