Batboard

I’m working on a voting system that consists of four MTR 2000 receivers, one MTR 2000 repeater in which three of the receivers and the repeater are connected to a Spectra Tac comparator via Telco lines and one receiver uses a microwave. When the system was narrowbanded we also set the levels which had not been set in years. After that the user is telling me that there is a loud tone or noise that is being broadcast over the air. On the dispatch console they were able to see what channel was keyed up when the noise was being broadcast over the air. I think at that time (at the dispatch console) they went through all the receivers and disabled them one by one until the noise went away. When the receiver is disabled there are no more problems. I checked the wiring from the problem receiver to where it’s punched down and is connected to the Telco lines; I also checked the wiring from the Comparator to the Telco lines on the other end. I also had the phone company check there lines from the building where the receiver is to the building were the Comparator is, all check out good. I had a spare SQM and that was swapped out for the one connected to the problem receiver. After that I set the wireline levels out of the receiver again, just to be sure. I enabled the receiver, it worked with no problems for two days and then the same problem accrued. I talked to the user and the noise is described as follows, noise is a very loud, static sound that comes over the radio that everyone, dispatch and field units hear. It’s so loud and over bearing it covers the transmissions from field units. Does anyone have any idea what may be going on and or have any suggestions on what my next step in isolating the problem should be? Thanks in advance for any help; if you need me to supply any more information please let me know.

Swap the suspected failed receiver to another location, and vice versa. If problem reoccurs at same site, you either have some interference to chase down, or there is still a telco and/or wiring problem. If the problem moves to the other site, you have a failed receiver.

See if the customer has a logging recorder hooked to channel. If so, have them send you a .wav file of the "noise".

A laymen's description of noise can be anything that irriates them! You need to hear it yourself to prevent chasing your tail.

MTR and Spectra Tac. I don't know your experience with SpectraTac or setting levels, but there are many possibilities.

#1- You said you have one site via microwave. Typically, the performance through a digital microwave/mux is going to be darn near perfect/flat. (Unlike telephone lines which have roll off). Without a "roofing filter", this site will rarely vote. (Because the telco sites will have less noise above 2200 HZ, and the voter will percieve these sites as more "quiet") A SpectraTac voter detects the audio on each SQM above ~2200 HZ, rectifies the audio and comes up with a DC value. The comparator "votes" the site with the lowest DC value.

#2- Even though the levels may not have been touched in a long time, there may have been a reason why they were set a certain way (which you may not be privy to). Setting levels for what you consider optimum, may have introduced an issue that someone prior was masking.

#3- Do the telco ckts have loopback cards? Are they set for a certain amount of insertion loss? Sometimes the cards are misaligned. Sweep the lines. Using a TIMS, generate a 404/1004 and a tone somewhere around 2300-2400 HZ. Levels should be within a couple of db. (I had a system with one telco line terribly misaligned, causing erroneous voting and tinny tx audio)

#4- What levels are you hitting the comparator at. Most systems shipped as -13db. What this means is status tone is to hit the comparator at 13 db lower than max system audio. This does NOT mean status tome should be measured at -13db at comparator. The relationship between max audio and status tone is the key. And if your telco circuits have inserted loss, you will need to take this into account. Most telco cards do NOT like to be overdriven with any audio greater than 0 db, or you run the risk of overdriving/distortion. EX. You may have a circuit with 10 db of loss. I usually start at the station with -10db of nominal audio (+- 66% of system deviation). This will put max audio out of station around -6 db. (This will prevent telco card over driving). 1004 should then arrive at comparator at -16db with max audio. Measure the status tone at the comparator, it should be at -29db. (Assuming no telco card alignment issues). Go into the MTR and use the line emphasis shaping to boost/attenute the MTR 2175 tone to achieve -29 db at comparator. At this point, it really doesn't matter what the level is leaving the station.

This is a two man job.

I am going off of memory, as I am about 7 years removed from the field, so I may be slightly off on my description/terminology.But I lived with these things for several years and set up several and inherited a misaligned one.

My two bits worth:
I have seen this before.
The SQM has a 13db pad in the front end (Rev B and above) that is why the level as measured at the SQM should be -13DBM0
If noise bursts occur on the audio link the GT decoder operates so fast that the input attenuator is set to the noise peak.
Eventually the attenuator is set so high that the GT cannot be detected, so the GT is Transmitted until the SQM times out, or is reset.

I have replaced the transistor switch with a pot, set the output to be identical with the others.

A noisy, unbalanced or "wet line" can hang up the system.