417.8 Repeating or Simulcasting RFD CMD 1

[cvrules90]

Excellent discussion! I like to see real valid technical discussions in a hobbyist radio forum and firmly believe that radio hobbyists really need to know and understand the basics of the internal workings of their equipment as well as basic RF behavior in the wild to get the most out of their hobby and perform educated and informed analysis of new data they collect using their gear. It is sad that the consumer scanner manufacturers cut so many corners but considering that the market is extremely limited and the majority of users so ill informed and desiring costs in the $200 to $300 range and balking at anything over $400 one can understand why we have what we have. In effect, for a standalone consumer scanner market in the USA we are currently limited to one major manufacturer up and running and one future one that should (assuming they do not change their minds) start selling units in the near future albeit using the ragged remnants of an earlier manufacture that imploded. Like zz0468 I am not holding my breath for anything truly revolutionary in terms of REAL RF performance (not the "bling" of "pretty features" based on relatively cheap and easy to design and manufacture digital microprocessor based devices). That said, I do see some minor "hope" in terms of that one currently extant and viable observably producing manufacturer listening to informed consumers' desires; the reports of some more informed and educated users concerning improved IF filtering in the latest models means some small success in that area - at least I'd like to think it does.

zz0468, 800, CQPSK, and C4FM (and others of similar ilk that may be lurking in the wings or that I failed to mention, etc. and so forth) - guys I really respect you all and am so glad you are around and willing to sacrifice some of your valuable time to participate in radio hobby forums such as this one, RR, and ScanDiego! We all benefit greatly from your knowledge and real world experience! Please don't be discouraged - some of us DO understand, value, treasure, and follow closely your collective advice and wisdom! Personally, I wish I could speak to you all in person some day!

I was an RF professional myself that fell on really bad times that I could not recover from and are not suitable to discuss here; suffice it to say, however, that although I was "taken out of the lab, the lab was not taken out of me" !

zz0468 is correct in his critique of CQPSK's original analysis regarding the original topic of this thread - the math seems to make sense but if you delve into the real workings of the superhet architecture you see some flaws; to restate what zz0468 said slightly differently, the IF output is a RESULTANT - it comes from the mixing of the local oscillator, LO, signal, one "source", and the RF input, second "source" (at least for the first IF conversion or isolated in terms of one conversion stage; of course the IF of one stage could be the source or "RF", in a sense, of the succeeding stage in multiple conversion schemes but, for the sake of this discussion, the usual culprit for such problems is the first conversion stage with the RF input being the source derived from the first RF amplifier connected to the antenna). For harmonics to occur, they need a source signal to start with. The source in this case could either be in the RF range or in the LO range. Spurs on either port can affect the resultant IF output. (To be fair to CQPSK, I have made the same mistake many times when just thinking on the fly and in a hurry - it is obvious to me that you do know what you are talking about and have the correct overall grasp of the issue) In most real world cases and certainly with regard to relative cheap and simply designed and constructed consumer level standalone scanner radios you have undesired "gunk" on both input ports of the mixers in all the conversion stages. That makes for lots of potential undesired "false positives".

To critique zz0468's analysis, on the other hand, you're guess that the 380MHz IF is only at play in "the higher bands" is incorrect; I am attaching a valuable document concerning how the IF conversion stages work in current GRE receivers that you can look at. You can see that the first IF will be at 380.8MHz and the first LO will be 798.6MHz to mix with 417.8MHz to yield that 380.8 first IF. The second LO will be 359.4MHz to yield 21.4MHz as the second IF; the third LO is a crystal oscillator signal at 20.945MHz so as to yield 21.4MHz - 20.945MHz = 455KHz, the final IF. Your math makes sense if the first IF was really 21.4MHz but it isn't - still, I can see a strong RF signal leaking into the second stage conversion mixer but the LO setting wouldn't be right (set for the 380.8MHz first IF input and not the tuned 417.8MHz RF input); it's LO is 359.4MHz and I'm not sure how it mixes with 460.575MHz or 460.6MHz to yield 21.4MHz as a resultant (you get 101.175MHz or 101.2MHz respectively). The image of the first IF input would be 42.8MHz + 380.8MHz = 423.6MHz, 37MHz away from the 460.6MHz interferer or 36.975MHz away from the 460.575MHz interferer. However, again, as we all agree, these receivers are full of nonlinearities and internal noise sources that can cause all manor of false positives; with enough design data and analysis I imagine we could deduce the cause.

My feeling is that the signal the OP is receiving is not "really there" in the sense that I believe it is a resultant of undesired internal mixing products within his radio - I know the 97 and it does not handle large signal levels well at all (as most later GRE designs don't). I would try using a lesser antenna and changing locations to check the validity of the signal as well as using radios of different designs with different internal frequency plans and overall designs (such as ICOM's and even Uniden's).

It is possible that it may be "real" in the manner Brandon has outlined, however. But my suspicion is that it is an undesired mixing product as CQPSK and zz0468 suspect.

-Mike

I agree with you. Always a good thing to undetand the brains of radio communication.

[zz0468]

DIINGDANGIT! I think I figured it out...maybe...

It's as good a theory as any, and it's going to end up being something complex like that. One of the main reasons for building a triple conversion receiver in the first place is for image rejection, but no superhet receiver will be 100% image free. In the case of an inexpensive wideband receiver like a scanner, triple conversion is necessary just for performance to be tolerable, particularly due to the cruddy synthesizers.

[cvrules90]

There are even radio stations that I believe have these images. KDGL is a good example. Broadcasts on 106.9 and has a repeater on 103.9.