Re: Advice on HP Function Generator Repair
- From: "Dave M" <masondg4499@xxxxxxxxxxx>
- Date: Wed, 15 Apr 2009 22:33:14 -0400
"Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote in message
news:jpvFl.29927$uK2.12686@xxxxxxxxxxxxxxxx
"Jake" <mechanicaljake@xxxxxxxxx> wrote in message
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On Apr 15, 1:18 am, "Matt J. McCullar" <mccul...@xxxxxxxxx> wrote:
Not having seen a picture of the function generator, I'm presuming you're
using an external oscilloscope to look at the FG's output.
Just a question: Have you been able to verify that the strange waveforms
are
actually coming from the function generator itself, rather than an
artifact
of... say, the oscilloscope probe and/or the oscilloscope itself not
being
able to handle incoming frequencies that high? Just don't want you to
waste
time trying to fix something that may not actually be broken. :)
If it is definitely coming from the function generator, then I'd agree
with
the previous posters: replace the electrolytic capacitors. They're over
20
years old now.
I am using a brand new Tektronix TDS2024B 200MHz 2GSa/s oscilloscope
with a fresh calibration.
Thanks for all the advice!
Not wishing to labour the point, but what you describe as happening to the
waveshapes, in that they are all heading towards a dirty sine wave the
higher up in frequency that you go, sure sounds like what you will get if
you glue a low-pass filter on the end of such a generator. Are you
absolutely sure that there is not any bandwidth filter switched in on the
'scope, and that the probe you are using is good for the frequencies that
you are looking at ? Is it a low capacitance probe for instance ? Looking
at anything above a few hundred kHz generally requires the use of a low
capacitance probe (typically a x10 attenuator probe, or a 'universal'
probe switched to its "x10" setting)
Arfa
In addition to the advice already given, I'd like to make a suggestion or
two. Make sure that the generator's output is terminated correctly (50
ohms?). A feedthrough termination is the generally accepted way to load the
output; place the termination at the scope input rather than at the
generator output. You don't normally use a probe in this setup; use a cable
of the proper impedance (RG-58 for 50 ohms; RG59 for 75 ohms). This will
give you a much better picture of the output signal. If you're looking at
the unterminated output through a low frequency probe, it's no surprise that
your square waves look like triangles and the triangles look like sines.
Next, if you don't already have one, try to get a service manual for it. I
looked in all the usual manual vendors' sites and didn't find any listings
for your 3114A, so it might take a bit of searching.
Cheers!!
--
"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra
Dave M
masondg44 at comcast dot net
.
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