0185417U05 (PD526-4-2) duplexer tuning issue

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gmrs122
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0185417U05 (PD526-4-2) duplexer tuning issue

Post by gmrs122 »

I recently picked up a used MTR2000 and a 0185417U05 / PD526-4-2 to go with it (yes, I know that many won't buy used duplexers, for very good reasons) and am trying to figure out if I can make the duplexer work. I took it to a professional radio shop for tuning and they told me that sure enough, one of the band pass rods (center one) on the low pass side appears to be stripped. I don't know if that means that he could not tune the pass for that cavity at all, or could not get it all the way where it needed to be. When I picked it up the tech was not in so I could not talk to him to determine exactly what the impact to the tuning was. My understanding is that on the Hi (RX) side the pass and reject tuning went fine, so my RX side should be fine. On the TX side the reject tuning went fine, but as I said, one of the three band pass rods was stripped.

I won't be able to put this on the repeater until this weekend to test it, but I am trying to get a feeling for the impact this will have. The previous setpoint for the cavity in question was 453.xxxx MHz and my freq is 462.xxxx. I believe that "all" this means is that the one cavity will not be tuned well to pass my TX freq resulting in some attenuation. I will have to see how much loss it is to determine how much of a problem it is. One thought I had is could I bypass that cavity by simply jumpering over it (it is the middle cavity of the three on the low pass side)? That would give me two cavities on the TX side (which I don't think would be too bad), but I would still have three on the RX side and keeping the TX freq and other junk out of the receiver is the primary concern anyway.

In case it matters it is a 5 MHz split, relatively low power at 50W. rural area, isolated very low RF site with only the single repeater.

Is my understanding of the impact correct? Would bypassing the bad cavity work?

Thanks
VE3HKB
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by VE3HKB »

You may wish to wait for the tech to return and have a talk with him.
Was he able to tune the bandpass despite the stripped thread? Usually the workorder would indicate if retuning was successful, and what the final loss and isolation figures were.
http://www.repeater-builder.com/motorol ... plexer.pdf

If the one bandpass stage was not able to be tuned, yes, you'd have more attenuation on the TX side. How much more is the issue. The PD526-4-2 is spec'd at 1.3dB transmitter to antenna. 1dB more may not be noticeable, but 10dB would. It all depends on if they were able to tune it and what the final figures are.

The difference in previous and desired frequencies only matters if the latter are outside of the operating range. 453 and 462 are both within the range of the 435-470MHz model, so that aspect should be OK.

Would it work with one cavity bypassed? - yes, but likely you would notice a difference in isolation. Remember, a duplexer's function is to connect the TX and RX to the same antenna, while isolating each from the other as much as possible. Each cavity performs a bandpass and reject function. More cavities = more insertion loss but more isolation. Just a guess, but I would estimate that bypassing one cavity on the TX side would decrease TX-ANT insertion loss by 0.3dB and reduce TX-RX isolation by at least 10dB. And that's assuming the unit was retuned in that configuration.

Real-world impact? Unknown. If it's located in a rural area, you may be dealing with weak incoming signals. If you can't hear mobiles above your own transmitter noise (desense), what's the use?

Before you consider any of these potential issues, find out what the condition of the duplexer is first.
Satelite
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by Satelite »

Hello:
I have a couple of celwave PD526 duplexers myself.
Heres my thoughts.
I am always catching guff from another friendly radio shop about using 6 cavity duplexers as in they say im wasting money on the 526 6 cavity as its over kill for the isolation I need.
Ok lets assume that's correct and in my case I do believe Im speced well above needed.
So if the PD526 gives you 100 db plus isolation and 75 is adequate then id say if he loses 10db isolation from 100 plus originaly to 90 db then what the hey hes good to go if its tuned correctly.
Id do it myself and run it = Bet ya itll operate just fine with less tx wattage loss due to one less can too while yer at it :-)
Satelite
Satelite
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by Satelite »

