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3rd Order Intermodulation Interference on Repeater Receiver

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BlueDevil

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What is the best way to reject 3rd Order Intermodulation Interference on a Repeater Receiver? Add additional filtering on the receive side of the duplexer?
 

cmdrwill

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"What is the best way to reject 3rd Order Intermodulation Interference on a Repeater Receiver?"
Ideally at the source of the mix.
 

RFI-EMI-GUY

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If it is a receiver mix, a band pass filter, a notch filter or both may be required. What are the specific frequencies involved?

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BlueDevil

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The repeater is a Tait TB8100 and operates on TX: 155.2500 and RX: 155.6850 and is getting 3rd order intermodulation interference on 154.8150.
 

prcguy

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Are you saying the repeater receiver on 155.6850 is getting interference when its transmitter on 155.250 and another transmitter on 154.815 are on at the same time? If so is the other transmitter located at the same site?
prcguy

The repeater is a Tait TB8100 and operates on TX: 155.2500 and RX: 155.6850 and is getting 3rd order intermodulation interference on 154.8150.
 

mm

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perfect 3rd order: (155.25 x 2) - 154.815 = 155.685 , the repeater input.

The fix may not be easy either because the non-linear location of the mix could be some distance away from the repeater and or the 154.815 tx which means that you need to trace the location where the 2 x 155.25tx output minus the 154.815tx is mixing and generating the third order product

Drive around with a high quality receiver tuned to 155.685 when the IMD is interfering and find the location where it is strongest.

Be aware that this is very tiring and time consuming to trace down,

I had one instance where the location of the mix was generated on a neighboring Mtn.peak located 1/2 of a mile away, I'll call it Peak B.

The mix on Peak B was caused by a rusty joint coming from an un-bonded corrugated roof joint but the two source transmitters mixing on peak B were both located on Peak A, located 1/2 of a mile away.

No receivers on peak B had any interference to them either, the only issue with peak B was that Damn rusty roof joint which acted like a diode mixer generating the third order product coming from peak A's 2 transmitters and re-radiating the Third Order PRODUCT back to the receiver on peak A.

Ugh, this was frustrating and hard to trace down, it took more than 3 weekends of driving around to find the location as there were 4 different Mtn. top sites spaced 1/4 to 3/4 of a mile in various directions.

It took some well filtered, commercial receivers with high dynamic range, programmed to the third order mix frequency to find the location.

Don't use scanners either as they overload easily and can fool you indicating false locations of the mix.

A high dynamic range analyzer or a Comm test such as an HP8920 also works if available but in our case it was easier to program up some good dynamic range mobiles which enabled several groups to search several different areas at the same time.
 

RFI-EMI-GUY

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The repeater is a Tait TB8100 and operates on TX: 155.2500 and RX: 155.6850 and is getting 3rd order intermodulation interference on 154.8150.
OK, we'll you have to first rule out a receiver mix. This is done by measuring the signal level of the mix by metering RSSI or limiter voltage. Then insert an attenuator, say 2 or 3 dB in the antenna port of the receiver and measure the level again. Use a signal generator to replicate those RSSI or limiter voltages. If the attenuated value drops exactly the same dB value of the attenuator, the IM product is generated externally from the receiver. If it drops nonlinear lyrics, say 6 or 9 dB, the mix is inside the receiver.

In the case you are describing, you could have IM generated within the transmitter itself. If so, measuring the product would require a spectrum analyzer and a notch filter set at the transmitter frequency to gain enough dynamic range to see it.

Receiver IM will call for a notch filter at the undesired frequency.

Transmitter IM will call for an isolation panel (circulators) and a harmonic filter. A notch filter at the undesired frequency may be required as well.

The IM product could also be generated in the antenna, the coaxial connectors and even in the coax cable itself if it is constructed of anything other than copper conductors. Low passive inter modulation Low PIM, antenna systems are best solution.

You might even have antenna support structure corrosion creating PIM conditions.

Is this a rooftop or tower? What kind of duplexer etc?

A third order mix is the worst to deal with.

Oh and the TX RX spread of 435 KHZ? Is your duplexer isolation up to this?

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BlueDevil

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Unfortunately I they will not let me be directly involved with the troubleshooting of this issue or the radio system in general. I definitely don't want to be in charge of our radio system but I would really like to shadow and be an assistant to the radio techs that we use. The radio techs we use are from a neighboring county and not a private business.

