Volante24

Finally I was able to play with a good spectrum analyzer and my
Hitec gear which in this case was Laser 4 TX and Supreme RX. I also
checked the GWS crystal sold for Futaba receivers (channel 22).

The numbers are the following. The carrier on TX is shifted about 1KHz
up compared to the nominal channel frequency,
72.10MHz + [channel#-11]*20KHz.

The original RX crystal for channel 33 in that receiver (it is double
conversion) oscillates on 61.750565MHz, that is the above channel
frequency minus 10.7MHz (first IF) plus 0.56kHz; the offset therefore
is roughly a half of the carrier offset in the Hitec TX, which makes
sense.

Now, the result for the GWS crystal. When plugged into the same Hitec
receiver it oscillated on 61.529550MHz (channel 22!), i.e. 0.56KHz
below the channel frequency minus 10.7MHz.

Thus, the difference in tuning of the receiver when using Hitec and
GWS/Futaba crystals is about 1kHz.

In fact, the frequencies do drift by about 100Hz for both TX and RX
oscillators; the drift may be more in a wider temperature range.

Now, whether 1KHz difference matters or not? I think, it depends on
the receiver. In fact, this 1 kHz is within the FCC mandated tolerance
(1.4KHz). The effect, if it is there, is not dramatic for sure. But
the final outcome depends on the receiver bandwidth. If they use
+-5KHz or wider filters, it should not matter. However, if the used
filters are very narrow like only 3KHz bandpass, some difference may
appear, depending actually on a particular tuning of both receiver and
transmitter. A factor of 2 in the range is the most I would generally
expect if the TX and RX are originally tuned well. If they are not,
one may even have a slightly better reception for the "wrong"
crystal.

Actually, the RF inductance in Supreme located nearest to the
removable crystal, shift the oscillating frequency. It looks like it
can be used to tweak the crystal frequency by half of a KHz or so, and
therefore probably can be used to offset the effect of a foreign
crystal, at least in this case. But this may require either good
meters, or applying some tricks with the receiver. (For instance, the
DC component of the output voltage on pin 9 of TA7761, "AF out" may
directly measure the frequency offset, or mis-tune between the TX and
RX, if it is not killed by the internal circuit -- need to be checked
experimentally.)

Nick

T28RON

My Feedback: (32)

You felows seem to know what U are talking about soooo here is a Q for U. I have a crystal that is covered with a white wrap around tag in black print that states 2.070 RX 14 on both sides. When I removed the tag, The printing on the crystal (one side) read, 61.370H and underneath that print reads KDS 5P. Can anyone tell me what brand of receiver this is for and is it a AM or FM or both. Tks.
Ron

fizzwater2

My Feedback: (61)

Sounds like a low-side injection crystal for a dual conversion receiver - it's 10.7 MHz offset from the channel, 72.070 (ch 14).

I can't tell you what receiver it came from, though.

T28RON

My Feedback: (32)

Tks dirty bird for that Maybe someone will know what receiver it goes to.

A.T.

Why not ask direct from KDS
FWIW:
Crystals are no longer interchangeable between brands, contrary to popular belief we do not recommend using another brand of crystal. They are totally different in cut, overtone value and a few other specs. e.g. Hitec DC RX crystals on 35.36 & 40MHz have a frequency 10.7MHz ABOVE the nominal frequency whereas Futaba DC RX crystals are 10.7MHz BELOW nominal frequency. Hitec FM TX xtals are cut at half nominal frequency whereas the Futaba FM TX xtals are cut at 1/3 nominal frequency.
Whilst you may get reasonable results in good conditions with Hitec xtals in Futaba RX, Hitec crystals must be used in Hitec RX for optimum results. Futaba single conversion RX crystals may appear to work but with proven less range and glitching, especially noticeable in damp weather or wet ground. Local radio technician was surprised but ran a number of tests with result he is also now convinced that there is a significant difference in the crystals as has also been confirmed by the local crystal manufacturer who supplies USA military and others requirements.
For further information, refer to crystal compatibility between brands
Regards
Alan T.
Alan's Hobby, Model & RC FAQ Web Links

T28RON

My Feedback: (32)

Thanks for that info however I was asking if the crystal could be I.D. given the info I submitted.