SDS100/SDS200: Desense or Overload Issues

[kruser]

Hi all,

I finally broke down and bought an SDS-100 the other day. I've been watching the threads here and it seems most like the radio.

I've been into radio for 50 years or so and have at least 50 scanner type radios not to mention all the commercial two way radios I've acquired over the years. It's a fun hobby.

Problem, my new SDS-100 is almost totally useless for me. I live near several large hospitals. In all their infinite wisdom, they all have 152.xxx MHz paging transmitters on their rooftop's. Why, I don't know as you can see each other from their rooftops. I live in pager hell to say the least.
Paging may be dead for the general public but for hospital use, it is alive and well.
These 152.xxx MHz signals bombard most older scanners that were not triple conversion from intermod. Many newer models with poor front end filtering and poor selectivity are plagued by desense mostly.
Desense is what appears to be killing my new SDS-100.

Our statewide P25 system is mostly a VHF system. They are adding 700 MHz sites to fill in gaps now. Using 700 MHz was probably because they can't come up with any more VHF frequencies. When they planned and built the VHF system, they used standard public safety frequencies as well as unused frequencies located in the government controlled part of the VHF spectrum as well as other parts of the band normally reserved for other users. Forestry was one and Paging was another big chuck of the band they obtained licenses in.
There are two towers on the statewide system well within my range. They both have voice and control channels in the 152 MHz part of the spectrum. Yep, the same 1 MHz chunk that was reserved for VHF paging!

My good receivers like Icom R9000's etc, work flawlessly but they also cost much more and can't trunk track a P25 system.

I did purchase good quality notch filters for the 152 and 158 MHz paging bands still in use today. These filters work very well to eliminate the problem for most radios. The worst radios by far are those made by GRE. Not only are they susceptible to paging signals, FM Broadcast radio stations will wipe them out as well.
For the others, the notch filters (PAR Electronics) work but they are also notching out the statewide frequencies I want to monitor. Because of this, I can't use an indoor antenna as a notch filter would kill the signals I do want. All I can do is use notch filters to try and reduce the powerful paging transmitter signals that are in the 152 MHz band. Notch filters do help as they get the paging signals down to a level most scanners AGC circuits can handle and work with. Notch too deep and you also lower the signal from the transmitter you want to hear. It's a fine balance!

So, I found that the SDS-100 front end is VERY poor when it comes to paging signals overloading it. I can monitor the paging signals and when they are quiet, my SDS-100 receives just fine. When the strongest of the paging systems goes on air, the SDS becomes totally quiet.
This is a shame as the statewide system was my primary interest.
Our local county system is near 100% encrypted so it's not worth monitoring.
I did try the county 800 MHz system for the few things that are still in the clear and quickly found that 152 MHz paging signals still desense the front end in the SDS-100. And that is with the PAR VHF notch filters inline!
So because of VHF paging, my new SDS-100 is almost useless in my area.
It is plaqued by desense issues from VHF paging signals mostly located in 1 MHz of spectrum from 152 to 153 MHz.

I've read several times that the SDS-100 uses a cheap TV Tuner in it's front end. If this is true, that is probably why it is letting signals at 152 MHz wipe out most other bands in the SDS-100. TV Tuners have always been a wideband device made for TV signals.
I do play with little TV Tuners for SDR based reception of aircraft location signals. They work but are also hit and miss due to the strong paging signals here. I get aircraft position data in between paging transmissions!

I was surprised to see that the SDS-100 is still affected by the paging signals when I have the notch filters inline when I'm monitoring 800 MHz systems but it is. I can monitor the paging signals and my 800 MHs signals are affected when the paging transmitter comes on the air.

Anyway, does anyone else live in an area full of high duty cycle and powerful paging transmitters also own an SDS-100 and have you figured out a way to help with this problem?
If notch filters do not work, I'm at a loss. I could get high pass filters made so I could at least use the SDS-100 at UHF and above but there is really not that much above VHF that interests me.

Any ideas out there?

I am planning on keeping this radio as I know it will work if I travel outside the area but I'd love to use it here at home.
Ironically, my older 996XT models still work mostly very well and my x36HP models actually work very well. Using yagi antennas helped me eliminate all issues caused by LSM. So I did not purchase the SDS-100 because of LSM issues. In fact, our statewide system is not even a simulcast system. All the VHF sites are single transmitter sites.

Stumped!

Thanks for any ideas.

