MSF5000 TX filter tuning question
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motorola_otaku
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MSF5000 TX filter tuning question
The repeater in question is a MSF UHF C44CXB. The receive tuning instructions were pretty straightforward, but the instructions for tuning the transmit prefilter and postfilter were a little confusing. The manual instructs me to use the tuning probe (I assume the same one used for injecting a signal into the receive prefilter) and tune each load for maximum/minimum reading on the "millivoltmerer". What millivoltmeter? How do I use a tuning probe with a BNC conector to connect to a voltmeter? I finally got it to work by tuning each filter to pass the transmit frequency, using an R2590 service monitor with the tracking generator option, and I get about 45 watts of transmit and .15 microvolt sensitivity with no desense, but I'd really like to know the correct way to do this.
You must be using service manual 6881092E80. They lifted those filter alignment instructions from the original UHF MSF manuals, but they seem to have forgotten the meter that was part of the original list of recommended test equipment. That was the Motorola S1339 RF millivoltmeter.
Motorola came up with this alignment probe and procedure as a way to field tune a multi-section window filter when test equipment available in the field was limited.
At the time the MSF was engineered the current state of the art in service monitors was the R2001 series and did not include tracking generator capability. These filters were undoubtedly tuned at the factory with network analyzers, but at that time a network analyzer was a fairly new concept and cost from $30,000.00 to $50,000.00. The engineers would have never envisioned folks in the field ever having one.
With todays test equipment I think you are better off by doing a swept alignment. I have compared the results of sweep testing a factory aligned [new - never touched in the field] filter to the factory curve sheets and then compared field tuning with the probe method and swept tuned method. If you are careful you will do better with swept tuning. You can easily match the factory tuned shape, bandpass, and insertion loss.
Before you touch any screws, sweep the filter and store a trace of its factory tuned SHAPE. That will be your target for your results on the new frequency.
Motorola came up with this alignment probe and procedure as a way to field tune a multi-section window filter when test equipment available in the field was limited.
At the time the MSF was engineered the current state of the art in service monitors was the R2001 series and did not include tracking generator capability. These filters were undoubtedly tuned at the factory with network analyzers, but at that time a network analyzer was a fairly new concept and cost from $30,000.00 to $50,000.00. The engineers would have never envisioned folks in the field ever having one.
With todays test equipment I think you are better off by doing a swept alignment. I have compared the results of sweep testing a factory aligned [new - never touched in the field] filter to the factory curve sheets and then compared field tuning with the probe method and swept tuned method. If you are careful you will do better with swept tuning. You can easily match the factory tuned shape, bandpass, and insertion loss.
Before you touch any screws, sweep the filter and store a trace of its factory tuned SHAPE. That will be your target for your results on the new frequency.
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motorola_otaku
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The part number for the manual I'm using is 68P81092E05-O, which covers all bands. If there's a more up-to-date version, I wouldn't mind ordering one.
Unfortunately, I've already detuned and retuned this one a few times, so the original factory settings are long lost. I was just sent a firmware upgrade package for this service monitor, which included a computer emulator that's supposed to have screenshot capability. I may play with that over the next couple of days and see if I can get some screenshots of where I have it set at now. Like I said, it's working just fine, but I was just curious as to what the "by the book" method was.
Unfortunately, I've already detuned and retuned this one a few times, so the original factory settings are long lost. I was just sent a firmware upgrade package for this service monitor, which included a computer emulator that's supposed to have screenshot capability. I may play with that over the next couple of days and see if I can get some screenshots of where I have it set at now. Like I said, it's working just fine, but I was just curious as to what the "by the book" method was.
The 6881092E05 is a good manual to have in your library. Also, 6881092E80 is the UHF 'depot' manual and is worth having.
Unfortunately, 6881082E10, the service manual that has all the real meat, is no longer available. You might watch for it on ebay.
Your radio seems to be working fine so your alignment is probably right on.
I have tried the General Dynamics screen capture utility for the R2670 and it works fine. The 2590 looks like a really nice unit - how do you like it so far?
Unfortunately, 6881082E10, the service manual that has all the real meat, is no longer available. You might watch for it on ebay.
Your radio seems to be working fine so your alignment is probably right on.
I have tried the General Dynamics screen capture utility for the R2670 and it works fine. The 2590 looks like a really nice unit - how do you like it so far?
