417.8 Repeating or Simulcasting RFD CMD 1

[brandon]

Is this full time or on occasion?

I have logged similar cross band patching on VHF federal.
163.0875 136.5 2008 Simulcast of OCFA 4C during wildfires.
168.2750 85.4 2007 Simulcasted traffic from 151.190 CDF Monte Vista during wildfires.

The above and your 2003 logs made sense because of the large mutual aid response for the incidents. I'm not aware of any major incidents in Riverside lately, so not sure of the reason 417.800 would be used other than some federal agency just doing some testing.

Or it could just be an image like others suggested.

[cvrules90]

Even some common frequencies go to birdies from what I"ve seen. Once channel 155.34 (HEAR) was a birdie.

[retiredFD07]

I appreciate all of the responses/ ideas/ theories. I figured it was some type of intermod but the strong signal and PL threw me off.

Someone asked about what radio I'm hearing this on. I'm using a Pro-97 with a roof mounted antenna, and I'm in Riverside. Never thought to check and see if I pick this signal up on a rubber duck antenna, or one of my other scanners.

<>< Mike

[zz0468]

Even some common frequencies go to birdies from what I"ve seen. Once channel 155.34 (HEAR) was a birdie.

Since birdies are an artifact of a specific receiver design, a frequency that has a birdie in one receiver won't necessarily have a birdie in any other type of receiver. That's also why many scanner models have a birdie list in the manual, because the manufacturers can predict which frequencies will cause a problem in that particular receiver.

[zz0468]

Someone asked about what radio I'm hearing this on. I'm using a Pro-97 with a roof mounted antenna, and I'm in Riverside. Never thought to check and see if I pick this signal up on a rubber duck antenna, or one of my other scanners.

The PRO-97 has a 2nd IF of 21.4 MHz, not 10.7. I think the first IF of 380.8 is only used for the higher bands. You use twice the intermediate frequency to calculate where the image will be... so assuming low side injection, 460.575-42.8 = 417.775, 5 KHz off from where it's being heard. I'd say the mystery is solved.

5 KHz off frequency might cause enough distortion that the PL frequency can't properly be decoded.

[cvrules90]

Since birdies are an artifact of a specific receiver design, a frequency that has a birdie in one receiver won't necessarily have a birdie in any other type of receiver. That's also why many scanner models have a birdie list in the manual, because the manufacturers can predict which frequencies will cause a problem in that particular receiver.

None of the scanners I've had listed that in the manual but it would be helpful.

[Mike_G_D]

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

[sp1989]

I remember back in the old days, when cell phones were in the clear, using the IF to decipher the image frequency to listen 'out of band' on a scanner.

[zz0468]

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.

LOL... this thread is definitely getting interesting now. So, Wow.

That 380.8 MHz first IF actually wanders a bit, depending on the frequency, and the 2nd LO wanders a bit to put the 2nd IF firmly on 21.4 MHz. It's actually a pretty clever scheme that accomplishes several things... it would be quite helpful for image rejection, it allows multiple channel step sizes without a fractional N synthesizer, and allows a very wide receive frequency range without requiring the synthesizer to cover the entire range itself, which would add to the cost.

It doesn't help salving this one, though. I spent a few minutes trying to work something out, but I'm not seeing any logical combinations that would allow for an image spaced at twice the second IF frequency, but that's assuming everything is clean and linear. Once we decide that it's not, you can throw out the rule book, and the receiver will spit out whatever it feels like. Which gets us back to square one.

Thanks, guys. I got to stretch a few brain cells on this.

[Mike_G_D]

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

Try this -

We have the first LO set at 798.6MHz and the second at 359.4MHz; now let's assume that the second IF filter and isolation just ain't that good and the 798.6MHz first LO gets through to the second mixer and tangles with the second LO at 359.4MHz (or, it might be that the second LO is getting into the first mixer...either way...). Ok, now we might have an inadvertent mixing product at 439.2MHz, right? So what might that little devil do to cause a little mischief? Let's say we have a perfectly innocent but pretty beefy 460.600MHz signal hanging around ending up getting in a tangle with that spur at 439.2MHz - guess what we get: 460.6MHz - 439.2MHz = 21.4MHz which just so happens to be equal to what the second IF filter is looking for! A 460.575MHz signal of similar level would yield a 21.375MHz signal at the second IF filter's front door - close enough for consumer low cost scanner work!

So, in essence, we have the unintentional mixing product of the first and second LO's which, in turn, mixes with an undesired strong RF signal and...VOILA!

Best I could come up with - I spent three plus hours dinking around with this earlier and kept banging my head against a wall...then thunk this up while on the exercise bike but had other stuff to deal with till I could use the computer and rethink it, though it finally took a glass of wine to help get me sorted out.

Might not be it but it fits the known data as far as I can see.

-Mike