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Dick Rollema wrote:
<blockquote TYPE=CITE><b>To All from PA0SE</b>
<p><b>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.</b>
<p><b>1. Most or all of them have no preselection before the mixer, making
them vulnerable to strong out of band signals. External input
selectivity must be added; for instance a band pass filter with at
least two or three high-Q tuned circuits.</b>
<br><b>The selectivity of the aerial system, as used for transmission,
may turn out to be sufficient, but only when there are no strong
broadcast or other transmitters in the vicinity.</b>
<br><b>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 Wandel & Goltermann SPM-12 as receiver in the "low distortion"
position. In the "low noise" position the instrument is hopelessly
overloaded before a suitable sensitivity can be selected.</b>
<p><b>(My SPM-12 is of the type that has the possibility of locking the
frequency to a crystal. Whether the stability is good enough for
QRSS I don't know because I only use normal CW. I also possess the
companion PS-12 signal generator that can be used on its own or driven
from the SPM-12. Between them they make a fine team for measurements)</b>
<p><b>2. The near selectivity is also limited. Though the pass bands are
no doubt as specified the filter slopes are not very
steep.</b>
<br><b>I used the SPM-12 to receive SAQ at 17.2 kHz but could not
get rid of interference from GBR at 16kHz. I had to insert an outboard
audiofilter between the SPM-12 and the headphones to obtain a clear signal.</b>
<p><b>3. The 25Hz IF filter rings objectionably because it was not designed
for pulse shaped signals.</b>
<p><b>I therefore very much prefer my Teletron LWF45 hybrid LF receiver
from the sixties with two valves in the RF part. 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. As the narrowest filter is 400Hz wide I still use the
outboard audiofilter as well. But the large signal behaviour of the RX
leaves nothing to be desired.</b>
<p><b>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.</b>
<p><b>73, Dick, PA0SE</b></blockquote>
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