XPR8300 Interference & Hang Time

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jasong224
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XPR8300 Interference & Hang Time

Post by jasong224 »

I have an odd issue with a VHF XPR8300 in analog mode. This install has been running largely problem free for more than 4 years. Recently at the end of a transmission the repeater would stay keyed with both TX & RX indicators illuminated indefinitely until you either power cycled the unit or repeatedly keyed and unkeyed an end-user radio enough times that it would eventually stop. As soon as someone transmits again the transient noise is right back. The transmitted signal post normal transmission is not normal "white noise", but I can't really confirm that modulation is present either. I can try to post a recording of the audio if that helps. I have setup a test radio to monitor the repeater input frequencies, but I can't ever hear the interference on it. An odd temporary solution I have found is to disable the hang time on the repeater. Normally I had the hang time set at 2-seconds, but I found that if I change it to "0", there is still a momentary burst at the end of a transmission, but it is quickly silenced. Assuming it could be a faulty repeater, I swapped in a nearly identical XPR8300, except this one is a 1-25W version instead of 25-50W and experience the same results. I also can confirm that the noise is present regardless of whether or not the unit is fed from AC or DC power. Aside from the post transmission noise we have no problems with the system. Audio is clear and coverage has not changed. The other components in the system consist of a TxRx 30-37-01a duplexer and an Andrew Decibel DB222 antenna fed by FSJ4-50B heliax coax.

Any suggestions on what to investigate next?
Thanks in advance!
Jason
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Wowbagger
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Re: XPR8300 Interference & Hang Time

Post by Wowbagger »

It sounds like you may have a low-level spur being generated by the transmitter, or a low-level IM product generated by the antenna. Can you perhaps put a tap inline with the receive port of the repeater and sniff that?
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
jasong224
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Re: XPR8300 Interference & Hang Time

Post by jasong224 »

Wowbagger wrote:It sounds like you may have a low-level spur being generated by the transmitter, or a low-level IM product generated by the antenna. Can you perhaps put a tap inline with the receive port of the repeater and sniff that?
I'll try to barrow a service monitor again and give it a shot. In reference to the transmitter spurs, would this be common to happen with two different repeaters in a system that was previously working just fine? Just to clarify, when you say "low-level IM" are we talking about intermodulation? If so, would that not affect the quality of our normal transmissions? Should there be any visible signs of damage to the antenna?

Sorry if these sound like frivolous questions, I'm just trying to get a better understanding of whats going on here.

Thanks,
Jason
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escomm
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Re: XPR8300 Interference & Hang Time

Post by escomm »

Can you post the audio? Is it like a grinding noise? I had one that had a similar problem, never could sort it out, but it went away when the repeater narrowbanded.
jasong224
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Re: XPR8300 Interference & Hang Time

Post by jasong224 »

escomm wrote:Can you post the audio? Is it like a grinding noise? I had one that had a similar problem, never could sort it out, but it went away when the repeater narrowbanded.
We moved to narrowband quite a while ago. I don't really think I would call it a grinding noise. I have uploaded an audio sample to the following location.
http://www.optimumts.net/audio/Repeater_Audio.wav

Jason
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escomm
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Re: XPR8300 Interference & Hang Time

Post by escomm »

Yeah man that's the same exact noise we had, and it went away with narrowbanding. Nothing in the hang time settings did anything. Changed PL, DPL, no love.
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Re: XPR8300 Interference & Hang Time

Post by Wowbagger »

A low level spur wouldn't necessarily impact TX - only a tiny fraction of the power would be in the spur. And an intermod product could happen due to a bit of corrosion - suddenly a wire becomes a diode, and you have a mixing product.

You could do a really quick test - get the system into the failure state, then unplug the RX side and see if it quiets down.
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
jasong224
Posts: 34
Joined: Tue Jul 15, 2008 12:46 pm

Re: XPR8300 Interference & Hang Time

Post by jasong224 »

I tried repeatedly to get the system to fail and produce the noise yesterday, but it wouldn't do it. We had a service monitor hooked up in-line with on the RX side and didn't see anything unusual. Also took the opportunity to measure power at the duplexer output, ~19W forward, ~1.5-2W back. Repeater is set at 25 watts.

escomm wrote:Yeah man that's the same exact noise we had, and it went away with narrowbanding. Nothing in the hang time settings did anything. Changed PL, DPL, no love.
We are already narrowband. I did try changing PL and it never helped here either.

Wowbagger wrote:A low level spur wouldn't necessarily impact TX - only a tiny fraction of the power would be in the spur. And an intermod product could happen due to a bit of corrosion - suddenly a wire becomes a diode, and you have a mixing product.

You could do a really quick test - get the system into the failure state, then unplug the RX side and see if it quiets down.
When it does it again, I'll try your suggestion of disconnecting the RX side and see what happens.

Thanks for the help so far.
Jason
Dorf411
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Re: XPR8300 Interference & Hang Time

Post by Dorf411 »

This is an interesting problem that we also have on a system with an identical problem that we can't seem to find the source of. On our system it is a MTR2000 that has been online for over 6 years with no problems. Suddenly we started getting the same intermittent indications as listed by the OP, we have been to the site multiple times. Usually the problem disappears before we get there, it seems as though it most typically happens when there is lots or rain or dense fog and moisture in the air but not always. Usually we have to make a change and then wait to see if the problem is solved or comes back.

Things we have done to unsuccessfully isolate or locate the problem:

- Hook up a mag mount antenna to the duplexer running out the door and it works fine.
- Use comm analyzer watching the frequency for a remote source of input on RX, nothing.
- Make all kind of adjustments to PL turn on/off and hang time, no change.
- Swap in a VXR7000 in place of the MTR2000, still bad.
- Swap out the cheap band reject duplexer for a higher end band pass duplexer, still bad.

I am beginning to think there is some loose hardware somewhere on the tower causing intermods, I think this all started 6-12 months after this tower had ice buildup that came down and caused damage to original antenna, ice bridge, shack, and much more.
jasong224
Posts: 34
Joined: Tue Jul 15, 2008 12:46 pm

Re: XPR8300 Interference & Hang Time

Post by jasong224 »

Dorf411 wrote:This is an interesting problem that we also have on a system with an identical problem that we can't seem to find the source of. On our system it is a MTR2000 that has been online for over 6 years with no problems. Suddenly we started getting the same intermittent indications as listed by the OP, we have been to the site multiple times. Usually the problem disappears before we get there, it seems as though it most typically happens when there is lots or rain or dense fog and moisture in the air but not always. Usually we have to make a change and then wait to see if the problem is solved or comes back.

Things we have done to unsuccessfully isolate or locate the problem:

- Hook up a mag mount antenna to the duplexer running out the door and it works fine.
- Use comm analyzer watching the frequency for a remote source of input on RX, nothing.
- Make all kind of adjustments to PL turn on/off and hang time, no change.
- Swap in a VXR7000 in place of the MTR2000, still bad.
- Swap out the cheap band reject duplexer for a higher end band pass duplexer, still bad.

I am beginning to think there is some loose hardware somewhere on the tower causing intermods, I think this all started 6-12 months after this tower had ice buildup that came down and caused damage to original antenna, ice bridge, shack, and much more.
I've done pretty much everything you listed except swapping the duplexer, but that is probably next on my list pending the suggested tests above. I think I have a source on a much higher end unit that I should be able to use at least temporarily. Fortunately this antenna isn't mounted too high so I've been able to easily check it, but found no signs of damage to it, the connectors or coax.
TreyH
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Re: XPR8300 Interference & Hang Time

Post by TreyH »

Sorta sounds like MotoTrbo interference ..
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