BCD396XT/BCD996XT: Bring your 3/996XT into the 21st century :)

[Mike_G_D]

Has this been confirmed?

I noted earlier in the thread how selecting NFM on my 996XT gives me "Heathkit" squelch, that is, very choppy squelch action with apparently no hysteresis. If the scanner is just increasing the audio gain then I wonder why the squelch behavior differs between NFM and FM? Ideas?

Also, please locate and use your Enter/Carriage Return key liberally. Our eyes will appreciate it greatly.

Ohhh, so THAT"S what that big fat key is used for?;-)

Yeah, sorry, you're right, I got carried away and just did a "data dump" without thinking of the readability - it was all one continuous thought in my head and it went to the keyboard in the same way; my apologies!

To address the question you had about the changing nature of your squelch control between FM and NFM modes, I can only make an educated guess. Since most FM squelch controls are noise gate based (see these wikipedia articles for a quick explanation: Squelch - Wikipedia, the free encyclopedia and Noise gate - Wikipedia, the free encyclopedia) my guess is that the enhanced audio gain in NFM mode (by whatever means Uniden is using) is changing the noise gate threshold or one of the other parameters like the release, attack, hold, and hysteresis and so you notice the effect. I must admit, I haven't noticed it at all on my Uniden BCT15 which is not modifed with the narrower filter(yet) - it is stock, as yours is.

I don't know why you experience this and I do not. I cannot explain that and have no idea unless it is firmware based as someone else suggested.

As to "have we confirmed that no actual bandwidth adjusting is used in the 15, 396, and 996 series radios when in NFM mode?" - well, if we go by Boatancher's analysis and looking at his photos of the internals there is only one "narrowband" IF filter in place so there simply isn't another one to switch to. This is why his replacement experiments have sparked so much interest and spurred this thread. I, myself, would very much like to do this for my 15 and intend to if and when other "life factors" will permit. I think this is a fantastic thread and really applaud him for creating it!

You can also do a rough test of your radio's IF selectivity (in analog mode) by tuning in a strong continuous signal - a NOAA continuous weather broadcast or strong trunking control channel are good choices. Then, using the direct entry and tune modes on your radio, slowly tune off frequency by the smallest increment you can and see when the signal really begins to fall off to low enough levels to close the squelch (or is low enough to set the squelch to a tolerably practical level). You should do this on both "sides" of the strong center signal (tune above and below it) as the IF filter may have different responses for each side (asymmetrical rather than symmetrical filter response).

If you have a GRE PSR500 or similar based radio (I can't speak for the 800 as I have no experience with it) you can do the above experiment first with FM then with NFM and will notice a dramatic difference whereas if you try it with a Uniden 15, 396, 996 series radio (stock, unmodified version) you will not notice a dramatic difference between FM and NFM modes. Unfortunately, if you do this with a GRE radio you will have to use the direct memory entry to program discreet frequencies offset from the center strong signal as the NFM mode will not work in "Tune" or "Search" modes. So, for example, you would program a channel with 162.400MHz (center) then succeeding channels as 162.405MHz, 162.395MHz, 162.4075MHz, 162.3925Mhz, 162.410MHz, 162.390MHz, etc., out to about 25KHz or 30KHz away or so and just manually switch between them.

-Mike

 

[sodjan]

> Anyone have a few to spare? I would like to try this Mod.

Hi. I'm the original seller on eBay of these filters.
They was sold in sets of 6 filters, but I had 5 of them
leftover and I have now listed then on eBay:
5 Pcs Murata Ceramic Filters 450 kHz 4 5 kHz Bandwidth SMD | eBay
Happy bidding!

B.t.w, I have got feedback from all but 2 buyers now, so it seems
like most (probably all) shipments has gone well.

 

[n4yek]

> Anyone have a few to spare? I would like to try this Mod.

Hi. I'm the original seller on eBay of these filters.
They was sold in sets of 6 filters, but I had 5 of them
leftover and I have now listed then on eBay:
5 Pcs Murata Ceramic Filters 450 kHz 4 5 kHz Bandwidth SMD | eBay
Happy bidding!

