Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.

Michael wrote in message
...

Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.




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
http://home.graffiti.net/diverse:graffiti.net/

On Thu, 30 Apr 2009 07:23:28 -0400, Michael
wrote:


Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

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



Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.

Michael wrote:
Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.

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

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

"N_Cook" wrote in -
september.org:

Michael wrote in message
...

Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

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.

Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.




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
http://home.graffiti.net/diverse:graffiti.net/






--
Jim Yanik
jyanik
at
kua.net

PeterD wrote in
news
On Thu, 30 Apr 2009 07:23:28 -0400, Michael
wrote:


Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

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

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.



Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.





--
Jim Yanik
jyanik
at
kua.net

Jim Yanik wrote:

On Thu, 30 Apr 2009 07:23:28 -0400, Michael
wrote:

Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

snip

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

On Thu, 30 Apr 2009 07:23:28 -0400, Michael
wrote:


Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.

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

wrote in :

On Thu, 30 Apr 2009 07:23:28 -0400, Michael
wrote:


Hi,
I regularly repair and restore vintage CB and ham radios. Older CB
radios often have had their crystals drift off frequency usually to a
lower frequency. When I use my frequency counter to align the crystal
back to it's intended frequency via a trimmer capacitor the frequency
changes as soon as I connect the counter. In other words what good
does it do to adjust the PLL reference oscillator to 10.240 MHz if the
frequency changes slightly when I disconnect the frequency counter?

Is there a way to minimize or eliminate the oscillator frequency
change when the counter is connected? My frequency counter is a Protek
B-818.

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

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 73 de N5BZ k

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

remove ch100-5 to avoid spam trap

bz wrote:
r.

wrote in message
...

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.




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

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.

remove ch100-5 to avoid spam trap