I have created an Excel file which will calculate four common spurious interferer products when a desired receive frequency is entered.
Just open the file in Microsoft Excel 2007 or later (it won't work on earlier versions). Enter your tuned frequency in the space indicated (it's locked so that's the only cell you can alter anyway).
One example would be tuning your scanner to 417.575MHz and hearing a signal that is really on 460.375MHz which would come from the spurious result of a difference mixing product of the first and second local oscillator signals.
Another example would be tuning to 170.2375MHz and hearing paging signals that are really on 931.8125MHz. This one comes from the spurious result of the image from the first conversion stage.
Still another example is getting a strong Motorola control channel on 795.5MHz that is really on 863.2MHz. This is the spurious result from the third harmonic of the first local oscillator signal.
Anyway, you get the idea. It's not, by any means, exhaustive, there are many other possibilities but the four I have started with are fairly common.
I created this for a user on another site who was frequently hearing odd signals where he shouldn't be and was confused by what he was hearing. I've cleaned it up considerably and simplified it for more general use.
I fortunately had access to a detailed document concerning how the GRE scanners of recent vintage setup their internal frequency conversion stages. Because I only have that info for GRE radios I cannot do the same for a Uniden or other brand (they are not all created equal!).
So please remember that this is only valid for GRE (now Whistler) made scanners and their Radio Shack equivalents. It will not work for Uniden models and it may not work for ALL GRE radios of earlier and future designs. It should work for GRE made Radio Shack scanners like the Pro-93, Pro-95, Pro-96, and Pro97 as well as the all of the newer digital P25 capable models and GRE PSR series scanners though I am not sure about the Pro-18 or the GRE PSR800 (though I expect the RF designs to be basically the same, they may not be).
So it's just something to play with if you find you are hearing something where you don't think it should be and at least want to check on the four common possibilities.
You can ignore the rest of the stuff on that page concerning the internal conversion setup data unless that interests you; it is used to perform the calculations for the spurious stuff, though.
I may update it in the future if I find more products that I can accurately define and note down. And if I ever find the necessary data for Uniden scanners I would make a similar document for them.
Hope it proves useful for at least some of you!
NOTE TO MODS: Because this pertains equally to GRE made Radio Shack scanners as well as GRE branded and new Whistler branded models I thought it best to put it in the General Scanning Discussion sub section rather than the Radio Shack, GRE, or Whistler sections of the RR forums. Also, even though it does not pertain exactly to Uniden models it may prove enlightening to all users - the Uniden's use a similar architecture in terms of the conversion frequency plan as far as I can tell, I just don't have the details like I do for the GRE system. So I think it should remain in the General Scanning Discussion section if at all possible.
-Mike
Just open the file in Microsoft Excel 2007 or later (it won't work on earlier versions). Enter your tuned frequency in the space indicated (it's locked so that's the only cell you can alter anyway).
One example would be tuning your scanner to 417.575MHz and hearing a signal that is really on 460.375MHz which would come from the spurious result of a difference mixing product of the first and second local oscillator signals.
Another example would be tuning to 170.2375MHz and hearing paging signals that are really on 931.8125MHz. This one comes from the spurious result of the image from the first conversion stage.
Still another example is getting a strong Motorola control channel on 795.5MHz that is really on 863.2MHz. This is the spurious result from the third harmonic of the first local oscillator signal.
Anyway, you get the idea. It's not, by any means, exhaustive, there are many other possibilities but the four I have started with are fairly common.
I created this for a user on another site who was frequently hearing odd signals where he shouldn't be and was confused by what he was hearing. I've cleaned it up considerably and simplified it for more general use.
I fortunately had access to a detailed document concerning how the GRE scanners of recent vintage setup their internal frequency conversion stages. Because I only have that info for GRE radios I cannot do the same for a Uniden or other brand (they are not all created equal!).
So please remember that this is only valid for GRE (now Whistler) made scanners and their Radio Shack equivalents. It will not work for Uniden models and it may not work for ALL GRE radios of earlier and future designs. It should work for GRE made Radio Shack scanners like the Pro-93, Pro-95, Pro-96, and Pro97 as well as the all of the newer digital P25 capable models and GRE PSR series scanners though I am not sure about the Pro-18 or the GRE PSR800 (though I expect the RF designs to be basically the same, they may not be).
So it's just something to play with if you find you are hearing something where you don't think it should be and at least want to check on the four common possibilities.
You can ignore the rest of the stuff on that page concerning the internal conversion setup data unless that interests you; it is used to perform the calculations for the spurious stuff, though.
I may update it in the future if I find more products that I can accurately define and note down. And if I ever find the necessary data for Uniden scanners I would make a similar document for them.
Hope it proves useful for at least some of you!
NOTE TO MODS: Because this pertains equally to GRE made Radio Shack scanners as well as GRE branded and new Whistler branded models I thought it best to put it in the General Scanning Discussion sub section rather than the Radio Shack, GRE, or Whistler sections of the RR forums. Also, even though it does not pertain exactly to Uniden models it may prove enlightening to all users - the Uniden's use a similar architecture in terms of the conversion frequency plan as far as I can tell, I just don't have the details like I do for the GRE system. So I think it should remain in the General Scanning Discussion section if at all possible.
-Mike
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