Celwave Duplexer Tuning Question

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Celwave Duplexer Tuning Question

Postby cusackmusic » Wed Apr 07, 2010 3:56 pm

I have a VHF Celwave 636 series VHF duplexer. I haven't tuned a duplexer since 1994, so I'm a little rusty. Anyway, I put my IFR1200 on "tracking med", and tuned it to the lowest notch at the opposite frequencies. When I get done, I have a few issues/questions:

1. The TX radio now has a sensitivity of 2KuV. If I "fine tune" it by transmitting the RX frequency into it from the common port, and tweak it to lower sensitivity, I can get it to barely RX at all. But when I go back to the duplex mode on the IFR, the notch os nowhere near what it was. I recall tweaking them in like this back in the day, but I don't recall the notch coming up. What am I doing wrong (or right)?

2. The TX power is attenuated by A LOT. 20W in, 12W out.

3. What is the difference between tracking low, trakcing med, and tracking high on my IFR? I have a service manual, but can't seem to find an operation manual on the thing. It appears that I can fine tune better in tracking high?

Any thoughts?

Thanks,

Jon
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Re: Celwave Duplexer Tuning Question

Postby IMBEJames » Wed Apr 07, 2010 4:33 pm

Refresh my memory... is this a notch-notch type mobile duplexer?
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Re: Celwave Duplexer Tuning Question

Postby cusackmusic » Wed Apr 07, 2010 5:15 pm

Yes, it is a 6 cavity notch, 3 for each side.
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Re: Celwave Duplexer Tuning Question

Postby tvsjr » Wed Apr 07, 2010 7:20 pm

Low, medium, and high should change the amount of signal the tracking generator generates, though I can't remember the levels for each setting. Connect the duplex out to the antenna in and measure it if you'd like. To me, it sounds like you have your notches in the wrong place, although I don't really understand what you're saying. Why are you measuring sensitivity on the transmit radio? It's transmitting, not receiving.

Generate into the TX port and receive on the antenna port (terminate the unused port with a 50ohm load). You want the notch to be on the RX frequency. Tune the cans to get the deepest notch on the RX frequency. The idea here is to minimize the amount of power generated on the RX frequency by the TXer. Jump up to your TX frequency and measure the insertion loss - expect 1.2-1.5dB.

Next, generate into the RX port and receive on the antenna port (again, terminate the unused port). Maximize the notch on the TX frequency - minimize the amount of power from the TXer getting into the RXer. Switch to your RX freq and measure insertion loss - again, expect 1.2-1.5dB.

What RX-TX split are you trying to use? I hope the answer isn't 600KHz... the PD/Celwave 636 has a minimum RX-TX separation of 4.5MHz.
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Re: Celwave Duplexer Tuning Question

Postby Jim202 » Thu Apr 08, 2010 2:47 am

The main error that most people make when trying to tune a duplexer is that they don't keep
all the ports terminated while doing the tuning. The best way to do this is to use a pad in
series on all the ports during the whole process. I use at least a 6 db pads. Put them on
each of the ports right at the duplexer. This way, no matter what cables you move around,
you will always maintain a 50 ohm load on the port.

You tune the RX side for a notch in the TX frequency. Then tune the TX side for a notch
at the RX frequency. You may end up having to use a receiver as the device your using
to do the notching with. In most cases, the receiver in the IFR will not go down low
enough. You may also have to use a low power TX as a source (2 watts or less), depending
on just how good of a duplexer you have.

Over the years, I have seen some real hog jobs done on duplexer tuning. If they are being
moved from a much higher channel set, you may also have to change the cable lengths
between the cans and going to the T used for the antenna port.

Band pass, band reject filters make poor duplexer cavities for repeater use.

Jim




cusackmusic wrote:I have a VHF Celwave 636 series VHF duplexer. I haven't tuned a duplexer since 1994, so I'm a little rusty. Anyway, I put my IFR1200 on "tracking med", and tuned it to the lowest notch at the opposite frequencies. When I get done, I have a few issues/questions:

1. The TX radio now has a sensitivity of 2KuV. If I "fine tune" it by transmitting the RX frequency into it from the common port, and tweak it to lower sensitivity, I can get it to barely RX at all. But when I go back to the duplex mode on the IFR, the notch os nowhere near what it was. I recall tweaking them in like this back in the day, but I don't recall the notch coming up. What am I doing wrong (or right)?

2. The TX power is attenuated by A LOT. 20W in, 12W out.

3. What is the difference between tracking low, trakcing med, and tracking high on my IFR? I have a service manual, but can't seem to find an operation manual on the thing. It appears that I can fine tune better in tracking high?

Any thoughts?

