Does IFX really do anything?

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Stevenme

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I found a couple of birdies on my 396XT and thought the IF Exchange could be used to shift the birdie off frequency. However, when I turn IFX on from the keyboard, the birdie is unchanged. Does IFX really do anything or am I missing a step? Thanks!
 

DaveIN

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All I know about it is how to turn it on or off.

The Twiki only has this to say about the icon on the display: IFX icon; You switched to the intermediate frequency (IF exchange) and Func + tap 4 to turn on the IFX in the 346XTHoldMode. It also appears in the various mode displays, but that's about it.
 

FlashP

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Yes, it does something. If you're hearing a signal that really isn't on the displayed frequency (such as an intermodulation product), it likely will remove it. If you're hearing a stray signal that's actually on the frequency, or bleeding through from an adjacent one, it won't do anything.

Unforturnately, this function is only available while holding on a channel - so it won't eliminate nuisance hits while scanning. To be truly useful, it would need to be part of the programming options for each channel.

Flash
 

UPMan

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It affects all instances of the selected frequency in the scanner, whether in scan or search.
 

Stevenme

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Well for the 3 birdies I've found on my 396XT, it has no noticable affect. The strongest one is 31.05Mhz which is full signal without an antenna connected. Trust me, I know the difference between an external signal and a birdie, and this is a birdie being generated inside the radio. Turning IFX on does not reduce the signal what so ever.
 

DaveIN

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If you only found three birdies (and only in the VHF low band) I'd say the engineers did well.

As for the birdies you found, are they actual frequencies in use in your area, or is the an experiment only?
 

W6KRU

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Well for the 3 birdies I've found on my 396XT, it has no noticable affect. The strongest one is 31.05Mhz which is full signal without an antenna connected. Trust me, I know the difference between an external signal and a birdie, and this is a birdie being generated inside the radio. Turning IFX on does not reduce the signal what so ever.
I have no signal at 31.05 on my XT. ???
 

Stevenme

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Three birdies out of 1275Mhz is excellent! My question is why doesn't IFX work to shift the IF and lessen the internally generated signal? I thought that's what it was for, no?

If others are not hearing a birdie at 31.05, then where do you hear them? They are easy enough to find, just remove the antenna and set the frequency to 25Mhz and start searching up. Both forget to turn off the broadcast screens.
 

W6KRU

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Three birdies out of 1275Mhz is excellent! My question is why doesn't IFX work to shift the IF and lessen the internally generated signal? I thought that's what it was for, no?

If others are not hearing a birdie at 31.05, then where do you hear them? They are easy enough to find, just remove the antenna and set the frequency to 25Mhz and start searching up. Both forget to turn off the broadcast screens.
I didn't search. I just put the frequencies in manually. I suspect that IFX doesn't do anything for you because these aren't birdies.
 

Stevenme

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Well what is your definition of a birdie? Mine is an internally generated signal inside the receiver. My criteria is that the signal is the same strength with the antenna on or off. I also move the radio to different locations to verify the signal strength does not change.

Please tell me your criteria, I'd love to learn somethiing new.
 

W6KRU

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Well what is your definition of a birdie? Mine is an internally generated signal inside the receiver. My criteria is that the signal is the same strength with the antenna on or off. I also move the radio to different locations to verify the signal strength does not change.

Please tell me your criteria, I'd love to learn somethiing new.
In my opinion a birdie would exist on the same frequency in every scanner of the same model. Maybe you can explain to me how two identical radios would be different in this aspect.

You have some sort of local interference IMO. It must be quite strong since it occurs with the antenna disconnected.
 

Stevenme

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I'm answering the same questions on two posts.

If it is an external signal as you propose, then why is it the exact same signal strength (1 bar) with or without the antenna (Uniden stock antenna)? I can go anywhere inside my house or outside without the antenna and the signal does not change even slightly.

That's how I know.
 

W6KRU

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I'm answering the same questions on two posts.

