Hi all,

I have a colpitts crystal oscillator with a 16.000 MHz parallel
resonance crystal that I'm turning into a FSK modulator with the
addition of a varactor / varicap diode. Two questions:

Originally, I had the varicap diode in parallel with the crystal. This
caused the oscillation to significantly attenuate (10 dB or so) as I
pulled the crystal 1 - 2 kHz. I found a reference design of a Hartley
oscillator that had two varicaps in series with a resonance capacitor,
so I decided to give this a try. This works a lot better, attenuating
my 1Vpp oscillation about 100mVpp. Why does the series configuration
work so much better?

I'm at home where I only have an analog tektronix 2213 60MHz scope (no
spectrum analyzer). How can I measure the frequency deviation / pull of
the crystal with the series configuration? Any ideas?

Cheers,
Chris

Apparatus wrote:
. . .
I'm at home where I only have an analog tektronix 2213 60MHz scope (no
spectrum analyzer). How can I measure the frequency deviation / pull of
the crystal with the series configuration? Any ideas?

If you have another crystal, build a second oscillator. Run the two into
a mixer followed by a simple lowpass filter, and measure the resulting
difference frequency.

Roy Lewallen, W7EL

Wouldn't it be easier to use a frequency counter? If one is not
available, but a general coverage HF receiver or transceiver with a
digital readout is available, you could zero beat the oscillator, note
the reading on the display, cause the oscillator to shift to your other
frequency, zero beat that signal, note the display reading and subtract
to find the difference in frequency.

Scott



Roy Lewallen wrote:
Apparatus wrote:

. . .
I'm at home where I only have an analog tektronix 2213 60MHz scope (no
spectrum analyzer). How can I measure the frequency deviation / pull of
the crystal with the series configuration? Any ideas?


If you have another crystal, build a second oscillator. Run the two into
a mixer followed by a simple lowpass filter, and measure the resulting
difference frequency.

Roy Lewallen, W7EL

"Scott" wrote in message
news
Wouldn't it be easier to use a frequency counter? If one is not available,
but a general coverage HF receiver or transceiver with a digital readout is
available, you could zero beat the oscillator, note the reading on the
display, cause the oscillator to shift to your other frequency, zero beat
that signal, note the display reading and subtract to find the difference in
frequency.

I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?

On Tue, 29 Nov 2005 18:53:20 -0800, "Joel Kolstad"
wrote:

"Scott" wrote in message
news
Wouldn't it be easier to use a frequency counter? If one is not available,
but a general coverage HF receiver or transceiver with a digital readout is
available, you could zero beat the oscillator, note the reading on the
display, cause the oscillator to shift to your other frequency, zero beat
that signal, note the display reading and subtract to find the difference in
frequency.

I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?

When the frequencies are close, a CRO is a great zero beat detector.

In article ,
budgie wrote:

I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?

When the frequencies are close, a CRO is a great zero beat detector.

Yup.

It's a bit easier if your crystal happens to be on a frequency which
lets you zero-beat against an AM-modulated carrier signal, such as WWV
(during the early part of each minute, when it's sending a tone
burst). You don't have to try to pick out the beat frequency
directly... instead, you listen for the tone to waver in and out, and
tweak for the slowest wavering.

It's not hard to get the zero-beating accurate to within 1 Hz or
better with a bit of patience.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Joel Kolstad wrote:

I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?


Use an analog comparator to drive a LED so you can see the difference
when you can no longer hear it. Add a counter if you want to get fancy,
and you can count the number of beats per minute, hour or even day.
--
?

Michael A. Terrell
Central Florida

From: "Joel Kolstad" on Tues, Nov 29 2005 6:53 pm

"Scott" wrote in message

Wouldn't it be easier to use a frequency counter? If one is not available,
but a general coverage HF receiver or transceiver with a digital readout is
available, you could zero beat the oscillator, note the reading on the
display, cause the oscillator to shift to your other frequency, zero beat
that signal, note the display reading and subtract to find the difference in
frequency.

I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?

