I have some questions about RC crystals

[aeroflop]

I'm hoping to connect with someone who can tutor me a little on how crystal freqs are calculated for the various brands of RC receivers. For instance, here's a cox/SANWA Rx, AM, dual conversion, part # 8219. I'd like to know what actual crystal freq would be specified for that particular circuit to get it on ch 72.030, or 72.150, or 72.670 for instance? I have a fuzzy awareness that there is something about harmonics involved and that you have to know the IF freq. and whether the if is above or below the final freq. Thanks for reading my post.
Bill aka AEROFLOP

[jeffs555]

The receiver crystal will be above or below the transmitter frequency by the value of the first IF. The normal first IF is 455kHz for single conversion, and 10.7MHz for dual conversion. The single conversion receiver crystal is normally 455kHz above the transmitter frequency, and the dual conversion crystal is normally 10.7MHz below the transmitter frequency. This is the normal convention, but there could be exceptions.

[vintage1]

Don't worry about harmonics: a third overtone crystal may have a fundamental at say 34Mhz but itr will be specified at 72MHz on the can.

All Cyrstals are not created equal and neither are all circuits that use them To an extent these are tailored to suit teh indetnded crystal, so beware of substituting 'other brands'

Its a happy accident, rarher than a rule, thant most brands of recievers work with most brands of crystal, but to save pain never rely on it unless you have a frequency counter and the knowledge to use it.

[aeroflop]

Does that apply fairly equally to AM and FM applications?

[SlopeKing]

Yup... but the old interchanging of tx and rx crystals seems to only apply to the older hitec Ranger sets, and the futaba 2ER

[Bruce Abbott]

On the TX side, FM is more critical because the crystal frequency is 'pulled' by varying an external capacitance. This is a simple method of creating FM modulation, but it relies on very tight crystal tolerences. The crystal is made so that it will resonate at the correct frequency only with the specified capacitance, which is is not standardized!

AM and FM TX crystals cannot be interchanged. AM TX crystals resonate directly at the output frequency, in Overtone Mode (a tuned circuit is used to force oscillation at a harmonic of the crystal's natural frequency). FM TX crystals resonate in Fundamental Mode, and then one of the higher harmonics is picked off for output. This produces a significantly different frequency than you would get by operating the same crystal in overtone mode.

At the RX end, AM and FM crystals are similar. There are four possible flavours, being the combinations of single or dual conversion, and whether the crystal frequency is above or below the channel frequency (+-455kHz or +-10.7MHz). Some older sets had slightly different IF frequencies, but I doubt many of these are still around.

An AM single conversion RX will usually work with a crystal that is 455kHz above or below the channel's frequency (since the difference frequency will be 455kHz in both cases), but an FM RX will demodulate to the wrong 'shift' if the crystal is on the wrong side of the channel. With Dual conversion the situation is worse because of the large frequency difference (21.4MHz), which may prevent the crystal from working at all in the RX's sharply tuned oscillator circuit.

Unfortunately, apart from a lack of standarization, most manufacturer's don't publish their crystals' specs. If you can't measure the frequency directly, then you can only make a guess based on usage. Futaba are particularly confusing, as they don't even state whether a receiver is single or dual conversion! (and there's that low-band/high-band issue on 72MHz...).

GWS have a useful compatibility guide in their CRYSTAL COMPARISON CHART (http://www.gws.com.tw/english/product/receiver/crystal%20comparison%20chart.htm). For example, we can see that a GWS TX crystal is compatible with Futaba on 72MHz, but not on 40MHz. However, they say that a Hitec 40MHz single conversion RX is incompatible, but I have found them to be OK (frequency wthin 300Hz). OTOH, Hitec dual conversion RX crystals are indeed not compatible, as they are cut for operation at 10.7MHz above the channel frequncy, wheras GWS, Futaba and JR are 10.7MHz below.

[aeroflop]

I'm getting some fine help here, thank you all very much. In my initial post I gave a particular Rx as an example. I am looking for a Xtal for that Rx. I just tried an Airtronics Xtal that was 10.7 below (61.970 /72.670) It didn't work. The heck it of is, I'm working with at least two unknowns here. I don't know that the Rx is OK and I don't know if the Xtal I tried is OK. So the next thing to find out is if the the COX/SANWA DC Xtal freq is supposed to be 10.7 above or 10.7 below. In other words, do I have the right Xtal? If the answer is yes, then I could send it to be tested. If the Xtal tests OK and it probably is, then the Rx must be toast.

[Rodney]

I hope you know that in the USA, it is not legal to change out transmitter crystals unless you are a licenced repair facility that has the necessary equipment to make sure the transmitter meets all FCC specifications after the crystal has been replaced.
I just reread your first post and notice you are talking about receivers only. Sorry about my comments as it is legal to change receiver crystals.