I, too, noticed that little omission from the tuning instructions. Luckily I had an older manual that told me to attach the tuning probe to an RF millivoltmeter and move it around to the various sampling holes in the filters. Then you adjust it more or less like you adjust the receiver's front end: peak the first coil, null the second, move the probe over, null the third, etc.
I found that the post-filter tuned very nicely with this method, as long as you follow the instructions to detune the slugs first and make only one pass. If you go back and try to re-peak and re-null the filter a second time, there's all sorts of interaction that takes place, and you end up chasing your tail.
The pre-filter, while just as easy to tune, did not provide adequate results when I finished it. The station (C74CXB) would not key up if I set it for more than 50 watts out of the antenna connector. I could key it up and ramp the power up to 100 watts (not recommended) but it would not key up at most power settings. I hooked my spectrum analyzer and tracking generator up to it and found a bit more loss than when the factory did it, plus the bandpass had a "tilt" to it - there was more loss at the higher end of the pass band than the lower end. I repeaked it with this setup and got back to just under 2dB loss, and the power limitation went away. Since then I re-did the post-filter the same way, but got no improvement, and actually had a slight bit of desense, so I retuned it with the probe/meter method and left it. I think the screw-on dummy load I used when tuning the pre-filter was probably a different impedance than the input stage of the power amp, but I can't say for sure.
As mentioned earlier, both tuning methods work. It's also a lot easier to get to the filters if you have the outer skin off the station, and an N elbow connector doesn't hurt either !
Bob M.
I found that the post-filter tuned very nicely with this method, as long as you follow the instructions to detune the slugs first and make only one pass. If you go back and try to re-peak and re-null the filter a second time, there's all sorts of interaction that takes place, and you end up chasing your tail.
The pre-filter, while just as easy to tune, did not provide adequate results when I finished it. The station (C74CXB) would not key up if I set it for more than 50 watts out of the antenna connector. I could key it up and ramp the power up to 100 watts (not recommended) but it would not key up at most power settings. I hooked my spectrum analyzer and tracking generator up to it and found a bit more loss than when the factory did it, plus the bandpass had a "tilt" to it - there was more loss at the higher end of the pass band than the lower end. I repeaked it with this setup and got back to just under 2dB loss, and the power limitation went away. Since then I re-did the post-filter the same way, but got no improvement, and actually had a slight bit of desense, so I retuned it with the probe/meter method and left it. I think the screw-on dummy load I used when tuning the pre-filter was probably a different impedance than the input stage of the power amp, but I can't say for sure.
As mentioned earlier, both tuning methods work. It's also a lot easier to get to the filters if you have the outer skin off the station, and an N elbow connector doesn't hurt either !
Bob M.
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motorola_otaku
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I don't have any hands-on experience with anything other than the 2590 service monitor to compare with, but I like it. It makes tuning the Celwave mobile duplexers in the GR series repeaters (my only other hands-on experience with a repeater before now) a snap. It doesn't have any digital options, of course, but for analog-only use it'll do almost anything you'd want it to. The firmware upgrade was supposed to correct a bug in the wattmeter, but the install utility program had a bug of its own.
I noticed too that the prefilter semed a little touchier than the postfilter. All it took was a half-turn on one of the slugs to make the difference between transmit and no-transmit. And concerning the 2590, I'd recommend using the antenna port versus the RF I/O port for an input for tuning; it makes a much neater and apparently more accurate picture.
I noticed too that the prefilter semed a little touchier than the postfilter. All it took was a half-turn on one of the slugs to make the difference between transmit and no-transmit. And concerning the 2590, I'd recommend using the antenna port versus the RF I/O port for an input for tuning; it makes a much neater and apparently more accurate picture.
I hooked some test equipment to my MSF5000 station today and did some measuring. The station is tuned for TX of 440.500 MHz and RX of 445.500 MHz, the standard 5 MHz commercial offset. This is with the factory installed internal duplexer/filter.
There's about 50 dB of attenuation by the post-filter of the receive signal at 445.500 MHz. This is on the sloped part of the upper passband, and there's still a lot more attenuation visible as you go further away from the TX frequency.
There's about 15 dB of attenuation of the TX freq, per section, of the RF tray filters, for a total of 90 dB of attenuation of the 440.500 MHz signal.
At 60 watts (about +47.8 dBm) coming out of the antenna connector on 440.500 MHz, the receiver's preamp will see that signal almost 90 dB lower after it runs through all six tuned filter stages in the RF tray.
