Filters

[xusmarine1979]

I'm not sure if this has been answered before, but is there a filter I can use for the cell freqs for my PSR 500? I do not use it as a base radio, just portable with the RS 800 antenna but the cell towers are killing me when I go to work everyday.

Sorry, meant to put this with the Antenna's. But if anyone has any recomendations then by all means.

 

[kb8rvp]

I have tried looking into this and the problem is that the cell tower freq is right next to the 800MHz freq you are trying to pick up so if you filter out one you filter out the other. I contacted GRE about a similar question and this was the question and the responce:

Just a general question about sensitivity/selectivity. I notice compaired to my Pro-2096 the PSR-600 seems to be more susceptible or prone to loose the data channel on 800MHz when I am anywhere near a cell phone tower when monted in a car. I understand you are working on a new replacement model. Would this issue be able to be resolved with filtering in the new model? Also have you ever thought of making a scanner just for 800MHz digital trunking that would cover 700-800MHz? For now I am forced to keep using my 2096 in the car and just use my PSR-500 and 600 at home. I have the same issue with the handheld if I am going to be in an area with cell towers nearby I have to take my Pro-96 instead of my PSR-500. Wondering if you focused on a 800MHz scanner , maybe you could resolve issues with sensitivity and simulcast.
It was nice to see everyone in Dayton.

Hi Mike,

This is a difficult issue to resolve with filtering. Some of this interference may be coming from Nextel sites, which cannot be filtered at all because they use frequencies that are interleaved in the same band with the frequencies that you want to listen to. Some of this will be resolved when 800 MHz Rebanding is completed, but even then, the Nextel frequencies are only being moved to the 866-869 MHz range, which is still in the receive passband of the current models.

800 MHz cellular systems operate from 869-894 MHz, which is very close to the cutoff of the current receive passband, so from a design perspective it would be difficult to filter this range without adversely affecting the performance of the desired band.

After 800 MHz rebanding is finished, we may be able to begin looking at improved filtering to begin the cutoff at 866 MHz and further attenuate the 869-894 cell bands. This may help some.

I think the PSR-600 is just a bit more sensitive than the PRO-2096, which may exacerbate the overload issue some as well.

Best regards,

Jack


GRE Support Team
GRE America, Inc. - Powered By Kayako SupportSuite

 

[xusmarine1979]

Thanks for the reply. You pretty much answered my question. I love the sensitivity of this radio but I hate the front end. I am just trying to avoid using the attenuater and switching antennas everytime I leave the house, especially while driving. While up on the highway the stock antenna works fine to help reduce some. But if I go into the city (street level) or home the weaker signals are on the fringe.

What makes it frustrating is my county's CC is 868.4000. But most of the digital TG's use VC's of 855.0000 or lower. It's the CC and the analog TG's that are using 866.0000 and higher that are giving me the trouble.

One thing I do, is I'll hit "tune" and program 868.0000. I'll drive or walk around and looking at the signal bar I can see how much noise I'm getting. While at home I get none, which is great. But I listen to the county north of me, with is a M36 system, and it's a little weak. But if I walk around a little while listen to the cc all the sudden I'll stark hearing an EDACS cc on that same frequency.

While at work I'll have 868 programmed and there is a cell tower within view and the signal bar is fully pegged, while inside the stock antenna works best. But pretty much anything while outside is useless. So, if anyone has any ideas or thoughts I'm all for it. Thanks again.

 

[fineshot1]

Jon - if I were you I would call Par Electronics and ask about a custom filter.

The cell spectrum you would need to attenuate is from 869 - 894 Mhz.

They could probably make you a filter to attenuate most of that spectrum.

PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts


Good luck & be safe....

 

[xusmarine1979]

Thank you kindly and likewise.

 

[tbiggums]

I'd really like to find a solution to this problem as well. I've noticed pretty much any scanner is useless when it's within several hundred feet of an 800 MHz cell site. Turning on the attenuator helps, but it still doesn't work near as well as a real 800 MHz commercial radio.

