Message-ID: <5.1.0.14.0.20020523112845.00a3e130@POP3.freeler.nl>
Date: Thu, 23 May 2002 12:52:12 +0200
To: "LF-Group" <rsgb_lf_group@blacksheep.org>
From: "Dick Rollema" <d.w.rollema@freeler.nl>
Subject: LF: Selective level meter as receiver
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<b>To All from PA0SE<br><br>
I endorse  what has been said before via the reflector: Selective level
meters are excellent for the job they were made for. But as LF receivers
they have limitations.<br><br>
1. Most or all of them have no preselection before the mixer, making them
vulnerable to strong out of band signals.&nbsp; External&nbsp; input
selectivity must be added;&nbsp; for instance a band pass filter with at
least two or three high-Q tuned circuits.<br>
The selectivity of the aerial system, as used for transmission, may turn
out to be sufficient,&nbsp; but only when there are no strong broadcast
or other transmitters in the vicinity.<br>
My nearest strong transmitters are at MF at a distance of 40km or so and
I find the selectivity of my aerial system sufficient for use with
a&nbsp; Wandel &amp; Goltermann SPM-12 as receiver in the &quot;low
distortion&quot; position. In the &quot;low noise&quot; position&nbsp;
the instrument&nbsp; is hopelessly overloaded before a suitable
sensitivity can be selected.<br><br>
(My SPM-12 is of the type that has the possibility of locking the
frequency to a crystal.&nbsp; Whether the stability is good enough for
QRSS I don't know because I only use normal&nbsp; CW. I also possess the
companion PS-12 signal generator that can be used on its own or&nbsp;
driven from the SPM-12. Between them&nbsp; they make a fine team for
measurements)<br><br>
2. The near selectivity is also limited. Though the pass bands are no
doubt&nbsp; as specified&nbsp; the filter slopes are&nbsp; not very
steep.<br>
I used the SPM-12 to receive SAQ at 17.2 kHz but&nbsp; could not get rid
of&nbsp; interference from GBR at 16kHz. I had to insert an outboard
audiofilter between the SPM-12 and the headphones to obtain a clear
signal.<br><br>
3. The 25Hz IF filter rings objectionably because it was not designed for
pulse shaped signals. <br><br>
I therefore very much prefer my Teletron LWF45  hybrid LF receiver from
the sixties with two valves in the RF part.&nbsp; It has five tuned
circuits before the mixer that track the oscillator and excellent IF
filters at 30kHz with 11 tuned circuits and a 6/60dB shape factor of
better than two.&nbsp; As the narrowest filter is&nbsp; 400Hz wide I
still use the outboard audiofilter as well. But the large signal
behaviour of the RX leaves nothing to be desired.&nbsp; <br><br>
The passive outboard audio filter provides choice between 200Hz and 30Hz
bandwidth, centred at 1000Hz . The 30Hz filter does not ring because it
is a linear phase design.&nbsp;&nbsp; <br><br>
73, Dick, PA0SE</b></html>