DIYbanter - View Single Post - Frequency of crystal ocillator changes when counter attached,

bz

wrote in
:

bz wrote:
r.

Or hook the transmitter to a well shielded dummy load, use a 't'
connector to hook up your counter (thru a proper attenuation network)
and adjust the xtal frequency while keying the transmitter.

That may not tell you whether the 10.240 MHz oscillator is on
frequency. Many CB's have a separate 10.695 MHz crystal oscillator
that mixes with the signal from the VCO to generate the transmit
frequency. If the 10.695 MHz oscillator is off freqency you will be
knocking the 10.240 MHz oscillator off frequency to compensate. On the
receive side the 10.240 MHz signal often mixes with the common first
IF frequency of 10.695 Mhz to make the second IF frequency of 455
kHz.

10.695 kHz - 10.240 kHz = 455 kHz

Also the 10.240 MHz frequency is often doubled and mixed with the VCO
output frequency to mix down to a frequency low enough for the pll
chip to process in the phase detector.

In a nut shell you may be fixing the transmit frequency only to knock
off the second IF frequency.

Agreed. However I would expect most mixer oscillators to have test points
available that allow attaching suitable test equipment for adjustments.

Another possibility is to use a receiver, tune in the oscillator signal
and use a second variable frequency oscillator to zero beat with the first
oscillator. You can count the test oscillator, set it to the correct
frequency and then adjust the mixer oscillator for zero beat by listening
to the receiver.

Now, it is possible to build a small, inexpensive, 'direct conversion'
'software defined radio' (SDR) kit covering a wide range of frequencies,
and use it, along with a laptop computer, to do all kinds of testing that
would have cost a fortune a few years ago. Completely constructed versions
are also available. Google for 'software defined radio'.

In a nutshell if the 10.240 MHz oscillator is knocked off to
compensate for an off frequency 10.695 MHz oscillator it will throw
the receive IF frequency off.

Of course, many more complex radios would have a carefully defined
'alignment procedure' that should be followed in order to make sure that
everything works properly. Adjusting anything 'out of sequence' or without
the proper equipment could result in operating outside of specifications
and in violation of FCC regulations.

--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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