I did buy a cheap used 800 MHz mobile duplexer a while back, and tuned the low-pass section to pass 851-861 (all the public safety users in my area have already rebanded) and provide 20-30 dB of attenuation at the 800 MHz cell band, but was somewhat disappointed at the results. I did help some, but not near as well as I had hoped. I suspect the scanner's poor internal shielding allows the 800 MHz cell signals to enter the unit downstream of the duplexer. My tests were with a PRO-197, but I would expect the GRE and Uniden mobile units to be similar.

While they do have metal cases, the front panel is mostly plastic, so I can see how an 800 MHz signal can get in easily. Maybe one of these days I'll get some brass stock, and try and make some custom shields for the front end of my PRO-197. Unfortunately, this would be tough to do with a portable scanner, with the all plastic case, and less space to work with...

 

[gmclam]

My experience has been similar with the PSR-300. If I use the RS800 antenna and I am next to one specific cell tower, I must use the ATT function to continue to receive from a nearby 800MHz TRS. If I use the stock antenna, my external mag mount or my PRO-97, the problem does not occur.

I understand the benefit of using the RS800, especially if that's the only band you are monitoring. But what happens if you use the stock antenna? For me I find it is a combination of ALL of these things before the cell tower overload kills reception of the 800MHz control channel:
- PSR-300
- RS800 antenna
- within 1/4 mile of (new) cell tower
- ATT off

 

[ButchGone]

The cell tower interference issue affects Unidens and GREs. The culprit is Nextel and their iDEN system that's in the 800Mhz band. In Tennessee they were supposed to have finished their end of rebanding by moving their iDEN sites above 861 Mhz, but they have not. They are moving slower than molasses on a winter day. Last word I heard was they were supposed to get out of the 851-860 segment in October. We'll see. They were supposed to have done this years ago. If you look at the FCC licenses for Nextel you'll find they are blasting signals in the thousands of watts range so scanners don't have a chance around them.
When I drive near a cell tower that does not have a Nextel site, I have no issues at all with either the Uniden or GRE radios. Most of the cellular systems are higher up in the 800 band or 2 gig range.
I think the Uniden and GREs work about the same, as my driving around tests show the Uniden sometimes works better than GRE near some sites while the GRE works better than Uniden around other areas.
BG..

 

[SCPD]

The only problem with cutting off at 866 MHz is that there are systems here in the 866-869 MHz range and would be adversely affected.

I am in Canada ... but I don't think that we are that different than in the US. I personally have never had problems with cellular towers and my PSR 500.

So .. we should be cautious about what we wish for.

 

[KK4HG]

The cell tower interference issue affects Unidens and GREs. The culprit is Nextel and their iDEN system that's in the 800Mhz band. In Tennessee they were supposed to have finished their end of rebanding by moving their iDEN sites above 861 Mhz, but they have not. They are moving slower than molasses on a winter day. Last word I heard was they were supposed to get out of the 851-860 segment in October. We'll see. They were supposed to have done this years ago. If you look at the FCC licenses for Nextel you'll find they are blasting signals in the thousands of watts range so scanners don't have a chance around them.
When I drive near a cell tower that does not have a Nextel site, I have no issues at all with either the Uniden or GRE radios. Most of the cellular systems are higher up in the 800 band or 2 gig range.
I think the Uniden and GREs work about the same, as my driving around tests show the Uniden sometimes works better than GRE near some sites while the GRE works better than Uniden around other areas.
BG..

When did Nextel start using "thousands of watts"?

 

[N1SQB]

GRE has always known that their receivers have hot or extremely sensitive front ends. My suggestion would be something in the way of a per channel/frequency multi-level/step attenuator. In other words, an attenuator with adjustable levels of attenuation that can be assigned by the end user, without affecting the entire radio. I know some scanners let you assign the attenuator on a per channel basis. However, the key here is multi-step adjustability. IE...0ff-5db-10db-15db,20db,ect... I'm thinking 40db would be the cutoff point or somewhere close to that. Then with some good filtering in place, you would have a pretty good scanning receiver. I'm no tech genious but it stands to reason. To go from "off" or no attenuation to 10 or 20 db right from the start with nothing in between is a problem. I don't understand why some manufacturers have not come up with this.