 

[W2MB]

Back in it's heyday, paging transmitters were famous for being dirty (not meeting spectral purity requirements). It is possible that the strength of the transmitter's spurious emissions exceed FCC requirements and are actually on your intended receive frequency. There are also various types of mixing issues that can occur at commercial sites which can cause strong signals on other than intended frequencies. This could be why your band reject filtering isn't working, that is the signal is actually on your intended receive frequency. The scanner also could have intermodulation or image rejection issues. One way to really narrow down the possibilities would be with sophisticated site test equipment such as spectrum analyzer.

 

[ka3jjz]

This may sound counter-intuitive, but did you try scanning at these sites with no antenna whatsoever? Or using a real small duck for the 2 meter band, like for example

MFJ Enterprises Inc.

Mike

 

[kruser]

Back in it's heyday, paging transmitters were famous for being dirty (not meeting spectral purity requirements). It is possible that the strength of the transmitter's spurious emissions exceed FCC requirements and are actually on your intended receive frequency. There are also various types of mixing issues that can occur at commercial sites which can cause strong signals on other than intended frequencies. This could be why your band reject filtering isn't working, that is the signal is actually on your intended receive frequency. The scanner also could have intermodulation or image rejection issues. One way to really narrow down the possibilities would be with sophisticated site test equipment such as spectrum analyzer.

Dirty paging, no way! Na, I totally agree. Paging has to be the worst behind illegal CB amps running behind way over modulated CB radios.

I do have test equipment. Doing a spectrum scan reveals nothing bad at all.
I also messed with a signal generator. Not wanting to interfere with anyone, I conducted my signal generator tests between 152 and 153 MHz.
If I crank the SG's power output up, that alone is enough to wipe out the SDS-100 front end. Mind you, the SDS is hooked to an outdoor discone at least 50 feet away from the SG. At the SG, I simply hooked a VHF portable's antenna to its output. As I crank up the SG output on say, 152.900, it wipes out any signal the SDS is trying to receive on 152.600 or 152.660, both control channels for the nearest sites to me for the statewide system.
Crank the SG output up to +1.0 or higher dBm output on 152.900 and bye bye CC signals on either CC frequency.

I seem to have to offending paging transmitters near me that both wipe out the SDS. One at 152.240 and one at 152.690.
Both produce an +10 to +20 signal when looking at an Icom R7000 or R9000 signal meter.
I'd guess both paging sites probably run at least 300 watts out before any antenna gain.
I know they do this to brute force the paging signals down into the basement floors where the ER and most Imaging type tests take place in many hospitals. It seems like they would do better with an in house paging system or even some type of "leaky coax" type system where the leaky coax would be down in the lower floors. Nope, they must crank up the output on the actual transmitters!

I had a pocsag pager on 152.240 several years ago. It worked but after learning what I learner years ago, I am really surprised the pager did work. Talk about possible timing issues from simulcast. I guess pagers are much more forgiving than a P25 control channel is to a receiver.
When I looked at the hospitals around me, I was located darn near in the center of three hospitals. Each one has a paging transmitter on their rooftops. You would think anyone outdoors would see severe signal cancellation from each of the transmitters. I wonder what they use to keep them synchronized as timing could also mess up paging signals and cause what we see today from LSM from simulcast P25 sites.

When either of the strong paging sites go hot, the SDS signal meter shoots up to full bars for maybe a half a second or so before its front end seems to give up. When it gives up, the signal meter in the SDS drops to nothing but of course you have no reception.
Almost immediately after the paging transmitters shut down, the SDS acquires a signal from the CC and starts decoding again.

The PAR notch filters are too wide to be tuned to notch just one of the paging signals without also wiping out the lesser signal CC's from the state P25 system or sites. I'd need a really tight filter to achieve reception when using the SDS. There's just not enough separation in the SDS to handle a paging signal on 152.240 or 152.690 when I'm trying to monitor the P25 sites CC's on 152.600 or 152.660. The nearby P25 sites for the state also both use voice channels in the same paging band. Voice channels are usually more around 152.510 MHz so dead center in the 1 MHz of space for this paging band!

If you are monitoring a weaker signal 800 MHz P25 site, it can sound exactly like it does when you are experiencing severe distortion from a simulcast site! Not good.
Strong signal 800 MHz signals will usually make it through and the SDS still decodes fine but weaker signals in the 7 or 800 MHz band can still be affected by the VHF paging sites.
For VHF, I don't think any strength P25 VHF signal would make it through when the SDS front end seems to shutdown after it can't handle the paging sites signal levels. The SDS seems very prone to desense from the VHF paging signals.

