Since I use my BCT15 almost exclusively for monitor AM Air, Im going to have to take it apart and see if it would be improved by this mod. Less noise on AM Air would be a godsend.
BCD396XT/BCD996XT: Bring your 3/996XT into the 21st century :)
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JamesO
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I actually modified one of my radios many years ago that had both 10.7 MHz & 455 kHz switchable filtering. Modified the switch to better match the IF filters that were cascaded and at the time actually had a spectrum analyzer available to use so I could actually see the filter bandwidth changes. I was able to narrow up the passband a small bit with the existing filters in the radio. Did seem to help reception a bit.
I find that the filter 3 dB bandwidth is usually key, these small IF filters ratings are usually rated at 6 dB and I find that using a narrower filter should only really have a negative impact on something that may be off frequency or if the radio base oscillator is off frequency.
I had been pondering this for some time, however, had not bothered to search around and see what filter options were available.
I have multple x96XT radios, so I may modify one and compare it against the stock radio and see what positive and negative impact the narrower filter may have.
A solid state switch circuit could be employed if it was necessary and the scanner logic d could switch the filters in and out and there would likely not be an impact in the switch speed, but not sure this is really necessary.
The real key would how does the slightly narrower IF filter impact P25 decoding on these radios.
I find that the filter 3 dB bandwidth is usually key, these small IF filters ratings are usually rated at 6 dB and I find that using a narrower filter should only really have a negative impact on something that may be off frequency or if the radio base oscillator is off frequency.
I had been pondering this for some time, however, had not bothered to search around and see what filter options were available.
I have multple x96XT radios, so I may modify one and compare it against the stock radio and see what positive and negative impact the narrower filter may have.
A solid state switch circuit could be employed if it was necessary and the scanner logic d could switch the filters in and out and there would likely not be an impact in the switch speed, but not sure this is really necessary.
The real key would how does the slightly narrower IF filter impact P25 decoding on these radios.
Have you ever tried an airband bandpass filter such as AOR sells?
They do work wonders.
The only problem is if you also monitor the UHF military AM air band. There are air band filters for that band also but I've never seen one that will pass both bands which is what you want if you are into both bands.
It's amazing sometimes at how much background noise from things like FM broadcast stations gets into your airband monitoring. You usually don't know until you insert an airband bandpass filter inline and find that signals that were filled with noise are now very clear!
The 15 does a pretty decent job as it is though. I also use mine for VHF airband with and without a bandpass filter and it works well.
Post back with your results if you do the mod. I doubt I'll do my 15 but may if it really helps.
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For your intended use on AM only, this modification should be very helpful and there should be no downside to it. For EDACS trunking I have concerns. The control channel for a 9600 bps system in the 800 MHz band still runs 5 kHz deviation as far as I know. According to Carson's rule the bandwidth of the control channel is 2x(4800+5000)=19600 or +/-9800 Hz. The 50G filter is +/-4500 Hz and should cause a lot of distortion. In the PRO-197 GRE used a filter similar to a 50F which has +/-6000 Hz bandwidth. It seems to work on EDACS without mistracking. I've ordered some of the 50G filters and will try the modification to see if it ruins EDACS tracking. If it does, maybe 50F filtering would be a good compromise. More information on Carson's rule and bandwidth calculations can be found at FM Systems Inc
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Ill check them out.
kellykeeton
Member
That would work but I wonder if the scan speed would suffer dramatically do to the switching. There is a lag time involved there.
The ebay guy from Sweeden must be wondering what the heck is going on. 5 orders in the last 12 hours
May take a little more shipping time to get them in our hands from Sweeden. The problem with local places like Digi-Key is they sell by the reel (350 pieces in this case). To get single amounts they charge a "reel cutting fee", and after all of the other charges you end up paying 5 times as much for one piece.
Wonder where Paul is on all of this?
Haha I was thinking the same thing - I got extras when they show up happy to mail for $2
Here's a link to the "guy from Sweden" on eBay...
