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Thread: Quantar roar or intermittent buzz saw sound

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    Default Quantar roar or intermittent buzz saw sound

    I had the opportunity to do a little research on the topic as I was programming and tuning a bunch of Quantars on different bands and tuning a bunch of duplexers around the same time. I had a couple of really old PD 522 duplexers that are 6 cavity but only one pass and one notch on the transmit side and 2 pass/2 notch on the receive side. This duplexer was designed when crystal controlled transmitters with tube PAs were the norm. All the Quantars I connected to the PD 522 tuned to spec roared like crazy which gave me a test bed to play with.

    I found adding additional band pass cavities and narrowing up the receive side had no change in the roar but adding just one 5" can set for .5dB loss (UHF in this case) to the transmit side was enough to kill the buzz saw and make it go away on the PD 522 duplexer. The skirts on this 5" can were about 23dB down 5MHz away at the receive frequency. Adding a high level moderate gain preamp at the receiver brought the roar back and adding additional cavities to the receive side did nothing but adding a second 5" band pass cavity set to .5dBloss or replacing those with a single 10" cavity set for 1dB loss would fix the roar. In another test I programmed different PL tones between transmit and receive and could not get the repeater to roar using just the shitty PD 522 duplexer and no additional pass cavities.

    In an unrelated activity I replaced an existing repeater of another type with a Quantar in a system with a master receive antenna, a window preselector filter with excellent skirts and rejection of the TX band and a master TX antenna about 25ft below the receive antenna. Several transmitters were combined with roughly 5" cans and single isolator to a star connector as a home made combiner. This Quantar roared immediately when put at this site. The vertical spacing could be better between TX and RX antenna but the main problem seemed to be the single pass can used in the TX combiner channel giving only 20-22dB of isolation between TX and RX frequencies. Adding a second can with appropriate length tuned cable between cans fixed the roar. In this case the RX preselector filter was a good 65db down at TX frequencies.

    Putting all this info together tells me the Quantar must have some broad band transmitter noise with PL modulation on it. I have not been able to see or verify that on a spectrum analyzer but its the only thing that can cause the roar. The PL decoder in the Quanter seems to be very sensitive and that coupled with A/D and or D/A conversions within the Quantar's audio path giving it some audio delay all add up to a feedback loop that is excited by the transmitter and its kept going by the sensitive PL decoder and audio delay. Not much different than acoustic feedback in a church PA system. The test where I programmed a different PL between transmit and receive resulting in no roar also proves to me that its the receiver picking up broad band transmitter noise with PL modulation that starts and feeds the roar.

    So how do you fix the Quantar roar or buzz saw? First make sure your duplexer is actually up to the task of snuffing any transmitter noise that will fall within the receive frequency. I don't have a number on this but 40dB or more down at the RX frequency is probably a good start. Some duplexers are just not up to this task. Next make sure your duplexer is properly tuned and meeting spec. If the roar persists then add additional transmitter filtering and a single pass cavity in the transmitter path seems to fix the problem in most cases I've seen. You will have some additional TX loss to the tune of .5 to 1dB but its worth it to make a Quantar play nice.

    If anyone has more info or comments on this topic, please chime in.


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    Have you swept the Quantar receiver preselector to determine its ultimate out of band rejection? I had a problem with low band Midland repeaters receivers not meeting their TIA603 spurious response out of band at about 1/3 of the operating frequency. In this case, the duplexer also had poor out of band response, essentially a wire from the TX and RX ports and the TX noise would enter at that spurious frequency. To recap, the desense problem was not the TX or duplexer, it was poor receiver spurous rejection. So no noise was seen at the expected operating frequency due to the notch. But the spurious response provided a path into the receiver.

    As I recall the receiver input frequency was to be in the 45 MHz range while the spurious response was in the 14.5 MHz range. After determining this, spectrum analysis showed that significant wideband noise in the 14.5 MHz range was generated in the PA and was readily conducted through the TX-RX pseudo bandpass duplexer. A different duplexer with improved bandpass performance resolved the problem. However the repeater model was rejected by the client for non compliance to TIA603D and the vendor subsequently provided an improved model.
    Last edited by RFI-EMI-GUY; 3 Weeks Ago at 09:52 AM.

