[R-390] R-390 AProduct Detector

Fri Mar 23 10:03:57 EST 2007

Could you provide a little more detail about implementing the Softrock
solution?  My quick look on the web says those kits haven't been
available for a while now...but it was a quick search.

    Computers don't seem to be a problem here...lol.

          Thanks...Dennis

On 3/23/07, Mark Huss <mhuss1 at bellatlantic.net> wrote:
> Not sure this is such a problem as you think. Two ways to generate AGC
> is (1) RF Voltage at the Detector. (2) AF voltage after the Detector,
> often called AVC. For AM, Nr 1 is considered best, as it is not affected
> by the modulation level because it acts on the received carrier wave.
> For SSB, however, there is no received carrier wave. Instead, Nr 1 often
> will be driven by the BFO injection level. Thus Nr 2 is the preferred
> way of controlling receiver gain. You can get around this by tapping the
> RF level before the BFO injection, but older receivers not designed to
> receive SSB had no reason to do this.
>
> With an External detector, however, things are different. If you set the
> receiver for normal AM reception, with a slow AGC time constant, the
> receiver AGC will act normally, substituting the sideband RF for the
> Carrier RF. It will be choppy and constantly varying in level as it is
> the modulation, but the Slow AGC time constant smooths this out. It
> would be the same as more modern receivers tapping the AGC voltage
> before the BFO Injection point. Attack time on the first syllable may be
> improved. And you have to have a long enough decay time to cover pauses
> between sentences. And you will have to use the receiver's filtering for
> most of the selectivity.
>
> The beauty of the Softrock solution is that it has its own AGC,
> implemented in software, with a range of greater than 60 dB (dependent
> on the dynamic range of your sound card). Taking a worst case example of
> two closely spaced signals a hundred Hz apart, and ignoring the 0.1 kHz
> position of the Bandwidth control. Receiver AGC will be controlled by
> the stronger, unwanted signal, the wanted signal suppressed, say 30 dB
> by AGC action. By setting your external detectors bandwidth to, say 100
> Hz, and off-tuning the external detector frequency (can you say
> 'Passband Tuning :-) '), the external detector's AGC will easily recover
> the suppressed signal. Or we can use an alternate technique not seen
> often any more. Set up a notch filter in software to take out the
> offending signal. Where the Receiver AGC is set by the unwanted signal,
> the Softrock AGC is set by the wanted signal, the unwanted signal not
> affecting the Softrock AGC.
>
> This is much like the excellent Hammarlund HC-10. In Hammarlund's case,
> they literally took a Hammarlund receiver, complete with bandpass
> filters, Q-Multiplier, Detector, AVC, and Audio Amplifier and boxed it
> up. 455 kHz (430-520 kHz, set by a trimmer inductor) is taken in.
> Converted down to their 60 kHz IF whose gain is controlled by its own
> AVC, and detected it. I believe the AVC is available, but seldom used in
> practice.
>
> I remember seeing someone who had a Drake 2B laying around unused. With
> a few wiring changes, he implemented an HC-10 clone. and Drake 2B
> receivers are going for about $180 on EBay. The HC-10 is going for +$300
> on EBay. The Softrock is by far the most expensive solution. $20 for the
> Softrock, and $700+ for the computer/Display/Sound card to use it. Of
> course, if you just happen to have a reasonably new computer sitting
> beside the R-390 that you use for logging/etc., the solution drops
> tremendously in price to about $40 in parts (connectors, box, power
> supply, cables, and kit). Plus it adds passband tuning, linear
> detection, a multitude of filter options, limited scanning capability,
> digital recording, CW and Digital Mode decoding, Propagation prediction,
> even an Atomic Clock(if it is connected to the Internet). Heck, with a
> little breadboarding and some programming, you can do away with that
> pesky Zero knob by counting the three oscillators and sending the count
> to your PC via the serial port! An eight-input mux, three 74HC4046's,
> and a PIC will do that. Tap the oscillators with #30 wire-wrap wire
> wrapped around the tube under the shield.
>
> 2002tii wrote:
> > Mark wrote:
> >
> >
> >> A forth [SSB option] is to add a separate SSB detector/BFO at the IF
> >> Out. Several Mil-Surplus detectors do this, as well as the Hammarlund
> >> HC-10. The latter has the advantage of providing notch filters and
> >> bandwidth filters, though it is expensive at about $300.
> >>
> >
> >
> >> The latter option just got a lot cheaper if you have access to a
> >> reasonably fast PC. A lot of Hams are playing with a little device
> >> called a SoftRock.
> >>
> >
> > One issue with outboard detectors is that they often don't feed an AGC
> > signal back to the radio, so the AGC won't be based on the signal you
> > are listening to. This is easy enough to do with the 390A, and I
> > believe some of the Mil units do it. It's not necessarily a horrible
> > problem, but it's not optimal, either, particularly if the outboard
> > detector has its own IF filters.
> >
> > I have always intended to design an outboard IF section with its own IF
> > filtering, selectable upper, center, and lower BFO injection, 2 or 3
> > tunable notch filters, a product detector with synchronous detection
> > capability, and an AGC feed. The ideal implementation would switch the
> > BFO offset with the IF bandwidth, so the upper or lower BFO would
> > always be just outside the selected passband. But I have always had
> > other, more pressing projects....
> >
> > Best regards,
> >
> > Don
> >
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>
>
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-- 
Dennis L. Wade
KG6ZI
Carmichael, CA