GRE Scanner Spurious Calculator

[Mike_G_D]

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

 

[Boatanchor]

Intermodulation interference is the real problem..

While possibly an issue in a very small number of cases (as you would have to have a relatively strong signal on one of these spurious response frequencies in the first places, which is unlikely as many of the spurious response frequencies are well outside of the front end bandpass filter response curves and in many cases, even outside of the LMR/Broadcast bands), a much bigger problem is that of in-band interference caused by inter-modulation products emanating from the scanner's pre-amps and mixers.

All of the digital scanners produced in the last ten to fifteen years, including the latest models from Uniden, share one common characteristic trait. They are designed from the ground up, in such a way that the RF/IF and even much of the AF sections are common between the handheld and the Base/Mobile versions. This is great from a manufacturing and servicing perspective, as it keeps development and manufacturing costs down and simplifies the servicing process.

Because of the common design, power consumption in the RF & IF stages has to be minimized to permit acceptable battery life in the handheld version. The problem is that very low power consumption in RF amplifiers and mixers typically has a negative impact on linearity and Intercept Point performance figures.

The more congested the bands become and the stronger signals become, the more intermodulation interference you are going to suffer. Plugging a scanner into a discone antenna in a metropolitan area, with potentially hundreds of very strong signals and FM broadcast channels being applied to the scanner's front end and mixers, driving them well past their linear operating range, is asking for serious trouble.
Adding Mast head/Low Noise pre-amplifiers is going to cause even more headache's, unless you live out in the boondocks and you don't have any transmitters operating within 10 miles or more from you.

As more and more systems go 'digital', it becomes even more difficult for the scanner operator to identify when this intermodulation interference is even occurring. Many people simply attribute poor digital decoding to weak signals, simulcast interference or some other unrelated problem, but in many cases they would be wrong!

Turning the attenuator on can help, but Why?

Adding 20dB of attenuation to the front end of the scanner reduces all incoming signal levels by the same degree, including those that cause the Intermodulation Interference. Reducing unwanted signals by 20dB can be enough to eliminate the intermodulation interference completely, providing all signals are reduced to the point where the scanner's amplifiers and mixers can remain operating within their 'linear' range.
That's all fine as long as the signal you want to listen to is not weak in the first place. If it is, then turning the attenuator on may make that signal disappear completely, placing you in a 'Damned if you do, Damned if you don't' situation..

Uniden and GRE/Whistler could go a long way towards fixing this problem in base/mobile scanners if they really wanted to. It would take a re-design of the active stages in the scanners front end and IF circuits to take advantage of new low noise, high IP3 components such as the PGA-103+ amplifier from Minicircuits. The power consumption would rise slightly, but in a Mobile/Base radio - Who cares if the power consumption rises from 250mA to 500mA!

 

[Mike_G_D]

The four spurious products in the file are due to internal mixing products and undesired interactions of the mixer, amplifier, and oscillator stages. They are quite prolific, actually, in GRE models! I know this to be a fact from both personal experience as well as those reported by others. I have done a fair amount of work in this.

Not all products will generate spurious responses in passbands as selected by the receiver when tuned which will cause immediate in-band issues. But some will. There are MANY variables.

This file is by no means exhaustive. To do so, you would have to know more details of the design and do a very exhaustive analysis of non-linear mixing products. Usually, in a lab, of course, we do quite a lot of in-circuit "sniffing" and present many different potential interferers to the inputs and measure the responses. But we have to start with the main usual culprits which include 1/2IF and images. Then we look at intermodulation first by analyzing the 3rd order intercept and P1dB of the dominate elements in the chain; typically we look at those qualities in the amplifiers as well as in the mixers which can get nasty to work with!

But the spurious elements I have noted in the worksheet are indeed quite prevalent and extremely valid, I assure you!

Do you know why GRE changed their 700MHz coverage with later firmware to cut out the 782MHz to 791MHz portion of the band? Plug in 785MHz into the worksheet and check out the third harmonic of the first LO spurious and see what that frequency is! Yes, right in the middle of the mobile portion of the cellular phone US AMPS band - not really important due to both the nature of the signals used now as well as the fact that that is the mobile portion BUT they had to comply with the FCC requirement to not receive those frequencies and this was a severe weak point in their design. I can absolutely guarantee you that I can readily spot many spurious of this kind from 800MHz trunked control channels and voice frequencies in the upper end of the 700MHz band - with just my rubber ducky whip on the handheld. The calculator in the sheet nails these exactly. The same goes for the 900MHz pager signals appearing in the federal portion of the VHF-HI band - I can nail those too and repeatedly call them up and experience them - it causes problems for monitoring some federal USFS frequencies where I live. The problem is, contrary to your statement that "...many of the spurious response frequencies are well outside of the front end bandpass filter response curves and in many cases, even outside of the LMR/Broadcast bands..." these, in fact, DO cause in-band problems and much confusion to unknowing users! The scanner is designed to receive many of the frequencies that these spurious issues fall on and these will often cause problems on completely disparate bands - what you are faced with when using a very wideband receiver designed with too many cost cutting measures in the RF and IF sections.

You also state "...a much bigger problem is that of in-band interference caused by inter-modulation products emanating from the scanner's pre-amps and mixers." Well, guess what that 460MHz stuff appearing in the 417MHz range IS?! That's right - intermodulation products stemming from unintentional radiations and mixing products from the radio's internal mixers and oscillators (and maybe pre-amps, too)! In fact, that 800MHz stuff appearing in the 700MHz band is pretty much "in-band".

The only one I have yet to experience personally is the 1/2IF problem as the design limits that way outside of most strong signal areas in most tuning cases. But you'd be surprised by how common the other three insidious demons are in these scanners!!

I get that there are many other interferer potentials, of course, but again, trying to document all of them really requires much more intimate knowledge of the design of the receiver, the layout of the pcb, the parts used, the shields and case design, etc. as well as real lab equipment to verify and test it all (and find the ones the initial analysis didn't get!).

By and large, you are "preaching to the choir" with me, as I think you realize. I agree on the RF designs of these scanners being somewhat underwhelming to say the least. But this is what most of us must deal with; I felt tools to help non-technical folks understand why they receive "weird stuff that isn't really there" would be useful.

I am intending this to be a quick and useful tool for testing four likely possibilities when picking up "something weird" that seems like it shouldn't be where it is.

Anyway, I have also created a version of the file that uses the Open Document Spreadsheet format (.ods) which can be used with the free LibreOffice Calc program for those who cannot afford or don't have Microsoft Excel 2007 or later. I have attached it with this post. For those who don't know, LibreOffice can be found here: Home | LibreOffice - Free Office Suite - Fun Project - Fantastic People.

-Mike

 

[Mike_G_D]

If any are interested, here is the document I used to derive the formulas used in the worksheet calculators. I can't remember exactly where I got it but I am pretty sure it was posted by someone on this site some while back. It makes for very interesting reading and gives quite the insight into the inner workings of the PSR and PRO series GRE-made scanners' PLL operation and conversion stages. I could not have developed the worksheet without this info.