EMRFD Message Archive 1504

Message Date From Subject
1504 2008-03-10 13:47:34 bkopski A Narrow Xtal Filter / DSO Experiment
A Narrow Xtal Filter / DSO Experiment

In recent years I have used different commercial and HB sweepers to
test and tune LC and Xtal filters using an ordinary 2 channel `scope
to display the detected filter output. The "detector" is my variant
of the popular QST Power Meter (June `01). My version is described
in QEX, May/June '02 and Sept/Oct '03 and simply adds some
versatility that permits easily manipulated, calibrated sweep
displays as herein.

With a conventional analog `scope in "XY" mode it is fairly easy to
display filter passbands wider than a few kHz. Such a BW still
allows a reasonable sweep speed and a stable, easily viewed `scope
display. However, narrower filters of a few hundred Hz BW require a
much slower sweep speed – a speed that presents an analog `scope
display of a bright dot traveling west to east leaving not much of a
passband outline that's visible to the eye. Ordinary `scope
phosphors simply don't have enough persistence for a useable display
of such a filter sweep.

Enter the DSO – digital storage oscilloscope! I've been toying with
getting one of these goodies for quite some time – for this and other
applications – and when I came across:


… I made the leap. Of course, I first downloaded the manual to make
sure of what I was doing and all seemed well. I had my new DSO in 2
days – and then it took 2 more to get cozy with it. But my careful
pre-purchase study did not prepare me for a major disappointment – it
turns out this instrument DOES NOT capture and save data when in "XY"
mode. Duuuuh!

A quick e-mail to the support folks at Saelig brought in a surprise
on their part as well – the gentleman there was surprised to find
this to be the case. So – could it be that all DSO's – or at least
many – can't capture in "XY" mode? A friend of mine pursued this
with a high-end LeCroy instrument at his work place and came up with
the same result. "The best laid plans …."

Not being willing to give up on this pursuit, I looked for a way to
use my new DSO in this filter sweeping assignment using the
normal "YT" mode – where it works fine in capturing and storing such
displays. The challenge was to somehow relate the scope sweep time
axis to the swept frequency.

I found that I could modify an existing ramp generator to sweep in
time-calibrated periods. It now has switch selected sweep speeds of
0.05 Sec to 10 Sec arranged in a 1-2-5 sequence. This HB box also
has fully adjustable "sweep start" and "sweep stop" ramp output
voltage limits. Thus, when used to drive (sweep) a varactor tuned
VFO/rf source, I can easily establish the stop and start frequencies
of the sweep. Coupling this established timing with the available
accurate DSO scan times, and triggering the DSO sweep on "EXT" from
the ramp generator trigger output, permits a very useable display
with amplitude on the vertical grid against a "frequency-equivalent"
horizontal (time) grid.

There are three associated photos (A-, B-, C-) that are screen
captures of some DSO displays. There are also pix related to the
test setup. Photos A and B show the filter passband on two different
vertical scales at a sweep time of 1 second while photo C shows
passband display distortion at a sweep time of 0.2 seconds – too fast
for this filter. Photos A and B appear unchanged when I tried a
slower sweep time of 2 seconds – so what's here is as good as it gets
for this filter. BTW – the total frequency sweep was 4 kHz.

The active DSO display is very useful during tune up of the filter.
I found that I could tweek source and load resistances, and filter
trimmer caps to get a quite good passband amplitude behavior. What
you see in the pix is much better looking than with just the cookbook
component values – at least for this crystal set. In summary, had
this been a 3 kHz filter instead of this nominal 300 Hz one, I'd have
done the same thing with my "regular" analog `scope and a camera.
The narrower filter would not be doable that way but this very
economical DSO as used herein does the job very nicely. BTW – I
still prefer my classic analog `scope for just about everything I do,
and I'm guessing that to get the same very comfortable performance in
a DSO would require far more kilobucks than I'm willing to part
with. Thus, I DO NOT recommend this economy DSO for your everyday HR

Cordially and 73,
1506 2008-03-10 17:38:01 Jason Milldrum Re: A Narrow Xtal Filter / DSO Experiment
Hi Bob,

If I'm understanding correctly what it is you want to do, it is
possible to do with a DSO. You just need to set the "Persistance"
setting to Infinite. This is the setting that allows a DSO to emulate
a CRT. Look at page 50 of your manual. I've used this same setting on
my Tek 1012 to sweep filters, and it works fine. It's not quite a
pretty as my 7603, but it does work.

Jason NT7S

1507 2008-03-10 17:38:03 Jim Miller Re: A Narrow Xtal Filter / DSO Experiment
Hi Bob

I was thinking about the same problem. I'm considering a somewhat different

1. Through a TBD D/A converter command a VCO setting.
2. Read the exact frequency by using a TBD counter. This eliminates VCO
non-linear response to voltage.
3. Read the output of the Power Meter in response. Or the spectrum analyzer
through a TBD A/D converter.

Since the speeds are slow the converters and frequency counters could
likewise be slow and therefore more accurate.

I've gotten exactly nowhere so far along this path except for thinking about

jim ab3cv