[Elecraft] scope suggestions?

[Don Brown]

Hi

I agree with most of this except 2 to 3 times the frequency should be =
enough. The problem is scopes are down 3 db at the rated frequency. This =
is how they have always been rated. This means a 1 volt Pk to Pk signal =
can read .707 volts at the rated frequency and still be in spec. Most =
scopes will indicate out to some higher frequency but they will just not =
be accurate for making measurements. So for HF work of less than 30 Mhz =
a 60 or 100 Mhz scope would be enough. Of course more is always better =
in this case.

 Remember the probe needs to be rated at or above the scope bandwidth =
also. A scope without a probe has a 1 meg ohm input resistance with 20 =
to 50 pF capacitor across the input. This will load many circuits and =
give false readings. The probe reduces the capacitance by a factor of 10 =
and increases the input resistance times 10 with the tradeoff of a loss =
of gain by 10. You would almost always use a probe on a scope for =
general work.=20

The Fluke scope meter is a great little scope but a little pricy. Tek =
makes several models similar that are good also although I still prefer =
an analog scope. I have a Tek THS-720 digital scope that is like the =
Fluke Scopemeter.  I use the THS-720 in the field and I have a 2465 4 =
channel 300 MHz analog in the lab. The 2465 was probably the best analog =
scope Tek ever made. You can find them for around $800-$1000 now in good =
condition. A lot of money but considering they were over $5000 new in =
the mid 80's to early 90's, not really that bad. The Tek THS-720 was =
$2200 new 5 years ago so you may be able to find one at a reasonable =
price now. This is a battery operated 500 Ms 100 MHz dual channel scope =
with built in DMM and will do waveform math and you can hold in the palm =
of your hand. Its about the size of a K2 and 1/2 as thick.

Keep in mind Tek has a policy of ending the support on a product 7 years =
after it is no longer in the catalog. At least that was the policy when =
I worked for them back 20 some years ago. A lot of the cheap Tek scopes =
on the used market like 465's,  475's and the 7000 series went out of =
the catalog 10 to 15 or more years ago. Parts are just about impossible =
to get on these because most of them were proprietary and Tek does not =
make them any longer. If you buy one of these you may want to get two so =
you will have one for parts

All of the new inexpensive analog scopes are made in China, Taiwan or =
Japan=20



Don Brown
KD5NDB =20
  ----- Original Message -----=20
  From: Ron D'Eau Claire=20
  To: elecraft@mailman.qth.net=20
  Sent: Wednesday, January 21, 2004 10:58 AM
  Subject: RE: [Elecraft] scope suggestions?


  One thing to keep in mind about scope "bandwidth" is that you normally =
want
  a bandwidth rating considerably more than the fundamental frequency of =
a
  signal you're likely to want to investigate if you want to see the =
detail in
  waveforms accurately. Something on the order of 5 to 10 times the =
highest
  signal frequency is a sort of "rule of thumb" I've always heard. So, =
for
  "typical" HF work, a bandwidth of 100 MHz or more is a good idea. =
Otherwise
  a lot of the small 'detail' - the squiggles and other artifacts that =
are the
  reason for investigating many waveforms - are lost because the =
frequency of
  the artifact is much, much higher than the basic frequency of the =
waveform.
  The result is that waveforms look much cleaner on a lower-bandwidth =
scope
  than they really are.

  A low bandwidth scope is fine for simply checking that logic is =
switching.
  Multiple traces are very handy for looking at the relative timing of =
two
  signals at once. Simple "logic probes" with LED's often can provide =
that
  information at much lower cost and smaller size. =20

  Good USED scopes are fairly cheap, but a good SMALL scope is a =
different
  matter.=20

  One pays for portability. If the budget allows it, Fluke has for years =
sold
  a very attractive little solid-state oscilloscope/DMM combo called the
  "scopemeter" (see http://www.tequipment.net/Fluke192Bscope.html for an
  example. They have a lot of models. My only connection with this =
vendor is
  that I've been a customer and they seem to have competitive prices).=20

  I drooled over one of these years past when I had to haul all of my =
tools,
  spare parts and test equipment up seven or more flights of stairs for =
every
  service call. I would have bought one except that my employer had a =
small
  3-inch CRT-type Tektronics 'scope available that did the job I needed =
-
  although it did weight more.=20

  I haven't seen the little 3-inch Tek scopes on the used market for =
some
  reason. It's been a decade since I hauled that 3-incher around. I =
don't
  recall its bandwidth.=20

  Ron AC7AC


--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---