I guess I expected a lot more from the SDS than I'm seeing. The SDS is actually way worse than any scanner I've ever used to monitor a digital radio signal with. To the ear, it sounds like simulcast issues all over again but it's not, it is desense issues as can easily be seen with spectrum analyzers or station monitors and other test equipment.
At least with the previous scanners experiencing simulcast issues you still heard bit and pieces but with the SDS, you hear mostly nothing when the paging transmitters are on the air.

I have tried the different firmware versions for the SDS but I can't say I found any change at all. I think I tried every firmware version ever released for the SDS.

And that's that for now. I doubt this can be fixed with firmware but who knows.

I did take a short drive today. When I was maybe 4 miles away or further, the SDS worked fine and never seemed to miss a beat when watching one of the two state VHF control channels I use for my testing.

 

[kruser]

This may sound counter-intuitive, but did you try scanning at these sites with no antenna whatsoever? Or using a real small duck for the 2 meter band, like for example

MFJ Enterprises Inc.

Mike

Do you mean actually going to one of the VHF sites and see how it performs with no or very little antenna on the SDS? No, I did not try that as they are not that close to me.
On an Icom R7000 or 9000, either of the two VHF state sites within range produce a signal on the meters of about S-7. Both receivers are equal. On a spectrum analyzer, both P25 sites appear clean and it also shows equal signal strength from both sites.
I just finished a rather lengthy post on this issue and today when I did take a short drive away from the paging sites, I did not need to go far in order for the SDS to receive either VHF state site again. I did not have any test equipment with me though so I can't say how much the paging signal dropped off after a mile or so away from the signal but it was enough to make the SDS work again.
The hospitals with the paging transmitters are about 2 miles away the way the crow flies.
If I were not in an apartment, I'd but up a VHF yagi and aim it away from the hospitals. That would probably work I suspect but being in an apartment, I'd be pushing my luck as a VHF yagi starts getting pretty big compared to my 12 element 850 MHz yagi!

If you have another thought in mind for doing tests at one of the P25 sites, let me know and I can make it happen but I suspect you are asking just to prove that the VHF P25 sites are putting out a clean signal.

 

[kruser]

I just watched how my 4 and 536HP models react on either VHF CC and they do not flinch when either or both paging transmitters come on the air.
Both HP models stay rock solid on the CC signals I tested with.

It's a darn shame as I actually like the display options of the SDS-100. I think I bought it for the display options more than anything as simulcast issues were not really that big of a deal for me.

I'll likely keep the SDS as my short test drive away from the paging sites today did prove that it works fine when not subject to high power VHF signals.

Next, I need to see what the SDS does when tuned to either the local or distant NOAA signals on 162.xxx MHz when the local VHF paging sites come to life. I expect the weaker NOAA signals may be wiped out on the SDS if desense is truly shutting its front end down.

Edit: Interestingly, the SDS keeps receiving the WX signals just fine when either 152.xxx paging site is on the air, even the noaa channels that barely deflect the signal meter on either Icom are still received on the SDS. Paging does not effect the SDS and WX signals (strong or weak) it seems.

 

[CQ]

#NotMe

My SDS100 (original and current) have no issues. Every other radio I own have zero issues.

The TRX-2 on the other had was nothing but paging intermod hell. It went back.

 

[325xia]

My 436 outperforms my SDS100 in almost every way. Except the Display. That’s the only reason I keep it.

 

[maus92]

My 436 outperforms my SDS100 in almost every way. Except the Display. That’s the only reason I keep it.

Which is the exact opposite of my experience, but then again I only monitor 700/800 simulcast systems in the DC / Baltimore region, mostly Motorola. People should qualify their statements so we can get some useful data for those are considering purchasing the radio.

 

[kc5igh]

Which is the exact opposite of my experience, but then again I only monitor 700/800 simulcast systems in the DC / Baltimore region, mostly Motorola. People should qualify their statements so we can get some useful data for those are considering purchasing the radio.

Excellent point, maus92.

My SDS100 is performing beautifully on a nearby Harris UHF simulcast system, which is the primary reason I bought it.

It does a good job on other, non-simulcast digital systems (P25, NXDN, DMR, and ProVoice), but it has a difficult time with analog signals, especially in comparison to my BCD325P2 and TRX-1. I haven't used my BCD436HP much since I bought the SDS100, so I'm not sure how it compares to that radio when it comes to analog signals.

