Frequency of crystal ocillator changes when counter attached,

You should be monitoring a PLL o/p and inferring the Xtal f

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

A *good* quality counter, used properly, will eliminate this problem.

Look for a buffered output from the xtal oscillator, rather than metering right off the xtal.

"N_Cook" wrote in news:gtc2tb$nec$ snipped-for-privacy@news.eternal- september.org:

pick off the signal with a scope(10Meg input Z) and feed CH1 out to the counter.or you could trigger the scope on an ext.reference signal like WWV and adjust the scope display for lowest drift.

PeterD wrote in news: snipped-for-privacy@4ax.com:

Nonsense;the counter probably has a low input Z and high input C,and together with the hookup lead/probe capacitance makes too much of a load. Using a scope with a 10X probe or FET probe may decrease the loading,if you have a CH.1 output to pass to the counter. Or perhaps a pickup loop might give a smaller load on the osc.

When a pickup coil doesn't provide enough signal, I use a broadband CATV distribution amp (~1MHz -> ~900MHz) which has surprisingly flat response and can also be useful as a scope preamp. These are often found cheaply at hamfests, local e-waste dumps, or from cable TV plants (shmoozing required). Use with a pickup coil or tank circuit on the input.

Michael

Use a buffer amplifier or analog PLL to stop the loading of the oscillator.

snipped-for-privacy@mucks.net wrote in news: snipped-for-privacy@4ax.com:

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.

I had a dummy load, I built into a juice can, that had a built in tap for the frequency counter. I used to carry these with me (along with the battery powered scope) when I serviced marine electronic equipment in the mid 70's.

bz wrote: r.

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.

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.

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.

Very nice description. I had forgotten some of the convoluted up and down-mixing schemes used by CBs to arrive at transmit and receive frequencies, and frequencies low enough to be handled by some of the early PLL chips. Reams of paper calculating crystal frequencies to produce different bands, and inter-channel offsets. Happy days. And SSB generation. Best not even go there ... !

Arfa

snipped-for-privacy@shadowstorm.com wrote in news: snipped-for-privacy@u10g2000vbd.googlegroups.com:

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'.

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.