Our radio system is a VHF Analog Simulcast Quasi-Sync II system with Tait TB8100 radios. We have 3 separate simulcast channels. We maintain 4 primary TX/RX sites and an additional 3 RX only sites. Our neighboring County also operates a 3 channel simulcast system. One of their channels is transmitting on the 154.8150 frequency in which we are getting the interference. Their transmitters are powerful and well positioned to push this frequency well into our County and hit all of our simulcast sites. The radio techs that work on our radio system are primarily employed by the neighboring county in which we are experiencing the interference. We contribute a partial FTE and share the radio techs.

PRCGUY: You are correct. We are getting intermodulation interference when our simulcast repeater channel operating on TX:155.2500 and RX:155.6850 is being uses while our neighboring county is using their simulcast repeater transmitting on 154.8150. It is pretty obvious that this is where the interference is coming from as you can even pick our parts and pieces of the conversation from the neighboring county. We did not experience this issue until we updated our simulcast system to the Quasi-Sync II with the Tait TB8100 radios. The same frequencies and repeater sites were being used by both counties prior to our upgrade.

RFI-EMI-GUY: I would agree that we are asking a lot of our filtering systems by only utilizing a 435kHz spread. Our radio techs seem to think that this is not an issue. At most of our sites we run all three simulcast radios through the same antenna system by utilizing a couple different duplexers other filters along with an antenna combiner so we can use just 1 antenna for all three radios. This seems like it would just suck the dB/gain right out of the system. It seems like a transmitter at 100watts would only have a fraction of that as ERP.

The last I heard the radio techs were waiting to hear back from Tait Engineers on a possible fix or solution. Seems like the solution might be easier than waiting to hear back from the Tait Engineers. However I am just a novice trying to learn and gain as much experience and knowledge as I can. Hence my post on this forum.
 

RFI-EMI-GUY

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BlueDevil If I understand correctly you used the same frequencies successfully without the interference before the Tait upgrade? The problem could be in the transmitter combiner system or the receiver multicoupler system. Did TX-RX provide the combines? They have always been helpful to me in designing these systems and knowledgeable as well. I agree it is a good idea to shadow the techs and learn all you can about the problem and solution. I would start out by drawing a detailed schematic of every component of the antenna combining system along with part numbers and tuning frequencies. Sometimes assumptions are made in the filtering design that turn out to be incorrect. The existance of 154.8150 MHZ from a neighboring system is one of those things the designer may have been unaware of. Your fix may be as simple as an additional notch filter at that frequency. And Isolators if missing or insufficient. Make that schematic, ask questions. Find out what testing is planned to isolate the mix point from the receiver, the transmitter, the antenna or other external source like a neighboring transmitter site. One thing you may want to guard against is the techs tweaking any factory set filters in the combiner without extensive discussion with the manufacturer. As PRCGUY said this can be frustrating. Especially if the mix is off site or you have multiple mix points. The VHF band is not properly allocated to facilitate duplex repeater operations. The industry and FCC dropped the ball on this year's ago when narrow banding was planned. They could have fixed it, but then folks like you would not have these headaches that Motorola and Harris can point to and ask why you didn't go to 800 MHZ for million$ more.

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BlueDevil

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The techs seem to have isolated one specific site that is a significant source of the problem. Shutting off the receiver at that site has almost completely stopped the intermodulation interference.

As far as shadowing the techs and learning as much as I can doesn't seem to be something the radio techs are very interested in doing.
 

RFI-EMI-GUY

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The techs seem to have isolated one specific site that is a significant source of the problem. Shutting off the receiver at that site has almost completely stopped the intermodulation interference.

As far as shadowing the techs and learning as much as I can doesn't seem to be something the radio techs are very interested in doing.
I am curious, is that site the closest one to the 154.8150 MHZ transmitter?
 

BlueDevil

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I am curious, is that site the closest one to the 154.8150 MHZ transmitter?
The site the shut that they attribute the vast majority of the problem to is not the closest site but it is the centralized hub where the computer and simulcast system and microwave system is all tied together and connected to our Dispatch center via Fiber.
 

RFI-EMI-GUY

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The site the shut that they attribute the vast majority of the problem to is not the closest site but it is the centralized hub where the computer and simulcast system and microwave system is all tied together and connected to our Dispatch center via Fiber.
Does this site have VHF base or RF control stations that the other sites do not?

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BlueDevil

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I have never had the opportunity or privilege of being able to visit the sit however I imagine that it does. It has more communications equipment at this site than any of the other sites. It is also the "brains" of the whole simulcast system. There is A LOT of RF noise at this site and in the general immediate vicinity.
 
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