Message-ID: <003a01c28bf9$1406c220$9945063e@laptop>
From: "Kevin Ravenhill" <hdq@btinternet.com>
To: "LF Group" <rsgb_lf_group@blacksheep.org>
References: <003101c28b20$d4b45e80$67967ad5@main>
Date: Thu, 14 Nov 2002 16:13:48 -0000
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Subject: LF: Re: Spontaflex
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Hello Hugh, Jim, Alan et al

Hugh M0WYE wrote:
>I wonder how you demodulate CW signals - do you allow the circuit to
>oscillate as you would do with a conventional regenerative circuit ? The
>basic Spontaflex seems to be an AM receiver.

Yes, works just as a "conventional" regen, receiving CW whilst gently
oscillating. It's a lot more sensitive in this mode than it is when
demodulating an AM signal (set just short of oscillation) - much more so
than I would expect, in fact. Have yet to work out why this should be -
there's certainly a number of quirks in that circuit!
The only real audio filtering I have at present consists of a 22nF capacitor
across my pair of SG Brown "Type F" 4kohm phones, the combination resonating
at about 750Hz. This has reasonably decent Q when used as the collector load
of the audio output transistor, which helps in more ways than one.

Jim M0BMU wrote:
>The small ferrite cored inductors that are fairly readily available can be
>used to make passive bandpass filters of several hundred Hz BW quite
>easily. If you don't mind winding some pot cores, and have some test
>equipment (audio generator & voltmeter), passive filters with bandwidth
>down to 50Hz are not too difficult. With limited dynamic range, you might
>be better to go for a passive preselector, which can also be made with
>several 100Hz bandwidth, if you can find suitable pot cores.

Alan G3NYK wrote:
> As to the effects of DCF39 ....(and it may be being useful as a BFO in
that
> circuit !!) it is possible at the LF frequencies to build a bridged-T
notch
> filter that will remove DCF39 with little loss in the 136 band. You will
> need a high Q inductor  as its is only 1030Hz above the band , so a Q of
> about 130 will still leave you with 3dB loss at the top of the band. It
> should be possible to make a circuit with a Q or 500 to 600, which should
> make the in-band loss almost negligible. There is even an example in the
AC
> analysis ZIP download (I think) using a 50uH inductor. The only difference
> might be it is easier to centre-tap the coil that use a capacitor "tap"
> (which was easier in the program)

Hmm, interesting (particularly the notch idea). It's obviously better to put
as much filtering as possible right up front, so I'll look at combining a
narrow RF bandpass filter with one or maybe even two bridged-T notches to
reduce the effects of the nearby commercials. It'll increase the overall
size of the receiver several-fold, but then you can't have everything! I've
got gain to spare in the receiver itself, so hopefully a little loss with
moderate coil Q should not be too much of an issue (in fact it might even
help). Will report back in due course.

73, Kevin G1HDQ