I just measured the resonance frequency of ten 10 MHz
crystals. I found a set of four within 30 Hz and
another set of four within 45 Hz.
How does this compare to other peoples results?
Here are my measurements in ascending order:
9,994,677
9,994,704
9,994,733
9,994,747
9,994,799 0
9,994,811 +12
9,994,821 +22
9,994,829 +30
9,994,874
9,994,887
You can see the chosen 4, which are within 30 Hz. I wouldn't have been able
to match a second set within 45Hz.
Note: these frequency counter readings are on my homebrew frequency counter
(
) which is installed in my 80/40m receiver
(). Previously in 30m
QRSS beacon experiments () I
built a simple 30m direct conversion receiver
() and calibrated the
frequency counter against the Moscow RWM timesignal on 9,996,000. My counter
was reading just over 1KHz too low at this frequency. So the measurements
above would need to be revised upwards by about 1KHz. But it's the relative
frequency which is important here anyway, and I haven't repeated the
calibration.
Did somebody already plot the passband shape of a
resulting 4 crystal filter?
How do you do that? Figure out the shape that is?
Farhan has spoken about theoretically determining the shape. I'd also like
to see experimental measurements. One way is to use a spectrum analyser with
tracking generator and very narrow bandwidths available. The filter shape
would appear directly on the screen. I built a spectrum analyser
(
) and someday I will make a tracking generator, but not yet (too many other
projects).
When I have time, I indend to build another logarithmic amplifier using the
excellent (but expensive) AD8307 chip. Using this with a voltmeter to
measure the logarithmic output, a frequency counter and a 30m VFO it would
be possible to take a series of measurements of the filter attenuation at
different frequencies and plot the curve. The log amp on my spectrum
analyser can be seen here:
/index.htm. You'll note that it's a very simple circuit. I think this will
provide a cheap way to obtain the passband curve.
72/3 de Hans G0UPL