MSF stands for More Signal (fascinating) Facts. As with all equipment, your mileage may vary.
Bob M.
There's about 50 dB of attenuation by the post-filter of the receive signal at 445.500 MHz. This is on the sloped part of the upper passband, and there's still a lot more attenuation visible as you go further away from the TX frequency.
There's about 15 dB of attenuation of the TX freq, per section, of the RF tray filters, for a total of 90 dB of attenuation of the 440.500 MHz signal.
At 60 watts (about +47.8 dBm) coming out of the antenna connector on 440.500 MHz, the receiver's preamp will see that signal almost 90 dB lower after it runs through all six tuned filter stages in the RF tray.
MSF stands for More Signal (fascinating) Facts. As with all equipment, your mileage may vary.
Bob M.
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motorola_otaku
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- Joined: Tue Jan 13, 2004 7:03 am
*bump*
I'm having a problem with my machine now where after a user unkeys, the repeater locks up in a loud feedback-like squeal. I uploaded a .wav file of what it sounds like: there's a loud MODAT blast at the beginning and the sound wavers in & out as I move the mic away from the radio, but it should give you an idea of what I'm dealing with.
http://rapidshare.de/files/16481937/squ ... F.wav.html
A full retune of the station didn't fix it, and there weren't any "whiskers" present in the RX filter or either of the VCOs. I never did get a RF millivoltmeter to tune the TX pre- and postfilters per M's instructions, but I feel like I did pretty good with the service monitor. Here's what the prefilter looks like:
The assumption I'm working on now is that high SWR on the antenna end is forcing too much reflected power back into the station, because I can disable PL from the front panel and the squeal goes away. I'm about to tear the antenna down to check this (it's a Decibel DB404 fed with LMR600 on a 50' Rohn pole in the backyard), but I would be curious to see if anyone else has run across this problem and what it took for them to fix it.
I'm having a problem with my machine now where after a user unkeys, the repeater locks up in a loud feedback-like squeal. I uploaded a .wav file of what it sounds like: there's a loud MODAT blast at the beginning and the sound wavers in & out as I move the mic away from the radio, but it should give you an idea of what I'm dealing with.
http://rapidshare.de/files/16481937/squ ... F.wav.html
A full retune of the station didn't fix it, and there weren't any "whiskers" present in the RX filter or either of the VCOs. I never did get a RF millivoltmeter to tune the TX pre- and postfilters per M's instructions, but I feel like I did pretty good with the service monitor. Here's what the prefilter looks like:
The assumption I'm working on now is that high SWR on the antenna end is forcing too much reflected power back into the station, because I can disable PL from the front panel and the squeal goes away. I'm about to tear the antenna down to check this (it's a Decibel DB404 fed with LMR600 on a 50' Rohn pole in the backyard), but I would be curious to see if anyone else has run across this problem and what it took for them to fix it.
Well, the first thing to do is connect a good dummy load to the antenna port of the station and see if the problem goes away. If it does, then your feedline, antenna, or something external is causing the trouble. If not, then it's within the MSF5000 itself.
You did mention that the problem goes away when you DISABLE PL. That switch only disables the receive PL; I would expect you to have even more problems with the station in carrier squelch.
The power amp has a circulator inside it; that should take care of most intermod problems.
Check the receiver metering position 2. It should idle around 10uA and should NOT increase at all when the transmitter is keyed. Use the front panel XMIT control, since that won't enable transmit PL or any other audio. If the meter increases at all, you have desense, either from poor tuning of the internal filter/duplexer, or the antenna system.
I couldn't tell from your analyzer trace, but it seems to be too broad to me. In other words, the peak seems too wide and the sides don't go down steep/fast enough. I can check the pre-filter on my system that just happens to be down in the basement. The post-filter will be the one that will eliminate the most garbage from the PA output, and its tuning is more critical to the full-duplex operation of the system. I'll sweep that one on my system as well so we have something to compare.
Bob M.
You did mention that the problem goes away when you DISABLE PL. That switch only disables the receive PL; I would expect you to have even more problems with the station in carrier squelch.
The power amp has a circulator inside it; that should take care of most intermod problems.
Check the receiver metering position 2. It should idle around 10uA and should NOT increase at all when the transmitter is keyed. Use the front panel XMIT control, since that won't enable transmit PL or any other audio. If the meter increases at all, you have desense, either from poor tuning of the internal filter/duplexer, or the antenna system.
