[Lf] [Fwd: LF: Selective level meter as receiver]

[Andre Kesteloot]

Dick Rollema wrote:

> 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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