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July 16th 03, 06:23 PM

(Avery Fineman) wrote in message ...
[snip]
Stirling's description isn't quite good enough in the '74 article covering
the included "reciprocating detector." The principle is that of simply
filtering out the carrier, amplifying it, and mixing it back with the
incoming carrier-plus-sidebands. At the output the carrier, mixed with
itself, becomes a DC level. The sidebands mix with the amplified-and-
limited/filtered carrier to result in the original audio.

I assume he is now a Silent Key - his call sign W1SNN is now
unassigned. Reading the preceding patent by Badessa, it seems more
complicated than what you describe but perhaps it boils down to that -
the complexity is in understanding the technique with which the
carrier is "synthesized" (his word). What you describe is a
"quasi-synchronous" detector; Badessa/Olberg talk about synthesizing a
carrier for suppressed carrier AM based on the carrier phase reversal
at zero crossings in the modulation envelope - the fundamental carrier
extraction also works on AM with carrier without the phase reversal
and it's my impression that the product detector itself oscillates at
the feedback loop filter frequency in the absence of a lockable
carrier.

The way my implementation behaves, I would say it has a synchronous
BFO. It sounds similar to a conventional BFO/product detector except
within the lock-in range, where it locks on to zero beat. I'm not sure
I have it functioning as Olberg intended or that Olberg's design is
consistent with Badessa's patent. The latter seems to claim phase
lock, except when there is a beat frequency. Both are silent about the
phenomenon I observe of an audio null on DSB AM carrier signals at a
certain point within the lock-in range. A couple have suggested that a
90 degree difference between carrier and reference would cause this -
(however,)it's not evident on SSB AM with carrier (e.g. CHU at
3330kHz). If so, then it would seem that the phase difference changes
across the lock-in range and passes through 90 degrees at one point
only. Wouldn't this be typical of all synchronous product detectors?

A hint that audio nulling may indeed have been a negative factor in
the RD is in Olberg's HR'78 article on the 10.7MHz RD where there is a
phasing network that does not appear in the earlier low frequency
models.

[snip]

Olberg claimed that (in the '74 article) the "reciprocating detector" did
not handle overloads well. While I've not examined that in detail, nor did
I read HR in that year, I used the MC1330P in 8 simultaneous RF pulse
[snip]

I have been quite pleased with the dynamic range of my RD in
combination with the output levels from the 2nd last IF amp of my
DX-394. I do hear a strong click on the initial pulse of strong
carrier for CW Morse characters that I have not yet concluded is
related to the AGC action of older receivers to which attributed some
overload problems. My modified AGC should have an attack of between 2
and 10 ms and I have turned AGC off and still get the clicks, IIRC.

Offhand, after a long and thoughtful 20 minutes after reading Olberg's
'74 HR article, a narrowband (IF) filter could pass the carrier through to
any FM-style limiter IC, its output to a balanced mixer (differential,
double-balanced bipolar or FET) which could then output just the side-
band content of an AM unsuppressed-carrier signal. If the limiter gain
was high (that is, limiting threshold low), front end noise would get
through and appear at the demodulated output...but that noise band-
width would be only as high as the narrowband IF filter bandwidth.
Once a carrier was there, it would take over in the limiter and
demodulation would become essentially linear. Impulse noise
received could be reduced depending on the relative peak IF power
of noise pulse versus carrier...the limiter stage would take over and
pass the higher input level while suppressing the lower input level.

I have not compared my RD output to the envelope detector output under
controlled listening conditions. RD goes to my old stereo and Dynaco
A25 speakers; ED goes through the narrower bandwidth audio amplifier
of the DX-394 to a 4" speaker. When locked, RD sounds pretty clean!

I doubt that this "reciprocating detector" would be good for any FM
except quite narrowband FM...due to the bandwidth of the filter.
That was another claim by Stirling Olberg in the 1974 article. FM
carrier level would vary with modulation (although entire carrier plus
sidebands would remain constant) and that would mess up the carrier
reinsertion part of the "RD." Might be okay for PM. Have to do the
numbers to see if that is feasible and my 20 minutes was up. :-)

For FM demodulation, he claims that converting the narrow-band crystal
filter that follows the feedback filter to an LC circuit makes a
frequency discriminator. In his Companion Receiver project, he
switches between the two filters for the two modes. My RD works with
the AM modes with/without the crystal filter (actually mine's ceramic)
and with an LC 455kHz resonator in parallel with the ceramic filter.
With straight wire, the lock-in range is on the order of 400Hz; with
the others, it can be reduced to less than the nominal 50Hz tuning
steps of the DX-394. I have yet to find a NBFM signal.

All in all, interesting stuff to see and consider!
So far, it has been all of that and the parts cost is low.

Len Anderson
retired (from regular hours) electronic engineer person

Good to hear from you,

Tom