FWIW, I live in a rural, weak-signal environment, and it seems to me that successive SDS100 firmware updates have generally improved its digital performance and degraded its analog performance. That observation is not based on scientific measurements and could easily be attributed in part to my imagination or any one of a large number of other variables (weather, the radio systems themselves, etc.).

Happy monitoring!

-Johnnie

 

[kruser]

Which is the exact opposite of my experience, but then again I only monitor 700/800 simulcast systems in the DC / Baltimore region, mostly Motorola. People should qualify their statements so we can get some useful data for those are considering purchasing the radio.

My SDS100 suffers severely when trying to monitor a P25 site in the VHF band. It will not receive the site(s) at all until all paging transmitters also in the 152 MHz chunk of spectrum are completely off the air.

My SDS will continue to function on a VHF P25 site that has a much weaker signal for me that has a CC on 169.6625 MHz. Paging signals at 152.240 and 152.690 do not bother reception of the CC on 169.6625. The VHF site with the CC on 169.6625 is probably 30+ miles from me. When I monitor that site, the SDS keeps a lock on its CC when the nearby paging transmitters come on the air. But... when the site using 169.6625 has voice traffic and tells user radios or the SDS to tune to the voice channel, the SDS will not hear the chatter as the Voice Channel is also in the 152 MHz chunk of spectrum! If the paging sites are not transmitting when this site instructs a user radio to tune to the voice channel, the SDS will hear the chatter just fine but as soon as a paging site comes on the air, chatter is immediately lost and the SDS tries to acquire data from the CC again.

I did some crude tests this morning. I plugged in the two VHF troubled CC's as analog only channels so I could hear them on the SDS.
The analog CC audio sounded crisp and clear. But as I listened, I noticed what sounds like a burst of crackling static on either CC when either 152.xxx paging site comes on the air. This burst of static can only be heard for a fraction of a second and then the audio of either CC goes back to what sounds like nice and clear CC audio with no hint of noise or static to the ear.
I also watch the "Noise" value reported by the SDS and it is always around the 200 range regardless if the paging sites are on the air or not. So the SDS is not seeing the paging sites as noise over either P25 CC that I'm tuned to.

The thing that does change in the SDS display is the RSSI value.
When paging sites are off the air, the SDS shows a dBm level of -102 dBm or -68 dBm, depending on which site you have tuned on the SDS.

When either 152.xxx paging site comes on the air, the RSSI value displayed by the SDS immediately changes to -40 to -45 dBm.

With two paging sites here, it's really hard to find a time where both are off the air!
There are other paging sites here in the 152.xxx range that are much weaker signal levels, those do not affect the SDS reception.
And when the audio gets clear sounding while the paging site(s) are transmitting, the SDS RSSI stays at -40 or so and reception does not go good again until the paging sites are off the air.
That little fraction of a second burst of static I hear when monitoring either VHF CC does not seem to be the root cause of the problem as the RSSI stays way off even though the audio sounds like it cleared up just fine to the ear.
I don't know why the audio seems to clear up on the SDS but probably something being adjusted by the SDS AGC circuit. Whatever it is, it's not enough to allow your P25 signal to come back until the paging transmitters go off the air.

I also did some testing with 800 MHz P25 sites. While there are no paging sites in the 800 MHz band, there is a chunk of 900 MHz spectrum set aside for paging. That runs from 929 to 932 MHz I think. I have some really powerful paging sites nearby in the 900 MHz band. They do not seem to bother my SDS at all when they are on the air. I set the SDS to some strong and some weak 800 MHz sites and not problems.
The VHF paging sites also do not seem to bother the SDS when I'm monitoring an 800 MHz P25 site. I may have indicated the VHF paging sites were knocking out 800 MHz P25 sites in an earlier post on this subject but that was incorrect if I did.
I don't think we have any local UHF (450 MHz) paging sites in the area any longer so I can't test for problems there but I will say I don't seem to have any issues with NXDN or DMR sites that are in the 450 MHz band.
I've sat on a few both weak and strong signal UHF DMR or NXDN sites and did not notice any issues with either like I see with VHF P25 sites.

I can also scan or search the VHF rail band from 160 to 162 MHz on the SDS and have zero issues. No inter-modulation from the paging sites in the 152 MHz area at all. The rail band runs clear squelch so anything could cause it to open but it stays clean. I can sit on a distant rail channel and hear them talk just fine when the VHF paging sites are on the air so this problem does not seem to affect VHF analog signals at all. Well it does, that short little burst of static you can hear in a analog VHF channel but that is almost unnoticeable so I don't consider it a problem at all.