Difference?

[Linga]

Hi all,

Is there any difference between AM and FM crystals.

Superfically I don't think there can be a difference! but still why distinguish?

Linga

[Miami Mike]

No difference.

Here's a link to the Hitec HFS-04MG FM receiver:

http://www.hitecrcd.com/Receivers/HFS04MG.htm

Note that it takes crystal #66472XX (where XX is the channel).

Here's a link to the Hitec HAS-03MB AM receiver:

http://www.hitecrcd.com/Receivers/HAS03MB.htm

Note that it also takes crystal #66472XX.

[Bruce Abbott]

Yes, and No ;)

Receiver crystals generally work equally well for AM or FM. The main requirement (for both AM and FM) is that the offset between the channel frequency and crystal frequency matches the receiver's IF (+-455kHz for Single Conversion, +-10.7MHz for Dual Conversion). Since all currently available AM receivers are Single Conversion, a receiver crystal may be designated 'AM' even though it equally suits a Single Conversion FM receiver!

Additionally for FM reception, the crystal's frequency needs to be on the correct side of the channel frequency (above or below), otherwise the 'shift' will be wrong. Some receivers have automatic shift detection or are not sensitive to shift, in which case this doesn't matter (however, a Dual Conversion receiver may not like a crystal that's 21.4MHz away from where it should be!).

In a transmitter, the same crystal could produce either AM or FM modulation. However, to get sufficient FM deviation the crystal must be operated in Fundamental mode, wheras for AM it can be (and almost always is) run in Overtone mode. Since the Overtone frequency is not an exact multiple of the Fundamental frequency, a crystal cut for AM transmission will not produce the correct frequency for FM.

Another important factor is that the crystal (AM/FM/TX/RX) needs a certain amout of circuit capacitance to get the exact frequency. As each manufacturer's products have different circuitry, it pays to use the recommended crystal, unless you know that it is compatible. FM transmitters are particularly sensitive to this...

[Chippie]

Bruce, thanks for the info on crystals.....even tho' I've been around radio and electronics for 40-odd years, just goes to show that ' you dont know everything'........ ;)

[Linga]

Hi Bruce,

Thank you for clearing the doubt. Indeed it is a nice explanation.

Linga