I couldn't tell from your analyzer trace, but it seems to be too broad to me. In other words, the peak seems too wide and the sides don't go down steep/fast enough. I can check the pre-filter on my system that just happens to be down in the basement. The post-filter will be the one that will eliminate the most garbage from the PA output, and its tuning is more critical to the full-duplex operation of the system. I'll sweep that one on my system as well so we have something to compare.
Bob M.
Well, I saved some images of a repeater tuned to 440.500 TX, 445.500 RX.
Here's the pre-filter. About 29dB down at 5 MHz above the TX freq, 2.2dB loss on the TX freq. Vertical is 5dB/div, and the input signal level is the top of the screen.
Here's the post-filter. About 50dB down at 5 MHz above the TX freq, 2.0dB loss on the TX freq. Vertical is 10dB/div, and the input signal level is the top of the screen. The rippls might be due to the TEE cable and receiver front end which were attached to the output of the filter/duplexer.
Bob M.
Here's the pre-filter. About 29dB down at 5 MHz above the TX freq, 2.2dB loss on the TX freq. Vertical is 5dB/div, and the input signal level is the top of the screen.
Here's the post-filter. About 50dB down at 5 MHz above the TX freq, 2.0dB loss on the TX freq. Vertical is 10dB/div, and the input signal level is the top of the screen. The rippls might be due to the TEE cable and receiver front end which were attached to the output of the filter/duplexer.
Bob M.
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motorola_otaku
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Yeah, it was either the antenna or feedline. I put my DB420 back up and fed it with Andrew 1/2'' Heliax and everything is good times again.
Your tuning of the TX filters doesn't look too far off from what I was getting. Did you do those with a service monitor or "by the book?"
Oh, and I did end up making my own tuning probe.. worked like a champ.
Your tuning of the TX filters doesn't look too far off from what I was getting. Did you do those with a service monitor or "by the book?"
Oh, and I did end up making my own tuning probe.. worked like a champ.
I'm pretty sure I did it three times: first by the book, then with my SA/TG, and finally by the book again, kind of.
The one thing I changed was I used a good dummy load at the far end of both the pre- and post-filters (opposite to where the sig gen was connected). I wasn't happy using the output of the PA as a load. Now that I know there's a circulator inside the PA, that might be why my results were a bit better with the dummy load.
The SA/TG method is good for getting it close, but the tuning is still rather broad with that instrument. A return loss bridge might get it better, but I didn't try that method.
I did notice that our two pre-filter waveforms are quite similar; I thought it was going to have a steeper skirt. What threw me was the lack of horizontal scale; mine are both 2MHz/div (20MHz span). The 2MHz bandwidth of the filter/duplexer is quite evident on these traces.
By the way, I used an HP 3406A wideband sampling voltmeter, the official tuning cable, and an Agilent E4430B signal generator to do the tuning. As I recall, the tuning cable attenuates signals by about 30dB, i.e. if I feed a 0dBm signal into the front end, I can read around -30dBm in the first tuning hole of the front end.
Glad you found the problem. It would be nice to know which one was actually bad: the antenna or the coax.
Bob M.
The one thing I changed was I used a good dummy load at the far end of both the pre- and post-filters (opposite to where the sig gen was connected). I wasn't happy using the output of the PA as a load. Now that I know there's a circulator inside the PA, that might be why my results were a bit better with the dummy load.
The SA/TG method is good for getting it close, but the tuning is still rather broad with that instrument. A return loss bridge might get it better, but I didn't try that method.
I did notice that our two pre-filter waveforms are quite similar; I thought it was going to have a steeper skirt. What threw me was the lack of horizontal scale; mine are both 2MHz/div (20MHz span). The 2MHz bandwidth of the filter/duplexer is quite evident on these traces.
By the way, I used an HP 3406A wideband sampling voltmeter, the official tuning cable, and an Agilent E4430B signal generator to do the tuning. As I recall, the tuning cable attenuates signals by about 30dB, i.e. if I feed a 0dBm signal into the front end, I can read around -30dBm in the first tuning hole of the front end.
Glad you found the problem. It would be nice to know which one was actually bad: the antenna or the coax.
Bob M.
Re: MSF5000 TX filter tuning question
Does anybody have a copy of the service Manual #6881092E80? I need to tune but i am new at this and need it to tune.