Thanks,

Jon
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Re: Celwave Duplexer Tuning Question

Postby Bill_G » Thu Apr 08, 2010 5:00 am

Doh! 6db pads on every port so they are all terminated and you can move your cables at will - why didn't I think of that!? Thanks Jim.

Jon - You must have a 1200 Super S. I don't have a tracking gen in my 1200S's. But, I suspect tvsjr is correct - that tracking low-med-high are the output levels from the gen. You will need the high output to see the deepest notch. Set your sweep for 1mhz to rough it in, and then 100khz to set center.

1.5 to 2db insertion loss is totally normal. Definitely pay attention to the 4.5M split. You will not be able to get them narrower.
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Re: Celwave Duplexer Tuning Question

Postby cusackmusic » Thu Apr 08, 2010 6:08 am

I have an IFR1200S, with Option 5 (Tracking Gen).
TX 150.995, RX156.030

I tuned for lowest notch on TX freq in RX cavities, and RX in TX cavities. The thing that I maybe didn't make clear was that I injected the RX freq into the ANT port, Connected the RX radio to the TX port, and fine tuned it (less than 1/16th of a turn) to notch out the actual repeater's receiver sensitivity. I recall doing this years ago, but seems that you guys are saying not the thing to do?

Another question, IFR related: When I change the sweep while TXing from the IFR, the spectrum changes slightly left or right on the Spectrum screen. This makes me wonder, how can I be sure that the center of my screen is the actual frequency I'm trying to notch? Maybe the center of my screen is wrong, and that's why I had to fine tune? What RX sensitivity should I get, or should I not be able to generate enough signal from the IFR to open the receiver, as I think someone implied?

Does it matter which I TX into, and Out of on the IFR? I may have injected into the TX/RX, and monitored on the ANT port? I would think it wouldn't matter, since it's a notch. Theoretically, should I transmit into the ANT port while tuning the RX cavity, and transmit into the TX port while tuning the TX? That way I'm tuning the actual direction of the signal? Just a thought.

In any case, even if I'm off a little bit, I'm losing almost 50% power. Seems I'm between 2-3db.

Also, the RX cavities seem fine, I have a .2uV sensitivity on the Receiver. So I know I didn't tune them "backwards" anyway.

I'm going to try again and make sure I'm monitoring on the ANT port.
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Re: Celwave Duplexer Tuning Question

Postby motorola_otaku » Thu Apr 08, 2010 7:32 am

I have a little 1W N-male dummy load I use for port termination. And yeah, if you don't terminate the TX port while tuning the RX side and vice-versa it'll never come out right.
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Re: Celwave Duplexer Tuning Question

Postby cusackmusic » Thu Apr 08, 2010 9:15 am

Here's some pics from the IFR of both tuning images. The RX cavity seems to be a lot nicer than the TX. Is there something wrong with my duplexer?

http://www.facebook.com/photo.php?pid=5569013&l=314ae1918a&id=789168205
http://www.facebook.com/photo.php?pid=5569014&l=9c27a3a33d&id=789168205
I tried to reverse tune, but that made one set of screws go all the way in, and the other way out, so I'm sure I was right the first time. It gets confusing, because the RX side says 158.000, and I'm tuning to 150.995, and I keep second guessing myself. They are labeling the intended frequency, not the tuned frequency on these, right? The TX side was originally 153.000 I think?

I don't have any pads, and my cables aren't the best, but I'm hoping I can get close enough with what I have.

Thanks for all of your help so far.
Last edited by cusackmusic on Thu Apr 08, 2010 7:42 pm, edited 2 times in total.
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Re: Celwave Duplexer Tuning Question

Postby motorola_otaku » Thu Apr 08, 2010 5:11 pm

Fixed your image link:
Image

From that, it does look like one or two of your notches aren't on the target frequency. The end result should be a nice smooth curve on both ends without additional dips on either side and about 200 kHz of usable bandwith down at the bottom. Not having the opposite side 50-ohm terminated or using bad cables can/will also produce that result. For my duplexer tuning I use two 3' RG-142 cables with good nickel/silver/Teflon N-connectors.
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Re: Celwave Duplexer Tuning Question

Postby cusackmusic » Thu Apr 08, 2010 6:51 pm

Can you fix the other image link?

The extra notch from that picture is coming from somewhere else, not sure where. All three cavities were notched to the middle freq. I'll see if that extra notch moves around with one of the cavities, But I think it was just hovering there.

I'll also try to get better cables, anybody have a good source? Digikey has some 24" RG-142 N-N, for $20. But I'd have to use an adapter on the IFR side. It would be better than the 3 adapters I'm using now, I'm sure.
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Re: Celwave Duplexer Tuning Question

Postby jackhackett » Fri Apr 09, 2010 5:06 am

tvsjr wrote:Generate into the TX port and receive on the antenna port (terminate the unused port with a 50ohm load).


