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July 15th 03, 08:53 PM

In article ,
(Michael Black) writes:

"Tom Holden" ) writes:
"Michael Black" wrote in message
...

snip

When I did some searches on the internet last year, I sure didn't find
anything more about it. A handful of Usenet posts, none of which added
to the scheme or even an understanding of it.

Hence my posting. HR accepted 4 or 5 articles on the RD or receivers using
the RD by Olberg and there appears to have been considerable response. I
daresay that, if this Usenet group had been around then, there would have
been some hot and heavy debates about the Reciprocating Detector!

I didn't see much response. The whole thing seemed to be propelled
by Stirling Olberg, and I never saw anyone else use the circuit in
a ham magazine article. Indeed, other than any followup letters,
there were no references to his articles. And nobody really seems
to know what's going on in the circuit.

I have bound volumes of HR going only to 1973 and did look up the '74
article to refresh my memory.

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.

Motorola used the same principle in the MC1330P video detector chip
introduced in the early 1970s (1972?). The 1330 had what amounted to
a limiter ahead of the mixing stage. The so-called "capture effect" of
the limiter will output the stronger signal which, in this case, is the AM
carrier. For the "filtering" (as Olberg called it), the 1330 used a simple
resonant circuit tuned to the carrier frequency.

Analog TV, both in North America and elsewhere, sends video sidebands
as mostly one of them with just part of the other sideband, and with the
carrier reduced to roughly half of what it would normally be with AM.
[so-called "vestigal sideband" with just a vestige of the lower one in US
NTSC standards] The resultant reduced carrier level is still good enough
to grab, limit-filter, and reapply in a mixer to recover just the video of
the
TV sideband. In the MC1330 application with TV carriers in the 50 MHz
range, balanced internal mixing and assorted internal capacitances
allow about 6 to 8 MHz video bandwidth detected with hardly any of the
carrier leaking through. Slight lowpass filtering can get rid of the
carrier.

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
detectors in an airborne receiver for an R&D project at RCA. Those RF
pulses were all 1 uSec wide at different RF carriers between 55 and 64
MHz and there was very little pulse distortion at the output of any one
detector without its input SAW filter. In the RF environment the RF
pulses would have up to 30 db variation in peak envelope power. The
MC1330 detector circuit wound up as a rather linear detector of RF
input amplitude versus video output amplitude over that 30 db range.
I was quite pleased with the results considering an impossibly small
space allowed for 8 detectors (non-optimized hand layout PCB was
about 1" x 1 3/4" for each detector). :-)

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 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. :-)

All in all, interesting stuff to see and consider!

Len Anderson
retired (from regular hours) electronic engineer person