RadioReference on Facebook   RadioReference on Twitter   RadioReference Blog
 

Go Back   The RadioReference.com Forums > Scanners and Receivers Forums > Uniden Forums > Uniden Thread Archives

Uniden Thread Archives A depository of archived threads from the original Uniden forum.

 
 
LinkBack Thread Tools Search this Thread Display Modes
  #1 (permalink)  
Old 04-30-2013, 3:29 PM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default BCT15X input filter too wide for 7.5KHz NB?

I have programmed both legacy AAR frequencies and the newer interlaced frequencies into this radio. In my area, all railroad radio activity is only on the legacy frequencies but I am scanning both sets of frequencies. When scanning 'up', the squelch sometimes opens on the adjacent channel below the correct (legacy) frequency and when scanning 'down', the scanner opens on the adjacent channel above. A BC125AT programmed with the same freqs does not exhibit this behavior. Is there a setup I am missing such that the receive signal energy does not open squelch prematurely and lock onto an adjacent channel?
Sponsored links
  #2 (permalink)  
Old 05-03-2013, 9:06 PM
KevinC's Avatar
Moderator
  Premium Subscriber
Premium Subscriber
 
Join Date: Jan 2001
Location: Houston,Texas
Posts: 1,319
Default

Are they both using the same antenna? Or is the 15X on an external and the 125 on the stock rubber duck? Or..??
__________________
BC-210XLT

"Wise men speak because they have something to say; Fools because they have to say something."
  #3 (permalink)  
Old 05-03-2013, 9:48 PM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default

Good question. The BCT15X is using the stock telescoping antenna whereas the BC125AT is outfitted with an RH77CA. Since the original post, I have observed the `125 locking on inaudible birdies? that appear similar to this phenomena. I assume they are birdies because scanning stops but the back-light does not activate as it does when the squelch truly opens.

If I may restate the phenomena more simply, I have three frequencies A,B,C that are centered 7.5KHz apart with 'A' being the lowest freq and 'C' the highest. The actual receive signal is at 'B'. When scanning up, the `15X often locks on 'A' and when scanning down, it locks on 'C'. Same thing rarely happens on the `125 but as I said as an inaudible.

Thanks for your reply.
  #4 (permalink)  
Old 05-04-2013, 12:33 AM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default

Question: how good is the selectivity of the 125 and the 15x? Going back to the A-B-C freq scanning setup of the previous post, here is my thought. If I lock out a frequency and the selectivity of the scanner is really, really good, then I would expect the scanner to never open squelch for a transmission on that frequency. So, I did just that for both the 125 and the 15x. I locked out freq 'B' but left 'A' and 'B' in scan. Disappointingly, both scanners opened squelch on either freq 'A' or 'C' even though the transmission was on 'B'. Oh well.
  #5 (permalink)  
Old 05-04-2013, 5:11 PM
KevinC's Avatar
Moderator
  Premium Subscriber
Premium Subscriber
 
Join Date: Jan 2001
Location: Houston,Texas
Posts: 1,319
Default

A quick search netted zero results of the selectivity specifications of either radio.

But a search for this spec on GRE scanners was a wash also.

So, it is what it is I guess.
__________________
BC-210XLT

"Wise men speak because they have something to say; Fools because they have to say something."
Sponsored links
  #6 (permalink)  
Old 05-04-2013, 5:30 PM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default

Yeah. Sensitivity and selectivity are always competing specifications. Here, I think we have to trade off sensitivity by using a heavier squelch setting or locking out 'A' and 'C' altogether. Nevertheless, they are good scanners but it helps to know what to expect.
  #7 (permalink)  
Old 05-04-2013, 5:41 PM
Member
   
Join Date: May 2009
Posts: 1,750
Default

Here is the $60 question: Does the Uniden have a separate narrowband IF filter, or is it audio compensation only when in the NFM mode?
  #8 (permalink)  
Old 05-04-2013, 6:02 PM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default

