TX fundamental oscillator frequency?

[rl_gbg]

I'm looking for detailed information about 35 MHz TX RC crystals
from different manufacturers.

I've sofar found that it is common that the fundamental oscillator
in the RC transmitter is running on half the frequency marked on the
crystal. The oscillator signal is then followed by a frequency doubler.
But I've also found that there exists transmitters with oscillators
followed by x3- or x4-multipliers.

A FUTABA 35 MHz TX crystal made for channel 72 (35.120 MHz)
is running on 17.560 MHz in the fundamental oscillator according to
a schematic I've found.
How about other "crystal manufacturers" (GWS, JR, HITEC, SANWA)?

I've found some information on this page (CRYSTAL COMPARISON CHART)
http://www.gws.com.tw/english/product/receiver/crystal%20comparison%20chart.htm
but it does not contain any information about the principal difference between them.

According to that chart 35 MHz TX crystals from GWS and JR are "direct replacement" but crystals from FUTABA, SANWA and HITEC are not.

I would like to know more about 35 MHz TX crystal from GWS, SANWA and HITEC. Are they designed to oscillate direct (fundamental mode, not overtone) on 35 MHz?
Maybe some other frequency generation scheme with these crystals?

Regards,
Ronny

[vintage1]

A crystal cut to 11.6 Mhz can be persuaded to ocsillate at 35Mhz nor problem..

I am confused, because althoigh the crystals are generally cut as third overtone=35Mhz, they don't oscillate at the fundamental. A basic tuned circuit ensures that they sing 'soprano'...;) ..there is no frequency tripling going on as such - just getting the crystal to sing an octave and a third higher :D


What is more an issue is what capacitative load they will be 'on song' at..I've tried a 'different' brand crystal in my futaba, and it sang allright...but it was about 4 KHZ off!!!

[Mr.RC-CAM]

There are a couple of methods used in USA 72Mhz R/C Tx's. One involves an xtal with a fundamental freq of about 18Mhz . The osc is tuned to the second harmonic, which yields 36Mhz. This is then followed by a 2X frequency multiplier stage. The other method uses an xtal in the third overtone mode, which effectively triples its operating freq.

[dj2u]

Futaba 40MHz use 13.56MHz Crystals

[AndyOne]

I am confused, because althoigh the crystals are generally cut as third overtone=35Mhz, they don't oscillate at the fundamental. A basic tuned circuit ensures that they sing 'soprano'...;) ..there is no frequency tripling going on as such - just getting the crystal to sing an octave and a third higher :D


Vintage,

I have found that you can make a 3rd overtone crystal oscillate at it's fundamental frequency if you use an circuit tuned to the fundamental. I have even got these types of crystal to oscillate at 5th overtone by stability goes out of the window.

Of course, the fundamental frequency isn't necessarily 1/3 of the marked frequency as overtones aren't exactly the same as harmonics.

Andy.

[Bruce Abbott]

Much wider deviation is possible when running the crystal in fundamental mode (9 times greater than overtone mode). Stability is probably improved as well, since no tuned circuit is required to force the crystal to oscillate at the correct frequency, and less 'pulling' is required to get the same deviation.

I do not know of any FM transmitters that use overtone mode.

[Mr.RC-CAM]

I do not know of any FM transmitters that use overtone mode.I believe that is what Multiplex uses on their 72Mhz systems. But I could be wrong.