Hi Brett.
> John,
> Please describe the quadrature phasing part of your receiver.
In general terms, the design is very similar to quadrature phasing receivers
used for SSB operation, except that since the bandwidth in this case is very
narrow, the 90 degree audio phase shifting network becomes ridiculously simple
(a single all-pass filter).
The receiver uses a pair of switching mixers (I and Q), an L.O. network with 0
and 90 degree outputs, a 90 degree audio phase shifter in one of the audio
channels (I or Q), a summing network, and three stages of active audio
filtering/amplification overall.
The phasing circuitry is necessary to produce "single-signal reception" (no
annoying image response on the other side of zero beat).
Tuning is a bit critical as it is possible to tune across a CW signal in
between the "dits" and "dahs", and not even know a signal was there.
> On weak signals, I often find narrower filter settings of the K2
> make the signal harder to copy.
> Part of that might be my bad hearing and the higher pitch
> sidetone I like...
> The noise and the signal seem to blend into one pitch...
I agree. In narrow bandwidths, the noise-induced ringing in an HF crystal
filter can almost sound like some sort of narrow shift FSK!
> > Mechanical filters, which operate in an acoustical domain,
> > may have somewhat
> > higher insertion loss and lower dynamic range than properly
> > designed crystal
> > filters.
>
> They operate at 455 KHz, I don't think even dogs can hear that high!
Your TV set uses a Surface Acoustical Wave (SAW) IF bandpass filter that
operates at 45 MHz, where the acoustic energy traverses along a lithium niobate
substrate. The insertion loss is fairly high, but the group delay distortion
is significantly lower than a comparable filter using LC components.
> Was not the old heathkit hw7 a direct conversion receiver?
> I had a lot of fun with that rig...
I believe the HW-7 was direct conversion...
73, de John, KD2BD
Visit John on the Web at:
http://kd2bd.ham.org/
.
.
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