Hello:
I have a couple of celwave PD526 duplexers myself.
Heres my thoughts.
I am always catching guff from another friendly radio shop about using 6 cavity duplexers as in they say im wasting money on the 526 6 cavity as its over kill for the isolation I need.
Ok lets assume that's correct and in my case I do believe Im speced well above needed.
So if the PD526 gives you 100 db plus isolation and 75 is adequate then id say if he loses 10db isolation from 100 plus originaly to 90 db then what the hey hes good to go if its tuned correctly.
Id do it myself and run it = Bet ya itll operate just fine with less tx wattage loss due to one less can too while yer at it :-)
And the critical issue is keep the tx out of the rx and he still has 3 cans filtering out the tx from the rx on the rx input side so hes good id think.
Satelite
gmrs122
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by gmrs122 »

I finaly aquired the equipment to tune this duplexer myself. I bought a calibrated HP 8920A, which seems to work well enough. The notch tuning is quite straight forwards and I have been able to achieve roughly -105dB on both sides.

The issue I am having is the correct procedure for the pass band tuning using a RLB. I have an Eagle 150X3. With a 50Oohm terminator on the DUT port I get around -45dB so it looks good. The uncertaintly I am having is exactly how to connect to the duplexer. For example for the TX freq, should I connect to the TX side or the antenna connector (terminating the two other unused duplexer ports). Same question for the RX Freq, do I connect to teh RX port or the antenna?

What I did is the following:

@TX Freq - I connected RLB DUT to duplexer TX port and terminated the duplexer RX and antenna ports. I can tune this nicely down to around -40dB. I think that is the correct way to connect for TX. If I swap the DUT to the antenna port though the return loss is worse and is around -20dB at the TX freq and the previous tuning. So it matters which way it is connected.

For the RX side @RX freq I connected the RLB DUT to the antenna and terminated the other two. Again i can tune this just fine to around -40. But again, if I switch the DUT to the RX port, the return loss is worse. It is not symetrical.

I have seen conflicting info on the web. Some things say connect the RLB to the TX and RX ports while terminating the others. I also found one set of instructions that said leave the other branch unterminated.

Again, I think the way I did it (TX to TX port and RX to antenna port) is correct, but I am not sure.

Can anyone clarify the correct way to connect the RLB for pass band tuning for this duplexer?

Thanks
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Wowbagger
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by Wowbagger »

Ideally, you want to check return loss on all three ports - remember, "return loss" is another way to say "power acceptance", which is another way to say "impedance match".
You want the Transmit port to have a high return loss, so that all the power developed by the transmitter is accepted by the duplexer.
You want the antenna port to have a high return loss, so that all the received power is delivered to the duplexer.
You want the receive port to have a high return loss, because that means the receive port is matched to 50Ω, and will deliver all the power to the receiver.

That said: once you get everything adjusted to 50Ω, you will need to do a final in-place set of tweaks, as it is very unlikely your transmitter, receiver, or antenna are truly 50+0iΩ, and you want the duplexer to be matched to what it's actually going to be hooked up to. At a minimum, check the return loss between the actual antenna and the duplexer, and the return loss between the actual transmitter and the radio. However, if things aren't close to 50+0iΩ, your return loss bridge will lie to you, as it is a 50+0iΩ load.

Edit: I wanted to add: don't get too crazy about impedance matching, however. The difference between delivered power at -30dB return loss and -90 return loss is less than .1%; be more concerned about isolation between TX and RX than matching.
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.
gmrs122
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by gmrs122 »

Thank you, that was very helpful. Specifically your edit. I made me realized that I was focused too much on achieving the highest return loss number in one direction. I think it is better to get a return loss of -30 on all ports that -45 on one and -17 in the other direction. So with that compromise in mind I was able to get a return loss of around -32 on all ports. I was also able to get better than 105dB isolation and around 0.5dB insertion loss on both sides. All in all I am happy with the tuning.
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kcbooboo
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Re: 0185417U05 (PD526-4-2) duplexer tuning issue

Post by kcbooboo »

Usually with that duplexer, you tune each section separately. Disconnect all six cables. For each section, the DUT port of the RLB goes into one jack, and you terminate the other jack with a good 50 ohm load. Tune the passband to move the "notch" you see (best return loss) to the desired frequency. Then move the load and RLB to the next section. Do this for all six sections. Then tune the various notches without the RLB. When you're done, reconnect the jumper cables. Remember that the two going to the antenna TEE connector are different length than the other four. Assuming you have the correct length cables for the intended frequency, that should do it.

Bob M.
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