[Fwd: LF: / Measuring receivers]

[Andre' Kesteloot]

Hans-Joachim Brandt wrote:

> 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
> >