LF: Selective level meter as receiver

["Dick Rollema" <d.w.rollema@freeler.nl>]

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