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    A plot of a properly tuned preselector is in my post here:

    https://communications.support/threa...9591#post79591

    If, as CARC383 suggests, this is broadband TX noise then a band pass element on the TX output will be required to clean up the output spectrum.
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    I recently encountered a Quantar making a buzz saw noise as a result of an intermod mix. It was suggested to me that it was going into P25 mode (it's a conventional analog repeater), but that's not what it turned out to be. As the op noted, its an artifact of it's DSP audio handling. There's nothing wrong with the transmitter, no unusual wideband noise or anything like that. It's the radio's own transmitter being heard by it's own receiver, DSP processed, sent out to the transmitter and back into the receiver. Lather, rinse, repeat.

    It sounds for all the world like some digital format being mangled through an analog radio. A lesser radio would have gone into acoustic feedback howling. I confirmed the diagnosis by running a sample of the tx signal through a mixer and back into the receiver. I could recreate it at will. It doesn't need to be a clean mix product back into the receiver, just enough so some piece of the audio is in a loop.

    The fix is better isolation between the TX and RX antenna ports.

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    Quote Originally Posted by Astro Spectra View Post
    A plot of a properly tuned preselector is in my post here:

    https://communications.support/threa...9591#post79591

    If, as CARC383 suggests, this is broadband TX noise then a band pass element on the TX output will be required to clean up the output spectrum.
    It would be curious to sweep that preselector over a much wider range from the low to high image frequencies. Actually TIA603D has guidance on how far to test for spurious responses. This can be tested in the radio by tuning an RF signal generator slowly, or by calculating the potential spurious and image frequencies and testing those specifically.

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    Good analysis Car. Quantar growling / howling is well known on high power repeaters, and is usually an indicator of a transmission line problem. ie: serious impedence mismatch. The energy has got to go where the energy has got to go.

    It can also enter the station through unshielded auxillary wiring the console wireline. The wierdest entry point I've encountered was a site GPS ref that got NEMA over serial cable from an outside rcvr to generate 10MHz clock, 1PPS, *and* (bonus) the xmit PL. One broken drain wire caused all kinds of grief.

    For others reading, in my experience, setting rx pl to not equal tx pl is a good test to prove there is poor isolation between tx/rx on any repeater. It won't solve the problem. It just prevents continuous oscillation. There will be some serious degradation of the repeat audio envelope as people attempt to talk through. It is not the cheap and dirty fix.
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    Intermod mix is a potential cause. My testing in the first post here was closed loop test bench with no antennas involved but I forgot to mention a roar problem I was involved with over 10yrs ago on a US Coast Guard Quantar at a very busy RF site. Everything was checked hardware wise and we had tried some preselector filters at the receiver front end with no change. We then spent a lot of time monitoring the surroundings with a spectrum analyzer and eventually identified an amateur repeater in another building at the same site that coincided with the roar most of the time. We handed the information over and the amateur repeater owner was contacted almost immediately to shut down and it turns out they had no isolators which were required at that site. After changes were made to the offending amateur repeater the Coast Guard Quantar roar problem went from every day to very seldom like once a month. So an Intermod mix that includes the roaring Quantars own transmitter is absolutely a potential source.

    Quote Originally Posted by wa6jbd View Post
    I recently encountered a Quantar making a buzz saw noise as a result of an intermod mix. It was suggested to me that it was going into P25 mode (it's a conventional analog repeater), but that's not what it turned out to be. As the op noted, its an artifact of it's DSP audio handling. There's nothing wrong with the transmitter, no unusual wideband noise or anything like that. It's the radio's own transmitter being heard by it's own receiver, DSP processed, sent out to the transmitter and back into the receiver. Lather, rinse, repeat.

    It sounds for all the world like some digital format being mangled through an analog radio. A lesser radio would have gone into acoustic feedback howling. I confirmed the diagnosis by running a sample of the tx signal through a mixer and back into the receiver. I could recreate it at will. It doesn't need to be a clean mix product back into the receiver, just enough so some piece of the audio is in a loop.

    The fix is better isolation between the TX and RX antenna ports.

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    Quote Originally Posted by RFI-EMI-GUY View Post
    It would be curious to sweep that preselector over a much wider range from the low to high image frequencies. Actually TIA603D has guidance on how far to test for spurious responses.
    I think the IF is 106 or so MHz with high side injection so a sweep up to about 700 MHz would be in order. I'll try and do that today.
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    I am having the same issue with a VHF Hammy Quantar. I am going out this week to turn power back and see if it helps. It happens at random times it seems, more towards the evening time.
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    Significantly reducing power could impact the roar. I didn't play with power levels and all the testing I did and units in the field I played with were at or near full power. If you can round up a VHF pass cavity and place that in the transmit path I would bet a lunch your roar will go away. You might need a critical length of cable between the duplexer and extra cavity to keep the return loss at minimum and achieve the best skirts as the extra can will become part of the duplexer.

