One thing I have not seen mentioned so far is the probes that you will need
to use. The scope's 1:1 normal probe will place a fairly large capacitance
across the device you are measuring and the amount will depend on the
quality of the probe you buy. If the signals you intend to measure are
fairly large, then it is very well worth while using at least a 10:1 or even
100:1 probe as these put a much smaller capacitance across your signal
source. That means the scope has to handle a smaller signal. Original Tek
probes are very expensive and a new one might cost the same as that second
hand scope! The probes are sometimes powered from a special socket on the
scope, so be careful you buy one appropriate to the scope.
I should also point out that it would be unusual to want to see the
individual sine wave trace; you would normally only see a wide green bar
across the face of the scope representing the amplitude. This is a peak to
peak waveform and you will need to divide it by 2.828 to get to the rms
value. The time base setting is much lower than the waveform you are
measuring. By the way, scopes are rarely better than 5% accurate and do
drift, they are not meant for precision measurement.
Those modern scopes that do the calculation for you save a bit of time and
you might normally need another instrument to measure the rms voltage (again
with a precision probe). This all depends on the waveform being well within
the -3dB measurement limits. Of course you can still measure - even beyond
the -3dB limit if you have previously done a calibration with various
signals so you know the performance at higher frequencies. Just because a
scope is spec'd to only say 20MHz doesn't mean it cannot be used beyond
that: you just have to know what that performance is.
Don't expect to see sinewaves. If the signal is strong enough, you don't
even have to make contact. Just laying the probe nearby can get you an
indication and not upset the circuit too much. You can make *comparative*
measurements like this, ie adjust a control while watching the scope for
peaking purposes.
David
G3UNA
----- Original Message -----
From: "Howard W. Ashcraft" <[EMAIL PROTECTED]>
To: "Doug Person" <[EMAIL PROTECTED]>; <Elecraft@mailman.qth.net>
Sent: Sunday, November 26, 2006 10:26 PM
Subject: RE: [Elecraft] Recomendations on Oscilloscopes
Everyone has favorites, but the older Tek analog scopes are very solid and a
good deal. If you have the room, you can get amazing performance/cost from
7000 series scopes. I have a 7854 (400 mhz plus waveform digitizing) and a
7934 (500 Mhz Analog storage) 7000s run from 100mhz to 1Ghz in bandwidth,
and, depending upon the plug-ins chosen, you can do just about anything with
a 7000 series. Their primary drawback is size. Both of my scopes are on
carts because they would take up a big chunk of my bench, otherwise.
Tek also made a very nice series of "portable" scopes. The 100 Mhz 2246 is
very nice and readily available used. It has the added bonus of "smart
cursors" to easily measure amplitude and period/frequency with a single
button push, and I have found the triggering to be quite good. The 2400
series, and particularly the 2465s are also nice pieces of equipment, but
are more expensive due to their greater bandwidth.
What you "need" really depends on what you "want" and what you are going to
do with it. The higher bandwidth devices are useful if you need to look at
abberations in a signal that may be smoothed out in a lower bandwidth scope,
but may be unnecessary if you just want to trace a signal through a circuit
and measuring changes in amplitude or phase. Extra bandwidth is never a bad
thing, it just costs more, and may not be necessary.
Now to stir up the controversy, I used to have a Tek 465. This is a
venerable 100 Mhz scope, but frankly the 2246s that I have had worked better
and were much easier to use. So, I would recommend a scope more recent than
the 465/475 series. But there are lots of these still out there and are
very much liked by some. My one experience repairing a 465 may have colored
my view.
Howard W. Ashcraft, Jr. W1WF
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