B.t.w, I have got feedback from all but 2 buyers now, so it seems
like most (probably all) shipments has gone well.

This will be fun to watch.

 

[tumegpc]

Filter Modded scanners for sale

If anyone is interested I have a 996XT & HP-1 with the 50G filter mods for sale here. I plan on selling 2 396XT's that I have not modded but will include the 50G filter for those who would like to do their own. I will post those tomorrow.

 

[mibzzer15]

45 minutes left, selling for $47 already..... 5 Pcs Murata Ceramic Filters 450 kHz 4 5 kHz Bandwidth SMD | eBay

 

[UPMan]

I am investigating whether we might be able to provide this modification as a service. If it seems possible, I'll post a poll so that we can judge demand (don't want to turn on a program then not have enough parts to satisfy the demand).

Our engineers provide the following caveats regarding this modification:
Under actual usage, if you mainly use the scanner for NFM/AM/APCO Phase 1. →　It is O.K. But, when receiving FM, the signal might be distorted under maximum Deviation.

Technical details:
There exists "Carson's rule" for detecting enough occupied bandwidth(90%).

For example, with maximam Modulation/Deviation.
FM: needed BW is ±7kHz
NFM: needed BW is ±4.5kHz
AM: needed BW is less than ±4.5kHz
APCO P1: needed BW is ±4.2kHz
APCO P2: needed BW is ±5.25kHz

with no frequency tolerance.
If there exist 1ppm frequency tolerance, need to add 1kHz@ 1GHz ( 0.5kHz@500MHz like that)

Also, you have to pay attention to Group delay with Digital modulation.

 

[cg]

Even selling the part would be helpful. I was thinking about buying a 350 piece reel, the best price I could find was $1200. However, I doubt I could sell 240 @$5 each to cover the costs (or even come close). You may be able to buy much cheaper.

chris

(Since I have 20 radios that I would do, paying for someone else to do it would be way too expensive)

 

[krokus]

Knowing an approximate price would help people decide if they want to pay Uniden, instead of other options.

Sent via Tapatalk

 

[mibzzer15]

If Uniden is able to provide the service at an inexpensive price, (something less than $40+shipping cost) then I would be interested. So count me +1 for the poll.

 

[scottyhetzel]

If Uniden is able to provide the service at an inexpensive price, (something less than $40+shipping cost) then I would be interested. So count me +1 for the poll.

+2 996xt & 15x

 

[kd0ega]

opps.... kind of late

At this point, I just shipped my BCD996T yesterday to Uniden for a faulty headphone jack replacement and firmware up grade.....would be willing to send it back if I deem the price to be fair as far as a filter upgrade......that's shipping both ways, service and part.

With that be said, is there anyone who could sell me a filter (send p.m) just in case Uniden sits on this forever and/or I decide the total price that they do come up with to be exorbitant....I'd rather that Uniden take care of this, but, if that is not meant to be, I 'll take care of it myself.

 

[FeedForward]

Hi, new guy here. Amazingly long thread on this interesting subject of IF filters. I have a few philosophical comments if you will allow me...

IF filtering has always been a very important part of receiver design and this case is no different. I suppose the most significant single contribution to the field would be the Collins mechanical filter which was (is) available in various bandwidths. At the time of invention it had the best filter characteristic available. The big difference for scanner use is that the IF filter characteristic must be a compromise for the application at hand which would allow for varying types of modulation without undue complexity and cost.

Ideally I suppose that the IF bandwidth should be adjusted dynamically according to the expected modulation type. I believe we will see this feature in future SDR models since it is so easily optimized with software. I have never heard a side-by-side comparison of an analog scanner and an SDR scanner, but I have to believe SDR is the wave of the future especially for scanners. Audio output level will not be an issue because it can easily be addressed before the signal exits the digital domain. SDR engineers have told me, for example, that brick wall filtering comparable to a several hundred pole analog filter can be achieved. That kind of feat was never in my mind when I operated or tweaked a tube style communications receiver. That's progress for you.

