> Tom said:
> > A good meter is more reliable than a scope.
> What does that mean? Meters have a longer MTBF than Oscilloscopes?
Well, they do but that isn't what I meant.You are supposed to read
peak envelope power, a good meter will store the highest crest that
comes along.
It's incorrect to think you need to look at one cycle, or that a meter
can't catch and register the envelope accurately. The rise time and
fall time of the envelope is extended by the bandwidth of the
transmitter. You have at least a few thousand RF cycles to
"capture" the peak over, not just one.
If we assume the highest modulating frequency is 3500 Hz with a
sine wave (can be square, or it won't go through the SSB filter) the
time from start to full to zero is 1/7000 second. The storage
system in the meter has half the full rise time of that gradual slope
available to charge the storage capacitor.
1/14000th of a second is not real fast.
> The waveform envelope is limited by filters to 3000 Hz, an audio limit.
> But does not the FCC regulations state that the power is to be measured
> over one RF cycle? At 30 Mhz = 33.3 nano-seconds At 1.8 Mhz = 555
> nano-seconds
FCC regulations say no such thing. You have 128 full RF cycles on
160 meters to build charge to the peak, and over 2100 RF cycles
on ten meters.
The amplifier itself can not go from zero to full power in 555
nanoseconds or less, let alone the restricted BW of the transmitter.
> rectifier time-constant is 40 micro-seconds, then in 3 time constants most
> of the prior information will be bled off and the circuit will be ready
> for the next pulse. Some of the circuits I have looked at had 15
> millisecond time-constants. Somehow either I am missing something very
> simple or my logic is faulty! I think Rich is using a good system. The
> bandwidth of the scope is more than adequate and will not miss in-between
> pulses. The FCC says power is measured over one RF cycle.
Where does it say that? They say peak ENVELOPE power, not
cycle by cycle power.
Even if they did, what would it mean anyway? Even your amplifier
can't respond that fast! If you had a nanosecond time frame
envelope peak, your signal would be many megacycles wide.
Violating a power rule would be the least of your worries with a
signal like that!
When the FCC measures power, they use a regular peak reading
meter. The FDA accepts peak reading meters. Only when you get
into nanosecond pulses of radar at microwave can the rise and fall
times be as fast as you imagine.
73, Tom W8JI
w8ji@contesting.com
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