I think the 60Hz answer is a "narrowband ready" scanner as the 125 has been described really only means it can tune the mandated freqs. When I purchased the 125, it required a firmware upgrade to become narrowband capable. Unless the final filtering is done in the digital domain (after an analog to digital converter) it is very unlikely that it would be updated to narrowband via firmware. Your other point is good too. Lower audio levels would be realized. This is really noticeable when scanning with an older unit like a BC220. Once you have the audio at a normal listening level, the squelch tale becomes almost intolerable. With the 125 and 15x, the audio levels are better so you may have it about the compensation.
  #9 (permalink)  
Old 05-04-2013, 8:57 PM
KevinC's Avatar
Moderator
  Premium Subscriber
Premium Subscriber
 
Join Date: Jan 2001
Location: Houston,Texas
Posts: 1,319
Default

Quote:
Originally Posted by hertzian View Post
Here is the $60 question: Does the Uniden have a separate narrowband IF filter, or is it audio compensation only when in the NFM mode?
Good question! My money is on audio compensation only.
__________________
BC-210XLT

"Wise men speak because they have something to say; Fools because they have to say something."
Sponsored links
  #10 (permalink)  
Old 05-05-2013, 9:45 PM
Mike_G_D's Avatar
Member
   
Join Date: Dec 2002
Location: Escondido, CA
Posts: 864
Default

Based on my tests I have found that the NFM mode (or FMN) on a Uniden BCT15 (the only one I own - not the "X" model but I would guess the RF and IF stages are either identical or nearly so) only engages audio compensation and has no effect on IF filter response (i.e. "adjacent channel selectivity"). In contrast, the GRE radios I own do exactly the opposite - no noticeable (by me) audio changes but a definite change in IF filter response. Ideally, it should be both but in both cases (Uniden and GRE) corners were cut for cost reasons I guess.

I do have considerable RF lab experience so I do know what I am talking about; however, I currently lack the proper test equipment to really definitively test the issue although I know how to do so if I did have said equipment. Instead, what I did was use my "trained" ears and experience and did the best I could.

Try the following as a test:

Tune to a strong continuous broadcasting and modulated signal such as the NOAA weather channel or a trunking control channel. Now listen to the audio quality and loudness with the NFM/FMN setting engaged versus not engaged. If the radio is doing some form of audio compensation for the deviation then it should be noticeable - wider deviated signals (such as the NOAA weather broadcasts) will seem louder and may distort in the narrow setting. If you cannot hear any difference then the receiver may not be doing any audio processing changes.

Now, to test if the receiver is doing any IF filter changes, try tuning off frequency the continuous broadcast by some amount (try small steps at first like +/- 5KHz or 7.5KHz and increase if necessary depending on how strong the signal is - experiment) so that you are still hearing the audio but somewhat distorted - ideally you want to be just at the last frequency edge of being able to detect the signal. Now listen to that distorted off frequency signal in both FMN/NFM mode and standard FM mode. In the NFM/FMN mode if a narrower IF filter is being used you should notice a dramatic change such as the signal completely disappearing while if no filter change is done you should either hear no change or some audio change (due to the FMN/NFM audio processing change).

Again, in my case, using only the BCT15 and comparing it to my GRE gear, I detect no filter change in the Uniden when using the FMN/NFM mode but some audio change while I get the exact opposite in the GRE gear.

I first really noticed this when trying to use the BCT15 for local state fire communications on the newer narrowbanded frequencies. It was a real problem because there were multiple fires around me and divisions from two or three separate counties were operating in close proximity (RF-wise) to me. Many of the newer channels were being used in addition to the older standard ones simultaneously. Essentially, the newer channels were useless to monitor on the BCT15 because the adjacent channels "bled over" so badly. I had to use the GRE radios which had either no problem or far less of a problem (maybe some slight "bleedover" but not so bad I couldn't "squelch it out"). As far as I could tell from monitoring the users communications it did not seem that they were experiencing any "bleedover" issues themselves (which you would expect using better professional gear). Unfortunately, this negated my intended usage of the BCT15 which is otherwise a good unit and has much better front end performance than the GRE's (in the lab, we considered the IF selectivity as separate from the RF "front end" performance so that is how I am looking at things here). I do not know for certain whether the other Uniden models are similar But I would guess they would be. I do recall a user (and admin on this site, I believe) who, in the California forums wound up thinking he was hearing communications on one of the newer "narrowbanded" CDF channels when in actuality it was determined that he was hearing signals actually transmitted on a channel 7.5KHz away and I think he was using a 996(X or T I do not recall) so I think that at least most of the Unidens use a common IF design.

