Alex,
Yes! Exactly why I'm looking at as many schematics as I can find. The
time constants are everything. Most of the kludge jobs I've seen over
the years did a very poor job of catching the true peaks simply because
not enough attention was paid to the times.
First thing is to make sure the peak detector is fast enough to catch a
short signal pulse. But how short, that's the question. For SSB, you
might have pulses with a rise time of 83 microseconds and a duration of
not much more than that. Can the circuit capture that? Most can't. On CW
you surely need to be able to capture a single "dot" at any normal
sending speed. My "digital" power meters used on HF do that well enough
but I haven't have any analog meters here in the shack that do.
Next item is "hang time". How long does the circuit need to hold the
reading? That depends on what type of indicator you use. If it's an
analog meter like I *might* use in this project, then the internal
damping will effect the circuit design. Hang time is determined by the
time constant of the resistor and capacitor in the peak detector in an
analog implementation for the most part. However OP amps typically don't
like to drive a capacitive load. If you make the cap small, you need to
make the resistor large and long time constants will be limited by the
leakage of the components and circuit board. Large caps will also limit
the fast pulse capture due to finite output impedance of the op amp and
diode as well as the slew rate of the OP amp. Details Details Details
None of the circuits I've seen so far have any offset drift
compensation. Is this not needed? Or, is it just ignored like so many
other details.
Truthfully I have not personally evaluated the Bird adapter board, or
any of it's clones, but I have information from several others that I
trust that it falls short for amateur service.
73, Larry
Larry - W7IUV
DN07dg
http://w7iuv.com
Saandy Eban wrote:
> NOOO, IT'S NOT!!!!
> At least Heathkit had on the market a dual function wattmeter (published
> also in QST)and MFJ has also at least one model- not a tuner with
> SWR/Wattmeter included- but a stand alone meter that can also measure
> average or peak power. The trick is in selecting the right time constants
> for the averaging circuits: long for peak readings, short for average
> readings. Bad thing is that selection has to take into account the
> mechanical characteristics of the meter movement!
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