[Fwd: LF: receivers and filters]

[Andre' Kesteloot]

Toni Baertschi wrote:

> Hello Alan,
>
> for LF I'm using a commercial US-rig, the Harris RF590. I like it
> because of it's extremely high frequency stability, high immunity
> against strong signals (IMD), the 1Hz read out and the low tuning rate
> (100Hz per revolution) which gives me the illusion of a LF-band as large
> as 40m, hi.
> But I got the rig also with a Collins 500Hz/455kHz filter (commercial
> equipment manufacturers are specifying filter BW often at -3dB and this
> gives you the illusion to have a 300Hz filter, but in fact it is 500Hz
> at -6dB)
> For a better separation of the stations in the narrow LF-band, I
> replaced it with the FL53A from ICOM. This is a very steep XTAL-filter
> (unfortunately also very expensive!) and I took no special measures for
> impedance matching.
> Drawbacks of the Harris RF590 are: operation also a little bit
> complicated (commercial radio operators have other needs) and a blow by
> in the IF-filtering (at least in my version) due to poor design, but
> fortunately it was easy to modify.
> I also tried other commercial surplus from Racal, Rockwell-Collins,
> Watkins-Johnson R&S and Telefunken. They are all very similar when
> developed in the same period (they all had to fulfill the same MIL
> standards, I guess).
>
> As an additional audio filter I'm using the DSP599zx from Timewave,
> where I can go down to 10Hz BW (-3dB) if necessary and as matter of fact
> some CW QSO have only be possible by using this filter and BW between 10
> and 35Hz (as the QSO last weekend with G3XDV).
>
> Listening to such a filter is not pleasant. They usually don't ring but
> the filter noise is close to the tone of the expected signal.
> And of course there is a big gap between theory and practice: the
> improvement by such a filter is far from the theoretical value!
>
> Often I use instead of the DSP-Unit a passive home-brew LC-filter
> between
> the RX and the headphones. It's a bandpassfilter with a 11 element
> Chebyshev LP and a 9 element HP. not so narrow (200Hz), but very
> pleasant to listen, even under heavy QRN.
>
> Finally I have to say that it is not necessary to employ commercial
> receivers for good results. I've heard most of the UK stations also on
> my Drake R4B (old tube rig) with home-brew converter (just LPF, Osc and
> Diode ring mixer) and the above mentioned passive audio filter. The
> sound is very pleasant (one of the best AGC's I know - better than
> Collins and most of the mentioned commercial rigs!). The only
> inconvenience is the lack of frequency accuracy,
>
> 73 de Toni