6 Pcs Murata Ceramic Filters 450 kHz 4 5 kHz Bandwidth SMD | eBay
6 Pcs Murata Ceramic Filters 450 kHz 4 5 kHz Bandwidth SMD | eBay
SquierStrat
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Shoot me a PM when they arrive.. I'll take one
Boatanchor
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More notes
A couple more things to note on this topic..
Firstly, after replacing the filter, you will notice that the squelch control may need to be turned up a few notches from what you are accustomed to. The squelch will still close at 2 or 3 with no antenna connected, but you will find that it is a little more (too?) sensitive at the very low settings after the mod..
I have now modified my 396XT's in the same manner as the 996XT.
Once the 396XT is pulled apart, changing the filter is probably easier than in the 996XT.
As mentioned previously, this mod will improve weak signal NFM, AM and P25 performance..
It will not improve FM (+- 5Khz deviation) performance. In fact, under weak signal, very high deviation situations, FM performance may actually suffer a little.
Although, I have not found a situation yet where I have noticed any significant degree of distortion on voice FM (+-5Khz) signals, it is possible that 5Khz EDACS 9600bd control channel decoding may be degraded by this mod!
Remember that although these filters are rated at +-4.5Khz, they are not 'brick wall' filters. They have sloped side skirts that will permit additional deviation to pass, albeit at increased attenuation levels.
While this may be acceptable for voice comms, 9600bd EDACS data may not be so forgiving.
If you are sitting on the fence re this mod and you need to monitor a 9600bd EDACS Network, it might be prudent to await reception reports from someone who has replaced the filter and tested the performance on a similar network.
Ultimately, we won't know until others report back on the degree, or lack thereof, of any degradation on 9600bd EDACS. Since I don't have one of these networks nearby to test this on, I can't tell you one way or the other - It will be interesting to see the results
Someone mentioned replacing the stock filter with a '50F' filter, which is slightly wider than the '50G' filter I have specified. The 50F filter will work and it will be a little better than the stock filter on the narrow modes, but not as good as the 50G.
But again, it is a compromise. The '50F' filter may be another option if they can be sourced at a reasonable price. They may be rather scarce in the form factor required for the x96 scanners, as +-6Khz ceramic filters are somewhat rare at the best of times..
A couple more things to note on this topic..
Firstly, after replacing the filter, you will notice that the squelch control may need to be turned up a few notches from what you are accustomed to. The squelch will still close at 2 or 3 with no antenna connected, but you will find that it is a little more (too?) sensitive at the very low settings after the mod..
I have now modified my 396XT's in the same manner as the 996XT.
Once the 396XT is pulled apart, changing the filter is probably easier than in the 996XT.
As mentioned previously, this mod will improve weak signal NFM, AM and P25 performance..
It will not improve FM (+- 5Khz deviation) performance. In fact, under weak signal, very high deviation situations, FM performance may actually suffer a little.
Although, I have not found a situation yet where I have noticed any significant degree of distortion on voice FM (+-5Khz) signals, it is possible that 5Khz EDACS 9600bd control channel decoding may be degraded by this mod!
Remember that although these filters are rated at +-4.5Khz, they are not 'brick wall' filters. They have sloped side skirts that will permit additional deviation to pass, albeit at increased attenuation levels.
While this may be acceptable for voice comms, 9600bd EDACS data may not be so forgiving.
If you are sitting on the fence re this mod and you need to monitor a 9600bd EDACS Network, it might be prudent to await reception reports from someone who has replaced the filter and tested the performance on a similar network.
Ultimately, we won't know until others report back on the degree, or lack thereof, of any degradation on 9600bd EDACS. Since I don't have one of these networks nearby to test this on, I can't tell you one way or the other - It will be interesting to see the results
Someone mentioned replacing the stock filter with a '50F' filter, which is slightly wider than the '50G' filter I have specified. The 50F filter will work and it will be a little better than the stock filter on the narrow modes, but not as good as the 50G.
