"Ian Iveson" wrote in message
...

"Phil Allison" wrote


** If you can't see them on the scope how do you even know when
they are there ?

I measure with a voltmeter. Also, the scope's trace goes fuzzy.


** That contradicts my "if" condition .

10 Mhz makes no effect on a 1 Mhz scope.



If you don't know they are there then why would you use an
(RF) voltmeter ??

To check they are not there :-)


** You are assuming continuous RF oscillations - more usually ones sees
bursts superimposed on a audio wave.



This is in the context of audio valve amplifiers. When I check an
amp I put a 25MHz voltmeter across the load. If I get a significant
reading with no input signal, it is oscillating. If I need to know
the frequency of oscillation then I use a frequency counter. The
scope only gets connected when I know the amp is not oscillating.
Both these instruments are decent, ex-military units and fairly
recently calibrated. I can read them without using a ruler.


To check for ringing above 2MHz, I suppose I could sweep and use the
voltmeter, but I don't bother. Perhaps for guitar amps, or other
things with complicated voltage stages and crowded wiring?


Do you often discover anything worth finding above 2MHz? Maybe I
should save up for a new scope?


** I started out ( as a teenager) with a home brew 3 inch, 2 MHz all
tube scope 35 years ago ( still have it too) then a 10 MHz single beam and
for the last 16 years a 50 MHz dual beam.

SS amps can have parasitic oscillations up to 40 MHz or more, especially
if they use MOSFETS.

Probably 2 MHz is enough for someone who works on tube amps exclusively.




.............. Phil