6 replies to this topic

[Granby]

Got a HPI TF-3 AM transmitter, by mistake i bought a pair of 27mhz FM  crystals so i thought i would try them anyway and they work (move the  servos), is this OK or will i have any problems with maybe range or  anything else

[Guns]

im 99% sure it wont work!

[Matt B]

As long as it works, then I don't see why not, as I believe a crystal is just a crystal. as long as you don't have range or glitching issues then it should be fine. You might as well test them now that you have them.

[Garry]

From Wiki:

Quote

AM detectors cannot demodulate  FM and PM signals because both have a constant amplitude. However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection  which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.

So basically, you CAN use FM crystals in some cases (not all), but it'll be more susceptable to interference so I'd stick to using the right crystals for the right radio.

[Granby]

Cheers guys, will wait until i get a failsafe first then give the FM crystals a go

[Guns]

For around £50 these days you can go to 2.4ghz and get a failsafe in the Transmitter/rx combo. maybe worth looking at

[Aterlatus]

I'm unsure on this one because the crystal is essentially nothing more than an oscillator. The AM/FM modulation is done by other circuitry in the equipment. However, there may some other phenomenon at play such as the crystals using slightly different frequencies (even if labelled the same) for the different modulations.

Personally, I wouldn't use the 'wrong' crystals but if you've done your range and glitch testing and all appears to be fine I also see no reason not to continue.