Bill wrote:
>
>Ian said :
>
>> The real problem is that there are so many meters out there with poor
>peak-reading circuits.....
>Amen
>What is needed is a careful design of the filters associated with the meter
>output.
>Suppose one stacked two time-constants with one having a 0.1 second tau and
>another with say a 3 second tau? The fast one would ollow enough of the
>envelope to get through so that one could see what is going on. It would more
>or less follow the syllabic peaks. The second tau would be holding the
>average value, more or less. Some experimenting would be required to
>ascertain the most practical values.
Most peak-reading circuits do attempt to create this dual time constant.
The two problems are the charging rate of the capacitors, and
excessively long discharge times.
First, the modulation peak that you're trying to grab is only a few
microseconds long, and the caps must charge up quickly enough to have
reached >99% of the peak rectified voltage while the peak lasts. This is
usually done using an op-amp, so that the output of the RF detector is
very lightly loaded, and the capacitor is charged quickly by the output
of the op-amp.
Second, the 'hold' time must be long enough for the meter needle to drag
its way up to the peak reading, stop bouncing and hold its position long
enough for you to take a reading.
One op-amp will do it, and there are lots in circuits in various
handbooks and magazines. AFAIK, the peak-reading adaptor for the Bird 43
uses this type of circuit. It *cannot* be done without some kind of
active circuit, which is missing from the cheap-and-nasty "peak" reading
SWR/power meters.
The problem is that if you try to create a good hold time by simply
making the RC time constant very long, it takes an age for the meter to
drift back down again. It becomes far too slow to follow speech for
example.
As far as I can see, a better way to solve this problem is by a sample-
hold-discharge circuit, that will
1. Grab the peak value very accurately
2. Hold the peak value constant for a second or more, so you can read it
3. Quickly discharge the capacitor down toward zero, so that the meter
follows the modulation envelope more accurately.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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