I generate into the the antenna port, guess it's six of one.....
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Re: Celwave Duplexer Tuning Question

Postby Jay » Fri Apr 09, 2010 5:35 am

Enjoying this discussion...I've bought some cables recently for stuff like this from:

http://universalweb.com/CustomCablesUSA/index.html

Not to hijack the thread, but where can I get pads?

Jay
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Re: Celwave Duplexer Tuning Question

Postby Bill_G » Fri Apr 09, 2010 7:09 am

jackhackett wrote:
tvsjr wrote:Generate into the TX port and receive on the antenna port (terminate the unused port with a 50ohm load).


I generate into the the antenna port, guess it's six of one.....


I like to push the energy in the direction it will be used. IOW gen into the tx port & measure at the ant, or gen into the ant & measure at the rx. But, ultimately it works out the same. I have seen winky cables cause bad tuneups though. Very asymmetrical because of impedance weirdness introduced by loose grounds, dirty pins, and critical lengths.
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Re: Celwave Duplexer Tuning Question

Postby Bill_G » Fri Apr 09, 2010 7:12 am

Jay wrote:Enjoying this discussion...I've bought some cables recently for stuff like this from:

http://universalweb.com/CustomCablesUSA/index.html

Not to hijack the thread, but where can I get pads?

Jay



Tessco carries Bird pads. Pomono elec carries their own. They are available in various atten and pwr levels.
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Re: Celwave Duplexer Tuning Question

Postby Wowbagger » Fri Apr 09, 2010 8:35 am

I don't remember - does this duplexer have variable capacitors or tuning rods in addition to the tuning knob? I've seen that sort of pattern where you couldn't get the notches to line up because of the caps/rods being misadjusted.
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Re: Celwave Duplexer Tuning Question

Postby Bill_G » Fri Apr 09, 2010 8:58 am

Screw rods.
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Re: Celwave Duplexer Tuning Question

Postby GlennD » Fri Apr 09, 2010 3:44 pm

Remember the duplexer does not have receive and transmit ports. It has high and low frequency ports. The notch slopes are not symentrical. If you align them backwards there will be increased insertion loss.
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Re: Celwave Duplexer Tuning Question

Postby cusackmusic » Tue Apr 13, 2010 4:34 pm

OK, it turns out that little blip WAS my error. I think I zoomed in too far, and fine tuned it without seeing the notch move. When I re-tuned it, it went away. And it turns out my loss was a bad cable, which I repleaced with ANOTHER bad cable, which I replaced with a good cable. Very odd. One cable showed 10 Watts, on showed 18 Watts, and the good one shows about 23 Watts, with about 30 Watts in. This seems in line with what you expect out of the duplexer, right?

Thanks,

Jon
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Re: Celwave Duplexer Tuning Question

Postby cusackmusic » Wed May 12, 2010 1:18 pm

OK, Repeater has been installed almost a month now, and seems to be working well. After re-tweaking the duplexer, I get no distinguishable desense of the RX. I wish I had better RX though. In basements downtown, I can hear the repeater, but the RX (of the repeater) is so scratchy, that I'm not being heard.

Thanks to those of you that had constructive things to say, it's was very helpful!

Next project, re-program our pagers with the repeater as a backup paging system. I have to figure out how to make the pagers scan without missing pages. I know this can be tricky, so I'll have to do some more research before I implement this...

Jon
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Re: Celwave Duplexer Tuning Question

Postby Jim202 » Thu May 13, 2010 3:38 am

You probably won't be able to get the pagers to scan and not miss a page. The timing becomes the critical
issue here. One solution is to modify the paging encoding to have a long wait time before sending any tones
after coming up on the paging transmitter. This will allow the pagers to scan and hopefully lock on the active
RF page channel before the tones start to be sent. If they are receiving any activity on the non paging channel
when the page channel lights up, they will miss the page.

Most pagers with multi channel ability, don't have a priority scan built into them. As such, once they stop
scanning on a non page channel, they will stay on that channel for the duration of the activity there. Then
the scan will resume after the activity drops. If a page comes up when the non page channel is active,
you loose the page.

Jim




cusackmusic wrote:OK, Repeater has been installed almost a month now, and seems to be working well. After re-tweaking the duplexer, I get no distinguishable desense of the RX. I wish I had better RX though. In basements downtown, I can hear the repeater, but the RX (of the repeater) is so scratchy, that I'm not being heard.

Thanks to those of you that had constructive things to say, it's was very helpful!

Next project, re-program our pagers with the repeater as a backup paging system. I have to figure out how to make the pagers scan without missing pages. I know this can be tricky, so I'll have to do some more research before I implement this...

Jon
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