Manny

 

[gmclam]

GRE has always known that their receivers have hot or extremely sensitive front ends. My suggestion would be something in the way of a per channel/frequency multi-level/step attenuator.

You can set the ATTenuator on a per channel basis on these GRE scanners.

In other words, an attenuator with adjustable levels of attenuation that can be assigned by the end user, without affecting the entire radio.

The typical attenuator in these scanners is fixed at 20dB. If you attenuate less, you won't reduce the signal enough to kill the interference. If you kill the signal more, you'll also lose what you are trying to receive.

I'm no tech genious but it stands to reason. To go from "off" or no attenuation to 10 or 20 db right from the start with nothing in between is a problem. I don't understand why some manufacturers have not come up with this.

The issue is the "slope" of the filter. Passive analog circuits can't be designed with a "brick wall" that just stops passing at some frequency. Filters start attenuating at some frequency and increase their effectiveness as the frequency goes up or down. Attenuators reduce the signal all frequencies.

 

[Mike_G_D]

The typical attenuator in these scanners is fixed at 20dB. If you attenuate less, you won't reduce the signal enough to kill the interference. If you kill the signal more, you'll also lose what you are trying to receive.

I disagree with this point extremely strongly. I have a background in RF design and testing and can assure you that attenuation levels below the brute force 20dB only choice would be VERY effective and extremely useful!! Every 3dB reduction in received signal power is a halving of said power. Having a choice of attenuation steps per channel would be an excellent improvement - something like 3dB, 6dB, 10dB, 15dB, and 20dB would be nice. The more choices the better. One fixed choice of 20dB is almost like being given a choice between having someone yelling in your ear at ear splitting volume or shoving a baseball in their mouth with no choice in between! From previous discussions about this issue I know of at least one other on here, the Mod WayneH, who agrees with this; he keeps a stash of resistive attenuators handy of various levels to attach between his antenna and the scanner and can "fine tune" to some degree, the attenuation he needs for a given circumstance. Of course, that solution does not allow for adjustment per channel which is where adding such a choice to the scanner design with programmability per channel would be a major benefit.

Something else everyone is neglecting to consider here too is receiver RF amplifier dynamic range. Improving this significantly would make a major difference for the better even without any additional filtering. A big problem for consumer radio scanners is poor RF amplifier dynamic range. This is why you get "desense" and "overload" issues when there are strong in-band interferers present within the passband of the receiver's front end filter. Filtering of said signals is difficult or impossible unless you have a tunable notch filter with extremely steep slopes or some such. About the only practical approach to these issues for a wideband receiver is to improve the dynamic range significantly and have multiple switchable front end bandpass filters and/or switchable band selective RF amps. This does add cost, of course, but I would gladly pay over $100 more for said improvements.

I think it would be practical to improve the level of consumer scanner receivers' RF dynamic range moderately and add multiple choices of attenuation steps and do so with less than a $100 increase in selling cost - even without any additional filtering that would make a major improvement in our experience in terms of dealing with strong in-band interferers such as cell sites, IDEN sites, and paging transmitters.

-Mike

 

[N1SQB]

I disagree with this point extremely strongly. I have a background in RF design and testing and can assure you that attenuation levels below the brute force 20dB only choice would be VERY effective and extremely useful!! Every 3dB reduction in received signal power is a halving of said power. Having a choice of attenuation steps per channel would be an excellent improvement - something like 3dB, 6dB, 10dB, 15dB, and 20dB would be nice. The more choices the better. One fixed choice of 20dB is almost like being given a choice between having someone yelling in your ear at ear splitting volume or shoving a baseball in their mouth with no choice in between! From previous discussions about this issue I know of at least one other on here, the Mod WayneH, who agrees with this; he keeps a stash of resistive attenuators handy of various levels to attach between his antenna and the scanner and can "fine tune" to some degree, the attenuation he needs for a given circumstance. Of course, that solution does not allow for adjustment per channel which is where adding such a choice to the scanner design with programmability per channel would be a major benefit.

