[Elecraft] Keying Waveform Measurement

[Bob - W5BIG]

 Hi Steve,

Check the spec on your scope probe. (try Google). If it can handle the
voltage at high frequencies, that is the easiest way to go. 100 watts into
50 ohms has a peak 100 volts (the peak-to-peak is 200V but the probe only
sees +/-100 volts max).

If you need to make an attenuator, it doesn't have to be compensated if you
are only interested in the shape of the waveform and not it's exact
amplitude. As long as you use linear components (no toriods), the shape of
the leading and trailing edges will be proportional to the actual waveform
and the frequency spectrum is the same.  Even 50K or 100K film resistors
would work, with something around 1K to ground.

 As an alternative, it doesn't take much capacitive coupling to get a usable
signal.  A piece of wire near the dummy load might pick up enough signal
when your running 100 watts.  In this case, a small resistor to ground can
used to control the amplitude of the signal.

73/ Bob - W5BIG


> It had occurred to me that a cheaper strategy would be to run the rig into
> my Heath Cantenna (remember those?) and connect a regular high-Z
> compensated scope probe (the probe is good up to 100 MHz) across the dummy
> load resistance. Is there some gotcha to doing that? Maybe that is not
such
> a good solution; 100 watts RF into a 50 Ohm load will have a voltage of
> something like 200 Volts peak to peak, and I expect that that is way more
> than the scope could handle. I also expect that to observe full power,
> you'd need to construct a high-Z voltage divider to tie across the dummy
> load, being very careful to keep its reactance low.
>
>