Hi all,
concerning s-meter reading of SSB or CW the best method I have found so far is
the principle which has been used in the old Plessey SL621 integrated circuit, a
combination of two detectors with different time constants:
One very quick, even suited to let the AVC react on short qrn or noise peaks,
the second time constant longer, which can be increased to several seconds
by removing a parallel resistor from the capacitor when the wanted signal
disappears, with also a third time constant being started when the wanted signal
disappears, in order to discharge the long time constant capacitor rapidly if
the wanted signal should not return in between the time of the third time
constant, in order to put the receiver quickly back to full sensitivity when
there is no signal any more.
I have arranged the functions of the SL 621 around a LM324, including a few
diodes; the only thing which I have made differently is to take the AVC signal
from the I.F., not the A.F. as in the application of the SL621. The s-meter
indication will remain the same, no matter whether you speak as usually or
whistle into the microphone.
QRN will always be a problem, I think; in my circuit this should not matter as
long as the peaks of the wanted signal are higher than the qrn peaks, otherwise
the QRN peaks would take the lead, of course.
HW?
73 Ha-Jo, DJ1ZB
James Moritz schrieb:
Dear LF group,
OK on the loading coils - my current loading coil is over 500mm
diameter, so ready-made containers to fit it are hard to come by.
I'm suprised that people have had success with coils exposed to
the rain, I found with mine that this drastically reduced the Q once
water got in between the turns. The 20kV or so across the winding
is a bit of a worry in the wet, too. Do hardware shops in Germany
really sell litz wire, Geri?
Interesting to see details of G4JNT's portable measuring receiver
project; on and off I have been working on something similar
myself. A significant difficulty with measuring signal levels on LF is
how to get a sensible reading with on/off keyed signals and
significant levels of QRN. A strong carrier or QRSS signal is easily
measured with any old AC voltmeter on the audio or IF output - but
it is not always possible to arrange such a signal. However, with
manual CW (and presumably modes like BPSK), the varying signal
level makes it difficult to get a steady reading. I have tried using a
"quasi peak" type voltmeter (ie. a peak detector with a few ms
attack and much slower decay, similar to many RX s-meter
responses). This works well with CW, but any sort of meter seems
to have problems with QRN; since the peak QRN level is often
much higher than even quite strong signals, you get a few dB
variation even with long meter time constants.
Perhaps the most satisfactory method is to use an oscilloscope to
display the signal, since then you can see and identify the
modulation envelope and QRN spikes, but this is less than
convenient for portable use!
Another measurement that is important to make is the noise level,
but this is also highly dependent on the type of meter response.
Cheers, Jim Moritz
73 de M0BMU
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