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Avery Fineman ) writes:
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. While the MC1330 internal schematic is not well layed out for clear understanding, it is different from the reciprocal detector. Unique to the RD design is the fact that the filter is at the output of the differential amplifier, what it sees at it's input is not just the incoming signal, but the incoming signal modified by the output of the filter. That seems to be a key to the design. The MC1330 splits the incoming signal, with one path going to a mixer, and the other path going throught a limiter and filter which then feeds the other input of the mixer. It's behaviour is obvious, ie limit the incoming signal so it's you get a constant amplitude carrier and mix it with the incoming signal to beat it down to baseband, and shows up in plenty of designs, both before and after the Badessa patent. There was an article in Ham Radio for September 1970 about the MC1496 double balanced modulator, and Roy Hejhall specifically mentions it's use as an AM detector in a similar scheme. He said that a limiter wasn't even needed since the 1496 will limit with enough signal. The same thing is stated in the MC1496 datasheet though you have to dig a bit to find it since it's under the "product detector" heading. The filter in this scheme is optional, because not only is there the 1496 example but I've seen similar schemes with no filter. The same scheme shows up in that fairly recent QST article about a synchronous detector (it's been in the Handbook too), though there it's labelled as "quasi-synchronous" and it's merely a side circuit to the main part using a PLL. But if you look in old literature it gets the "synchronous detector" label. What is a puzzle is why Olberg did not reference such articles, because they were halfway there to explaining the reciprocal detector, and what is vague in his articles is what makes the RD different. One thing is certain. That "amplify the carrier, limit it and use it as a locally generated carrier" scheme will not work with SSB, unless the original carrier is not well suppressed. And it's not going to work with CW either, since beating a carrier, as you say, against itself will result in DC and double the carrier. There'll be no beatnote. Since the RD is claimed to work on these modes, something else has to be going on. Michael VE2BVW |
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