I've said it before - I wish I could have a Uniden front end combined with a GRE IF section in one unit! Oh well...

As an aside, I performed the same tests as describe above on my Wouxun dual band HT which is a Part 90 FCC certified unit for professional use and found that it seems to do the same thing as the Uniden in terms of not really changing any IF filtering when the receiver engages NFM mode but only changing the audio (discriminator settings, I am guessing); however, in this case the standard IF filter was far more selective than the one in the Uniden scanner so that this was not really a problem. I suspect that cheaper professional models (like the Wouxun) might do this in many cases but I am guessing that more expensive higher end gear likely does have special selectable narrow/wide IF filters.

-Mike
  #11 (permalink)  
Old 05-05-2013, 10:56 PM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default

Mike,
Great post and I like your suggestions. I think your IF test is similar to my idea of locking out the broadcast frequency and scanning/adjusting the adjacent channels. If the receive filters were well matched to the received signal width, the squelch would not open which brings up another interesting question: where in the conversion process is the power typically monitored for squelch operation?

I would be surprised if anyone went to the expense of including switchable filter banks in these scanners so the filter switching you see in your other radio I would think would be programmable FIRs based on user setup. At least that would make the scanner very configurable via firmware and very fast switching. Do you know what Uniden does when the mode is set to auto? Is it based on simple frequency band assumptions?
  #12 (permalink)  
Old 05-06-2013, 12:23 AM
Mike_G_D's Avatar
Member
   
Join Date: Dec 2002
Location: Escondido, CA
Posts: 864
Default

Well, I think in terms of RF hardware and generally tend to lean toward a switched IF filter in the GRE units. I don't think in terms of digital filters but I suppose it's possible at the final 455KHz IF (I have no schematic for the units so I can't say for sure). I just know what my results indicate and that is that there is definitely a filter change or at least a filter characteristic change.

As to the squelch - again, no schematic but typically the FM squelch is noise based and uses rectified high pass filtered noise coming from the FM discriminator to create a DC voltage that runs through a comparator - the squelch control sets the level of the comparator that determines the noise level at which to mute the audio in the receiver. As FM signals increase in strength the noise present in the discriminator output decreases.

As far as I know, all Uniden does in terms of the auto setting for modulation is use a built-in lookup table for determining the "normal" bandplan versus modulation setting. Nowadays, it should use NFM/FMN for most cases above 138MHz and below 512MHz with the exception of some military and/or government stuff, Marine, GMRS, and amateur radio and maybe some other miscellaneous stuff like wireless mics, telemetry, etc. I think GRE is lazier - they just set the mode to FM across the board except for AM in the aircraft bands. In fact, a pet peeve of mine is that they (GRE) do not allow you to change to NFM when using the service search pre-programmed bands (like "Fire", "Police", etc.) and also not when searching between user set limits! You can only engage the NFM setting when programming in memory channels in conventional mode. You can set the NFM mode while in trunking but I don't really think it is actually doing anything (the setting is ignored) and in P25 mode (same deal - no effect). It was verified on here by "someone in the know" (someone who worked for GRE who was in contact with one of the admins on this site) that the NFM setting on the GRE's has no effect on P25 signals (meaning that the IF response does not change) although the display will show "NFM" as being engaged. I have a test for that too that I talked about in that old thread which I won't go into here since we pretty much have the "straight skinny" on that issue anyway (as far as P25 is concerned).

-Mike
  #13 (permalink)  
Old 05-06-2013, 11:27 AM
Member
   
Join Date: Apr 2013
Location: Orlando Florida
Posts: 10
Default

Since my scanning interests are confined to ferroequinology, I have procrastinated on purchasing more scanners until the AAR commits to and deploys a trunk system. (Given Florida is a "red state" when it comes to vehicular monitoring, I wish someone made a conventional VHF only model just for me that I could argue before a judge cannot be "adjusted to" police transmissions.)

Back on topic, if a 6KHz mandate arrives first, it will be interesting to see what GRE may do, if anything, in terms of updating analog filters and/or anticipation of future mandates.

Thanks for the post.
 

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 9:37 AM.


Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
All information here is Copyright 2012 by RadioReference.com LLC and Lindsay C. Blanton III.Ad Management by RedTyger
Copyright 2011 by RadioReference.com LLC Privacy Policy  |  Terms and Conditions