The very narrow band FM (+-2.5KHz) will produce a lower audio amplitude than the older +-5KHz signal all other things being equal. This topic can go further in complexity for the really detail-minded person because the lower-higher audio output is really a characteristic of the discriminator and not necessarily a drawback of the modulation type. The discriminator has been used in one form or other since the beginning of FM itself. Successful operation of a ratio detector, quadrature demodulator, various mixer demodulators or PLL demodulators all depend upon a signal strength sufficient to achieve limiting in the IF section. Once the signal strength falls below limiting there is going to be some kind of noise added to the original modulation. That fact will also be true if the demodulation is handled in the digital domain, since occasional samples will be missing or incorrectly determined if the signal is weak. Some improvement will probably be possible via complex interpolation inside the chips, so we can look forward to some improvement there, I think. I'm saying I think because I won't be the one doing the math to figure all this stuff out - not by a long shot.

So in my personal opinion, the reduced bandwidth filter being discussed lowers spurious noise simply by virtue of the relationship between noise power, signal to noise ratio and bandwidth. But I might add that this effect would predictably apply to those signals which fall below the limiting threshold - i.e. weak signals. Having said as much, probably lots if not most of the signals received by a scanner will fall into that category, so the change seems to be a good one, all things considered. I was just pointing out that there are a lot of things to consider, and not just one thing in the IF bandpass. All those electrical adjustments in software have something to do with how a signal gets from RF down to audio, and again these parameters have to be a compromise in order for the scanner to be useful in all the expected modes of operation.

One of the posts mentioned the information available on the FM Systems site. I did not look exhaustively through the site and rules of thumb are always helpful.....but. I noted one comment on the site repeated here:
" In fact the "FM Advantage" in signal-to-noise ratio stems exactly from spreading the modulation over a greater bandwidth than Amplitude Modulation."

Excuse me, but someone needs to repeat basic electronics 101. Well, I guess we all could benefit from doing that, but let's correct this error. Perhaps the author was thinking of the method of spread spectrum, but in fact spread spectrum is not a method to achieve greater signal to noise ratio either. The "FM advantage" is and always has stemmed from the relative insensitivity to amplitude variations in the received signal. (compared with AM, for example). Again, we have to assume that the signal is strong enough to cause the IF to limit. Speading the spectrum on any type of modulation will open the window for the noise power vs. bandwith effect. What is achieved by opening up the bandwidth on any type of angle modulation is fidelity, and fidelity is not a goal of communications radio. Single sideband would be another example of how closing up the bandwidth of the transmitted signal (and receiver passband) lowers overall noise and contributes to greater intelligibility of the same information (using the same transmitted power level, and compared with an AM signal that requires more than double the bandwidth). Anyway, chose your engineering sites carefully, I guess.

Now as a practical matter, since the reqiured 4.5KHz filters are only available in lots of 350, maybe there is someone willing to make a few bucks by buying a reel and selling them a few at a time to interested scanner enthusiasts. Maybe this is already being done, unbeknownst to me. Anyway, great thread and interesting discoveries especially for those who are skilled in the replacement of SMD parts. The final word on any type of modification is whether the gear operates better and is more enjoyable to use.

Thanks!
FeedForward

 

[Mike_G_D]

FeedForward,

Be careful in analyzing FM noise versus bandwidth.

You are right that a narrower bandwidth (via filtering) reduces the received noise power, obviously, that the following circuit must deal with BUT actual analog audio Frequency Modulation itself does mathematically have a reduced SNR with higher bandwidth when the signal level is above a certain threshold of quieting ("noise threshold"). Below that level, it can be worse than an equivalent strength AM signal (one reason routine air communications still use AM over FM) but above that level it is considerably better - at least it is when the "modulation index" (peak deviation divided by maximum frequency of modulating signal) is well above a ratio of 1.0. Below that, you start to get less and less SNR improvement over AM. Using 2.5KHz deviation is about as low as one can go without extra fancy post processing and still get some SNR improvement in demodulated analog audio over the equivalent in AM (in fact, you've about lost most improvement over AM in that regard using that low of deviation even when keeping the upper frequency of the modulating analog audio signal below 2.5KHz which is about as low as you want to go and keep decent human voice reproduction).

