|Jul 31, 2010, 10:11 PM|
whats this frequency must be off to work with TX
I copied this because I always thought the
Rx and the TX crystals have to match and all of mine do at least whats stamped on them.
The frequency marked on the UM1 crystal that came with my R-4P or R-6N does not match my transmitter's frequency?
In order for a single conversion receiver to work with a given transmitter frequency, the receiver will require a crystal that has a frequency value that is +455 kc above the transmitter crystal. For example, channel 40 will have 72.590 marked on the transmitter crystal. The matching UM1 crystal will be marked 73.045 in order for the single conversion receiver to work.
can anyone explain this to me
|Aug 01, 2010, 12:09 AM|
That's because in the receiver the crystal controls a local oscillator that produces an RF signal at that frequency that is then mixed with the incoming 72.590 signal to result in a difference "beat" frequency that is 455khz.
It's like if you have a twin engine aircraft and the RPMs are not the same. You hear both engines but there's a wowwowwowwowwowwow beat frequency tone produced by the two being near but not at the same RPM. The 455khz is that same sort of deal. The follow on IF (Intermediary Frequency) filter stages are tuned to this 455 khz frequency to a very high degree so only beat frequencies from the proper incoming frequency produces the proper 455khz frequency.
The problem is that the mixing function in the receiver doesn't care if the 455khz comes from a 72.590 signal or a 73.500 signal. Either will produce the correct difference to get the 455Khz. That's where dual conversion receivers come in. Or just avoiding the higher frequency signal.
|Aug 01, 2010, 03:56 AM|
Joined Nov 2003
As Bruce says it's because the receivers we use are what's called "Superheterodyne" receivers (you can Google that if you really want to know how they work). They use an IF or Intermediate Frequency of 455KHz. Because of this the receiver crystal frequency is 455KHz different from the transmitter frequency.
What confuses things is that most manufacturers don't mark the actual crystal frequency on their R/C receiver crystals, instead they mark them with the channel number and frequency of the transmitter they're intended to work with. GWS and Multiplex are the exceptions who tell you the actual frequency (and usually the channel number, which is all most people ever look at ).
|Aug 02, 2010, 12:11 PM|
Joined Aug 2004
The 455 KC difference is for single conversion receivers only. Dual conversion usually uses a receiver crystal that is at a much greater frequency difference, about 11 MHz.
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