LF: Selective level meter as receiver

To:	"LF-Group" <[email protected]>
Subject:	LF: Selective level meter as receiver
From:	"Dick Rollema" <[email protected]>
Date:	Thu, 23 May 2002 12:52:12 +0200
Reply-to:	[email protected]
Sender:	<[email protected]>

To All from PA0SE

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.

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.
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.
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.

(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)

2. The near selectivity is also limited. Though the pass bands are no doubt as specified the filter slopes are not very steep.
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.

3. The 25Hz IF filter rings objectionably because it was not designed for pulse shaped signals.

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.

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.

73, Dick, PA0SE

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