Having the limiter stage prior to the demodulator stage does also add to the noise immunity of analog FM, of course but, again, as long as you keep a fixed high frequency point for your baseband modulating signal, increasing the bandwidth of the deviation will increase the SNR of the demodulated audio signal (approximately 6dB per bandwidth doubling in certain cases).

Wikipedia can contain errors and can be incomplete at times but, in general, I find it pretty accurate to a first approximation in these things and it usually quotes from reliable sources when it comes to engineering topics (well, at least most of what I've looked at, at any rate).

That having been said, look over this Wikipedia article: Frequency modulation - Wikipedia, the free encyclopedia.

You are not wrong when you think of reduced IF bandwidth resulting in reduced noise power - that is an obvious relatively easy concept to grasp. But, as you say, there are many details to really consider in a final analysis. When a wideband FM signal with a modulation index much greater than one is demodulated it presents a signal with inherently greater SNR than an equivalent bandwidth and level AM signal - it is in the actual mathematics of the analysis and is not linearly related to the other aspects of the receiver chain including the IF bandwidth (provided it is of sufficient bandwidth for the signal, of course) and not just due to the limiter preceding the final demodulation stage. Don't conflate the audio SNR of the demodulated signal with the RF SNR in the final conversion stage (they relate, of course, but, with FM, not linearly).

Rather than my rusty and tired brain trying to explain this, here are two places online that I felt explained it pretty well without a lot of math:

1) From "Wireless Communications Principles and Practice, Second Edition" - a preview on Safaribooksonline - Wireless Communications Principles and Practice, Second Edition > Modulation Techniques for Mobile Radio > Frequency Modulation vs. Amplitude Modulation - Pg. : Safari Books Online. see the second half of the last paragraph - "...in an FM system, it is possible to tradeoff bandwidth occupancy for improved noise performance. Unlike AM, in an FM system, the modulation index, and hence bandwidth occupancy, can be varied to obtain greater signal-to-noise performance. It can be shown that, under certain conditions, the FM signal-to-noise ratio improves 6 dB for each doubling of bandwidth occupancy. This ability of an FM system to trade bandwidth for SNR is perhaps the most important reason for its superiority over AM."

2) See post number 5 in this forum: Why FM has less Noise than AM? | Electronics Forum (Circuits, Projects and Microcontrollers).

All of that having been said, given the tiny deviation and low modulation index of what we are dealing with for the purposes of this thread concerning the x96 IF filtering - we're certainly not dealing with a modulation index of >>1! In said case, the SNR is likely far more dominated by the RF SNR exiting the final conversion stage (just like an equivalent AM signal) so, in effect, what you said is quite valid - that narrower filter will result in a less noisy demodulated analog FM signal.

UpMan - since I'm on the subject, could you give us the peak deviation and peak modulation frequencies used to derive those bandwidth numbers above? I'm not getting the same results using the Carson's Rule approximation and 3KHz peak baseband modulation frequency and peak deviations of +/-5KHz and +/-2.5KHz for the FM and NFM deviations, respectively. But then, that is an approximation and if you're using a far more exact analysis then that may account for the differences.

-Mike

 

[jturner0829]

anyone have an extra? I would like to mod my 996xt.
sounds like a great improvement to have for p25. p25 is all I have programed to listen to. my county only.

 

[kd0ega]

anyone have an extra? I would like to mod my 996xt.
sounds like a great improvement to have for p25. p25 is all I have programmed to listen to. my county only.

1) As far as the filter goes, I did some further checking into it and research on my own...away from this website....the filter,in some cases, would/could work best if I where monitoring an adjacent counties P-25 system...where as with in my county, like what you are doing with yours, the filter would not be necessary due to the location of the transmitters..I live between two that are aprox. 5 miles away in opposite directions....I have no clue how close you would be to yours or how many tower sites you have, but when I spoke to my go to radio tech man and we further researched the filter , it would have little to no effect on local county monitoring (taking Shawnee Counties P-25 system in to consideration) ......and since that is what I would be doing, the decision was made jointly not to pursue the filter issue anymore. The IF filter would be something that I would want with my scanners if I where wanting to filter weaker signals.... I believe that I am better off using a directional db gain antenna with rotator and attenuating the scanner....same as I do with my ham rigs.