If it is an external signal as you propose, then why is it the exact same signal strength (1 bar) with or without the antenna (Uniden stock antenna)? I can go anywhere inside my house or outside without the antenna and the signal does not change even slightly.

That's how I know.
I apologize Steve. You are correct. I did some more testing and you are correct about the 31.05 and 72.45 frequencies. The reason I didn't find this on the first tests is because the only way I see it is with the antenna disconnected. If I hold on the frequencies, the noise goes away as I screw the antenna on and it returns as I am removing the antenna. I tried several antennas and I can attach the sma-bnc converter but any anything beyond that and the birdie goes away.
 

UPMan

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The birdie might be wide enough that IFX doesn't shift far enough to eliminate it.
 

Stevenme

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The birdie might be wide enough that IFX doesn't shift far enough to eliminate it.
UPMan, since it has now been confirmed that 31.05 is a birdie on at least 2 XTs, would you be able to investigate why IFX does not work? I have only found 3 birdies on my XT and IFX has no detectable affect on any of them. So if the reason IFX was added to the XT was to eliminate birdies, I'd say it fails (at least in my case).

BTW, I love my XT and I think it's the best handheld receiver on the market. As with anything, there's always room for improvement!
 

DaveIN

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If it's a birdie, would shifting the intermediate frequency actually effect the result?
I personally do not have any active agencies on those three frequencies to verify the result:roll:
 

Mike_G_D

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"Birdies" is a broad term for a fairly complex list of possibilities. In the lab, we called them "self quieters". Again a general term for a broad list of possible causes. In general, these are internally generated signals which can affect some frequencies in a radio receiver. The goal is to minimize them sufficiently so as to keep the radio within spec across the desired range. The more complex the radio and the wider the expected frequency range the harder this is to do while keeping cost below a certain limit.

If the birdie is caused by some inadvertent mixing product containing some elements of the radio receiver's internal superheterodyne frequency plan then the IF exchange feature should help (I have wanted someone to put such a feature in scanners for a very long time and am glad to see Uniden finally do so!). On the other hand, some internally generated signals might not be dependent on any internal LO mixing products and so will not be affected by the IF shift. One possibility is something caused by the internal digital electronics. If, say, some harmonic of an internal clock frequency is noticeable on a frequency within the receiver's frequency range then nothing short of extra shielding will likely get rid of it. The IF exchange will only help if the internally generated signal is in some way dependent on the local oscillators' and intermediate frequencies of the radio's internal frequency plan (RF side and not the digital side). It can also help when images and some intermodulation products cause issues - it is a great feature to have in these cases! Again, I applaud Uniden for including this feature in there latest product!

Again, "birdies" or "self quieters" are a difficult enough design problem for receivers limited to a specific band (such as the cellular 800MHz band) but are a much greater problem for wide band receivers such as the 396XT or PSR500.

One other possibility for what is being discussed here might be some mild instablity in the front end amp of the radio. I say this because of the antenna affecting the strength. With antenna on apparently it reduces in strength while with it off it increases. If this is the case then it might be the front end RF amp or some succeeding stage going into oscillation when the antenna port load is shifted too far outside of nominal (I assume that is 50 ohms resistive). You could test this by attaching a good 50 ohm resistive dummy load to that antenna port and seeing if the "birdie" lowers in strength or even goes away completely. Designing wide range RF amplifiers and immediately attached succeeding stages (such as filters and mixers) is a challenge in and of itself. The active parts (amps and some active mixers, etc.) can have stability issues when the input and/or output impedances are not within a certain range relative to expected nominal. The goal for most wide range amps is to make them "unconditionally stable" so as to mitigate such effects. That is not always possible in all situations. If the amp goes into an unstable state then it can oscillate and therefor cause such problems as the one the OP noted. Again, if the interferer's signal level is changing with changing loads on the antenna port then this can be a sign of said issues. And this, once again, would likely not be affected by the IF exchange function. I have found a very similar issue on at least one frequency in my PSR500 also.

-Mike
 
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XTS3000

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XT got rid of the birdie/spir that the T had on the Tac channel in my town. Happy camper here.
 
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