But again, it is a compromise. The '50F' filter may be another option if they can be sourced at a reasonable price. They may be rather scarce in the form factor required for the x96 scanners, as +-6Khz ceramic filters are somewhat rare at the best of times..
Boatanchor
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My 50G filters have been shipped and when they arrive I will check on EDACS performance.
I measured the deviation on the 800 MHz control channels receivable here and found that on average it is 3 kHz. The lowest I found was 2 kHz and the highest 4 kHz. I'll be using the 4 kHz deviation control channel to test the modification.
Have you done any listening on VHF-LO, 30-50 MHz, with the 50G filter in place?
bear780ks
Member
Boatanchor I love my XT's my 396 and my 996) What I'm worried about is I have a new Computer a Laptop & one that is on Windows 8.1 Very new software sort of speak Now I know I Can get a USB for them to connect to the computer, but my thing is will Uniden's software Mesh with the 8.1 in a good matter..
On my old windows XP computer I had the old style hooks ups and things seemed to work OK) But with this new Cat = computer it nice, shiny faster than a bull in fly time.. but if can't work with the software It's boat in the water with out the Ores:roll:
So I guess that my wonder of the Day..
On my old windows XP computer I had the old style hooks ups and things seemed to work OK) But with this new Cat = computer it nice, shiny faster than a bull in fly time.. but if can't work with the software It's boat in the water with out the Ores:roll:
So I guess that my wonder of the Day..
My big gripe with stock NFM mode on the Uniden 996XT is that it becomes Heathkit squelch, no hysteresis so it's very choppy even on stronger signals. I don't know what Uniden is doing differently when programming a channel NFM as opposed to FM, but it degrades the performance of the squelch action.
Can you describe the squelch action with the narrower filter? I don't mean the level of the threshold, but how does it sound on weaker mobile signals with flutter? Is it very choppy or does it allow for smooth fade conditions?
Does it tend to snap closed even on solid, but weaker signals, with no change in signal strength/noise?
Thanks.
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JamesO
Member
I have 2-996XT and 2-396XT and I have not run into a squelch issue.
Mine is set to 2 on all of the radios and I have never had an issue.
What number do you have the squelch set to and is this problem on trunked or conventional systems? AM or FM modulation?
What version firmware are you running in your scanners?
I will be modding my radios in the near future. I will mod one and then leave one alone so I can see the pros and cons between the different filtering. I will see if the squelch will behaving differently. I do expect that the squelch level may need to be set differently as well.
AZScanner
Member
This is so cool - reminds me of the good old days I spent muddling my way (with lots of help) through Bill Cheek's Scanner Modification Handbook. Let us know if you mod that 785. I'm curious if a similar mod might breathe new life into my old 796 which has much of the same hardware inside. I'd be willing to try it since that scanner's going to be pretty much used only for analog stuff once the Phoenix RWC goes Phase II in a couple years.
-AZ
Let us know if you mod that 785. I'm curious if a similar mod might breathe new life into my old 796 which has much of the same hardware inside. I'd be willing to try it since that scanner's going to be pretty much used only for analog stuff once the Phoenix RWC goes Phase II in a couple years.-AZ
kellykeeton
Member
Again, it's not related to the squelch threshold, it's the lack of hysteresis on the squelch action AT ANY SETTING. The behavior of the squelch changes when the channel bandwidth programming changes from FM to NFM.
This is notable on just about the only thing left worth monitoring on analog FM, the railroads.
Well, like that's just your opinion man.
Boatanchor
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Personally, I would not do what you have done.
The 450Mhz IF section has relatively high gain at a high impedance. It will be VERY prone to noise pickup unless short connection points and excellent shielding is used.
I've got a feeling that you are going to negate any benefits you may have had from the narrow filter by extending a couple of 'noise antennas' out from those pads.
The only way to run multiple filters is to use a diode or chip filter mux arrangement (as used in other radios and the x36HP's).
Rick
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