 

[jturner0829]

ooops my bad.

thanks for the info kd0ega.
i live about 7 miles from the closest tower in my area but i believe i am also getting a signal from another tower that is outside the county i think maybe like 15 miles away or so. the system here has a weird layout mho.

 

[wx5uif]

Going to post an update here.

I have a BCT15, BCD996T and BCD996XT that I have modified.

I have been using the BCT15 running Unitrunker with a discriminator tap. Signal levels have improved noticeably using the mod.

The BCD996T is scanning a new multi site P25 system. The mod helped quite a bit.The weaker signal sites now come in with a much less error rate.

The BCD996XT I am unsure. I think there might be something wrong with the scanner. I'm using a 4 port amplifier into all three scanners. The 15 and 996T get full signal on most systems I can scan with them. On the 996XT I am getting barely any signal or very staticy on analog channels. I have swapped it to one of the other radio's antenna feed with the same result. I ended up swapping the filter back with the same result. I'm surprised that the radio sounds so horrible to be honest.

 

[JamesO]

Turn off the other radios on the splitter and see what happens.

Also make sure any unused splitter ports are terminated.

Might find something unusual, maybe there is an issue with the radio or the antenna connector on the radio??

 

[KD4UXQ]

I am investigating whether we might be able to provide this modification as a service. If it seems possible, I'll post a poll so that we can judge demand (don't want to turn on a program then not have enough parts to satisfy the demand).

Our engineers provide the following caveats regarding this modification:
Under actual usage, if you mainly use the scanner for NFM/AM/APCO Phase 1. →　It is O.K. But, when receiving FM, the signal might be distorted under maximum Deviation.

Technical details:
There exists "Carson's rule" for detecting enough occupied bandwidth(90%).

For example, with maximam Modulation/Deviation.
FM: needed BW is ±7kHz
NFM: needed BW is ±4.5kHz
AM: needed BW is less than ±4.5kHz
APCO P1: needed BW is ±4.2kHz
APCO P2: needed BW is ±5.25kHz

with no frequency tolerance.
If there exist 1ppm frequency tolerance, need to add 1kHz@ 1GHz ( 0.5kHz@500MHz like that)

Also, you have to pay attention to Group delay with Digital modulation.

How about using a 50F filter for ± 6KHz instead of the 50G for ±4.5KHz. It would meet the limits for APCO P2 and likely FM would be fine. Maybe not as much improvement as seen with ±4.5KHz, but possibly an improvement without compromising APCO P2. These are not the same form factor, but maybe they will fit? Murata 450kHz 6kHz Ceramic Bend Pass Filters 5pcs CFUCG450F | eBay
Comparing the one in my BCT15X these are slightly narrow. It looks possible to solder them in place and jumper to one pad with a strand of wire.
Has anyone removed the filter using a sharp tip pencil iron and prying it loose with gentile pressure from a jeweler screwdriver? This looks like a possible method. I have not attempted it yet. I did order the 50F filters hoping to try one in my BCD396XT.

 

[pro92b]

How about using a 50F filter for ± 6KHz instead of the 50G for ±4.5KHz. It would meet the limits for APCO P2 and likely FM would be fine. Maybe not as much improvement as seen with ±4.5KHz, but possibly an improvement without compromising APCO P2. These are not the same form factor, but maybe they will fit? Murata 450kHz 6kHz Ceramic Bend Pass Filters 5pcs CFUCG450F | eBay
Comparing the one in my BCT15X these are slightly narrow. It looks possible to solder them in place and jumper to one pad with a strand of wire.
Has anyone removed the filter using a sharp tip pencil iron and prying it loose with gentile pressure from a jeweler screwdriver? This looks like a possible method. I have not attempted it yet. I did order the 50F filters hoping to try one in my BCD396XT.

CFWKA450KGFA-R0 was the original 50G filter in the modification and it contains six ceramic elements. The CFUCG450F contains only four ceramic elements and that is why it is smaller. It will have inferior attenuation outside the passband. Is a CFUCG450F better than the stock filter, CFWKA450KEFA? I'm not so sure it is. Better to use a CFWKA450KFFA available at Mouser.