    2M is difficult with a 600KHz spread as a single 5" can may only have 10-12dB skirts at 600KHz with .5dB insertion loss and maybe 15-18dB at 1dB loss. That might be enough and it would be great to see some test results and add to the info here. Any preamp on that machine?


    Quote Originally Posted by moetorola View Post
    I am having the same issue with a VHF Hammy Quantar. I am going out this week to turn power back and see if it helps. It happens at random times it seems, more towards the evening time.

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    Quote Originally Posted by CARC383 View Post
    Any preamp on that machine?
    Negative on the preamp, that is one thing I highly frown upon. Most of the hams, that is one of the first things they want to do around here, "Install a preamp will fix the coverage issue" LOL.

    Thanks for the info!
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    I've done some real world research on preamps that I can post in the near future. When done properly the repeater as a system can improve upwards of 6dB or so in receive sensitivity with no ill effects and weak handhelds are picked up noticeably better. A low loss duplexer and low loss receive filter will usually have way less insertion loss than the front end filtering within the repeater and that feeding a good preamp will provide a lower system noise figure than the stock repeater. Good modern low noise figure, moderate gain, high IP1/IP3 preamps will have much better specs than what's inside a 30yr old repeater. The problem I see is its rarely implemented properly and people go stupid on preamp gain and shitty IP1/IP3 specs with no additional filtering to protect the preamp, trashing out an otherwise ok repeater. This topic needs its own thread.

    Quote Originally Posted by moetorola View Post
    Negative on the preamp, that is one thing I highly frown upon. Most of the hams, that is one of the first things they want to do around here, "Install a preamp will fix the coverage issue" LOL.

    Thanks for the info!

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    Quote Originally Posted by Astro Spectra View Post
    I think the IF is 106 or so MHz with high side injection so a sweep up to about 700 MHz would be in order. I'll try and do that today.
    sweep low as well.

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    Had time today so here's a Quantar T-band preselector on 494 MHz

    Image 1: start is 300 kHz stop is 1 GHz vertical scale is 15 dB per div

    IMAGE01.PNG


    Image 2: centre is 494 MHz span is 300 MHz vertical scale is 10 dB per div

    IMAGE02.PNG
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    From what I can see on the plots the insertion loss is a good 2dB and possibly 3dB if you calibrated the instrument and the 0dB line is honest. At about +/- 15MHz from center is maybe 40dB down. Not great but the Quantar preselector is a very small module for what its doing.

    If you have space in the cabinet for a couple of 5" 1/4 wave cans they will be about 40dB down at only +/- 6MHz at UHF and with 1dB insertion loss. Put a moderate gain low noise amp after that and you will have improved the system noise figure by about 2dB since the preselector cans and new preamp will determine most of the system noise figure. Keep the preamp gain in check and enjoy less noise on handhelds using the system.

    And thanks very much for the plots.

    Quote Originally Posted by Astro Spectra View Post
    Had time today so here's a Quantar T-band preselector on 494 MHz

    Image 1: start is 300 kHz stop is 1 GHz vertical scale is 15 dB per div

    IMAGE01.PNG


    Image 2: centre is 494 MHz span is 300 MHz vertical scale is 10 dB per div

    IMAGE02.PNG

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    looks like it is doing what it should do.

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    Quote Originally Posted by CARC383 View Post
    Significantly reducing power could impact the roar. I didn't play with power levels and all the testing I did and units in the field I played with were at or near full power. If you can round up a VHF pass cavity and place that in the transmit path I would bet a lunch your roar will go away. You might need a critical length of cable between the duplexer and extra cavity to keep the return loss at minimum and achieve the best skirts as the extra can will become part of the duplexer.

    2M is difficult with a 600KHz spread as a single 5" can may only have 10-12dB skirts at 600KHz with .5dB insertion loss and maybe 15-18dB at 1dB loss. That might be enough and it would be great to see some test results and add to the info here. Any preamp on that machine?
    I dropped the transmit power from 75watts to 30watts. It still happens, but to a lesser extent, IMO. Although the local hams, say it is about the same, as far as how often.
    It seems to raise it's ugly head, mostly when a net is going on.

    I have a cavity that I am going to tune up and try.
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