Something else everyone is neglecting to consider here too is receiver RF amplifier dynamic range. Improving this significantly would make a major difference for the better even without any additional filtering. A big problem for consumer radio scanners is poor RF amplifier dynamic range. This is why you get "desense" and "overload" issues when their are strong in-band interferers present within the passband of the receiver's front end filter. Filtering of said signals is difficult or impossible unless you have a tunable notch filter with extremely steep slopes or some such. About the only practical approach to these issues for a wideband receiver is to improve the dynamic range significantly and have multiple switchable front end bandpass filters and/or switchable band selective RF amps. This does add cost, of course, but I would gladly pay over $100 more for said improvements.

I think it would be practical to improve the level of consumer scanner receivers' RF dynamic range moderately and add multiple choices of attenuation steps and do so with less than a $100 increase in selling cost - even without any additional filtering that would make a major improvement in our experience in terms of dealing with strong in-band interferers such as cell sites, IDEN sites, and paging transmitters.

-Mike

I could not have said it better myself Mike! Thanks for explaining it in a technical way that I could not!
The RF dynamic range issue, as you have explained it, makes a lot of sense too.
There is certainly room for improvement in both areas.

Manny

 

[xusmarine1979]

I disagree with this point extremely strongly. I have a background in RF design and testing and can assure you that attenuation levels below the brute force 20dB only choice would be VERY effective and extremely useful!! Every 3dB reduction in received signal power is a halving of said power. Having a choice of attenuation steps per channel would be an excellent improvement - something like 3dB, 6dB, 10dB, 15dB, and 20dB would be nice. The more choices the better. One fixed choice of 20dB is almost like being given a choice between having someone yelling in your ear at ear splitting volume or shoving a baseball in their mouth with no choice in between! From previous discussions about this issue I know of at least one other on here, the Mod WayneH, who agrees with this; he keeps a stash of resistive attenuators handy of various levels to attach between his antenna and the scanner and can "fine tune" to some degree, the attenuation he needs for a given circumstance. Of course, that solution does not allow for adjustment per channel which is where adding such a choice to the scanner design with programmability per channel would be a major benefit.

Something else everyone is neglecting to consider here too is receiver RF amplifier dynamic range. Improving this significantly would make a major difference for the better even without any additional filtering. A big problem for consumer radio scanners is poor RF amplifier dynamic range. This is why you get "desense" and "overload" issues when there are strong in-band interferers present within the passband of the receiver's front end filter. Filtering of said signals is difficult or impossible unless you have a tunable notch filter with extremely steep slopes or some such. About the only practical approach to these issues for a wideband receiver is to improve the dynamic range significantly and have multiple switchable front end bandpass filters and/or switchable band selective RF amps. This does add cost, of course, but I would gladly pay over $100 more for said improvements.

I think it would be practical to improve the level of consumer scanner receivers' RF dynamic range moderately and add multiple choices of attenuation steps and do so with less than a $100 increase in selling cost - even without any additional filtering that would make a major improvement in our experience in terms of dealing with strong in-band interferers such as cell sites, IDEN sites, and paging transmitters.

-Mike

I agree. I don't need the full 20db, something adjustable would be perfect. If I use the full 20 here at work the signal becomes quite weak.

 

[gmclam]

0-20dB Variable attenuator

I agree. I don't need the full 20db, something adjustable would be perfect. If I use the full 20 here at work the signal becomes quite weak.

Then you should just install one of these.

 

[xusmarine1979]

Then you should just install one of these.

I've thought of that one, but for portable reasons I'm trying to keep the radio small enough to avoid damage to the antenna/BNC.