On zero-beating very low beat rates: The only perceived problem is
the stability of the receiver and manual control of finding an
"exact" zero-beat. Other than that, cranking up the audio level
will let you know - by the background hiss intensity changes
during zero beat - when the zero point is reached. With manual
tuning and a crystal-controlled BFO that could be done to about
+/- 0.1 Hz if the receiver is kept at an even temperature and
power line voltage kept stable. [the metrologist's patience is a
factor there as well]

There is a problem with modern receivers using PLL or DDS sub-
system tuning: The resolution of the control system (typically 10
Hz on HF receivers). That limits the precision of zero-
beating...unless the beat difference itself is measured with a
counter.

"Time interval averaging" has been used for 3 decades in frequency
and time interval counters to increase accuracy limits caused by the
+/- one count on the display. Statistically, that can be improved
by a factor of the square-root of the number of times it is
measured. For example, taking the square-root of 100
measurements will increase the accuracy by 10 times; 10,000
measurements will increase by 100 times, etc. That averaging is
automatic on base ten displays in modern frequency counters
made since the 1970s.

Example of determining accuracy of frequency standards beat against
WWVB on 60 KHz: An early H-P WWVB receiver and strip-chart
recorder read-out for phase difference against WWVB. A nice little
overlay scale was provided to lay on the strip-chart recording.
Find the slope of the phase comparison plot on the overlay and
determine the error of the local standard down to Parts Per Billion
no problem. [extreme example of "low-frequency" zero-beat...:-)]



Two years in Standards Lab at Ramo-Wooldridge Corp. in early 1960s.

From: "Joel Kolstad" on Tues, Nov 29 2005 6:53 pm

"Scott" wrote in message

Wouldn't it be easier to use a frequency counter? If one is not available,
but a general coverage HF receiver or transceiver with a digital readout is
available, you could zero beat the oscillator, note the reading on the
display, cause the oscillator to shift to your other frequency, zero beat
that signal, note the display reading and subtract to find the difference in
frequency.

I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?

On zero-beating very low beat rates: The only perceived problem is
the stability of the receiver and manual control of finding an
"exact" zero-beat. Other than that, cranking up the audio level
will
let you know - by the background hiss intensity changes during zero
beat - when the zero point is reached. With manual tuning and a
crystal-controlled BFO that could be done to about +/- 0.1 Hz if the
receiver is kept at an even temperature and power line voltage kept
stable. [the metrologist's patience is a factor there as well]

There is a problem with modern receivers using PLL or DDS sub-system
tuning: The resolution of the control system (typically 10 Hz on HF
receivers). That limits the precision of zero-beating...unless the
beat difference itself is measured with a counter.

"Time interval averaging" has been used for 3 decades in frequency
and time interval counters to increase accuracy limits caused by the
+/- one count on the display. Statistically, that can be improved
by a factor of the square-root of the number of times it is
measured.
For example, taking the square-root of 100 measurements will
increase
the accuracy by 10 times. That averaging is automatic on base ten
displays in modern frequency counters made since the 1970s.

Example of determining accuracy of frequency standards beat against
WWVB on 60 KHz: An early H-P WWVB receiver and strip-chart recorder
read-out for phase difference against WWVB. A nice little overlay
scale was provided to lay on the strip-chart recording. Find the
slope of the phase comparison plot on the overlay and determine the
error of the local standard down to Parts Per Billion no problem.
[extreme example of "low-frequency" zero-beat...:-)]



Two years in Standards Lab at Ramo-Wooldridge Corp. in early 1960s.

Joel Kolstad wrote:
"Scott" wrote in message
news
Wouldn't it be easier to use a frequency counter? If one is not available,
but a general coverage HF receiver or transceiver with a digital readout is
available, you could zero beat the oscillator, note the reading on the
display, cause the oscillator to shift to your other frequency, zero beat
that signal, note the display reading and subtract to find the difference in
frequency.


I'm told that 'zero beating' typically has accuracy not much better than some
tens of Hz due to the limited lower frequency response of the human ear... has
anyone tried zero beating a signal from above and below and taking the average
to get what might be a more accurate frequency estimate?


If you have the right kind of AGC you can listen to the noise getting
stronger/softer and go for the slowest variation -- this will get you as
fine a zero beat as you have patience for.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com