MSF5000 Circulator and filtering question

[k5rpd]

I recently acquired an MSF5000 (C74CLB7105AT) that I am trying to work up for an Amateur Repeater. The frequencies will be 443.575 transmit and 448.575 receive. I do want to use a 114.8 PL tone both decode and encode. The unit has the internal filtering installed along with the "Tee" cable (Green band). It also has the PA unit (TTE1452A) with a single "circulator." (TTE1432A)
I have seached for information about the circulator and haven't found what I'm looking for. I understand how to adjust the Micor circulator (http://www.repeater-builder.com/rbtip/m ... lters.html) and I'm wondering what to do to adjust the MSF5000 single circulator??
Here is an image of the unit in question:


I have labeled the 5 trimmer capacitors that are exposed. I didnt want to take the circulator out of the PA unless it was required. Anyone have any experience tuning one of these? The service manual does not give a detailed schematic of the unit, just the 50 ohm load and the resistor for monitoring temperature.

My second question: I am planning on using a two antenna set-up for the repeater IF the noise floor is low enough at the site to use a pre-amp on the receive antenna. (My plan is to install an antenna at the highest place I can install on the tower and then using a service monitor to measure the noise floor over time- and then decide whether or not to use a preamp and install a transmit antenna below the receive antenna.) How much of the transmit filtering do I need to keep intact, such as- Do I keep the prefilter between the IPA and the PA and eliminate the postfilter? Or can I eliminate both? What are the Pros/Cons of this type set up? I know the service manual shows a two antenna set-up that substitues the postfilter for a low pass filter and the prefilter is changed to a simple barrel connector to go from the IPA to the PA. I tuned the prefilter and postfilter today using a service monitor to sweep tune them- the prefilter peaked at 4.5dB of loss and the postfilter peaked at 4dB of loss. Thoughts?

Images of the MSF5000 I have (PA removed) are available at http://picasaweb.google.com/jjk5rpd/MSF ... directlink
Any suggestions will be greatly appreciated!

JJ Smith
K5RPD
Russellville, AR

[d119]

JJ,

I've put a lot of these repeaters on the air in the amateur band and have never adjusted the circulator, nor do I recommend it.

[kcbooboo]

I would not attempt to adjust/tune the circulator. To do it properly you need access to all three ports: input, output, and load, and be able to connect equipment to all three in any configuration. You DO need some sort of filtering after any circulator, and in the MSF that means a low-pass filter (with a cutoff around 600-700 MHz) or the output section of the internal filter/duplexer. I've seen people selling the low-pass filter on fleabay and it has a lot less loss than the internal filter/duplexer, especially if you'll be feeding two antennas or a real duplexer.

You should follow the alignment instructions for tuning the filter/duplexer. If you just try to peak it for maximum signal, you'll get a lot of loss, as you're seeing. When tuned properly, the input and output sections have at most 1.5-2.5dB loss.

You won't be able to use much gain with any of the commercially available preamps. The MSF can take about 6dB gain at most; anything greater than that will just raise the noise floor at the receiver. Unfortunately most preamps give you over 16dB gain, so what I've done (and so have others) is to add a 10dB attenuator at the output of the preamp. Yes, it's wasteful but that's all the receiver can really handle. The Micor preamps only give you 6dB gain, and Motorola knew what they were doing.

Bob M.

[k5rpd]

Thanks for the great info so far!
I was wondering about the loss of the prefilter and postfilter... I am using an Aeroflex 3920 to sweep the filters with the tracking generator. I did some reading about the proper set-up of the test and realized that I was not using the reference setting on the 3920 (eliminating the connector and cabling losses). When I retuned both filters, I came out with 2.5dB loss in the prefilter and 2.0dB loss in the postfilter. The prefilter loss doesnt bother me because of the output that the IPA is capable of. The postfilter- I would love for that to be lower of course! I figured out how to do screen capture as well:
Prefilter:

Postfilter:


The receive frequency is at -31.6dB from the transmit when using the postfilter. If I use the two antenna set-up on the tower I guess I'll have to make that up in vertical colinear seperation of the Rx and Tx antenna- shouldn't be a problem.

On the preamp issue: I plan on tower mounting the preamp (if used) at the base of the Rx antenna. I'll be using either LDF4-50 (1/2 inch hardline) or LDF5-50 (7/8 inch hardline) with a run of about 80 feet from antenna to receiver. Not too much cable loss involved so would only 6dB gain still be recommended or maybe more?
Thanks again,
JJ Smith
K5RPD

[kcbooboo]

I had closer to 50dB attenuation at 5 MHz away from the TX frequency. Check your spectrum analyzer settings; your noise floor is rather high. Use narrower filtering or a slower sweep speed to get 60-80dB visible on the screen.

I found that the receiver front end offered about 80dB attenuation of the TX signal, so when added to the post-filter loss, there should be enough to do the job.

As you're using low-loss cable, 6dB is more than enough gain to offset the system losses. The receiver will just choke if you give it more signal.

Mounting the preamp up at the antenna: I hope you like climbing it often to replace it. Think about lightning and other static discharges.

Bob M.

[Wowbagger]

Check your spectrum analyzer settings; your noise floor is rather high. Use narrower filtering or a slower sweep speed to get 60-80dB visible on the screen.

Seconded. Any time you start to see "grass" on a spectrum analyzer, especially when using a tracking generator, you are getting noise limited.

You need to turn up the tracking generator level. You are running -60dBm, so the top of that peak is in reality only -60dBm, and you don't have that much delta over the noise floor. I'd use about -30dBm for the tracker at least. I'd also cut in the 3920's preamp.

[k5rpd]

Thanks for the info on the 3920 settings- I'm still very new to using it and could use lots of advice on it. I did get on Aeroflex's website and download the various articles to learn it. I'll give those settings a try and see what I come up with.

JJ

[FMROB]

So, Im a bit lost here, and really want to try to uderstand this.

The original configuration on the MSF had that Tee device that acted as the "factory" duplexer, so to speak. I remember guys that heavily used the MSF talking about how that device incurred high losses and was a bit cumbersome.
I remeber most of those same "MSF" guys ripping out the tee equipment and usually setting up the MSF to a 526 duplexer (at the time manufactured by phelps dodge). For the hammy boxes, a pre amp was almost standard.

My limited experience with the MSF, we ran a angle linear gasfet preamp, at about 17db gain in connection with a 526 and I don't remember having an issue, even dumping that high of a signal into the RX side of the MSF. Now, this is going back a bit of time, and I couldn't even recall the site conditions or noise floor levels at the site, which assumably were low at our particular site. I do remember that the preamp made that box hot as a fire cracker, and this thing had quite an ear. One time when the preamp power supply gave out, we jumped out the preamp until we got a new PS, and what a difference.

So, I guess my question, in a round about fashion is why use the two antennas? Is it simply not to incur the loss of the duplexer? And even with a duplexer, what harm would it be to utilize the pre amp?

Thanks, Rob sorry for the long wind.

[k5rpd]

My thought on the two antennas is to avoid the losses in a duplexer and to be able to run a preamp on the receive (mounted at the antenna) if the noise floor is low enough. My plan for now is to mount an antenna as high as I can on the tower and then measure the noise floor and possibly even go ahead and hook up the MSF5000 in it's standard configuration using the internal "duplexer" filtering. If I can tell a noticable difference in the receive quality by hooking the antenna directly to the receiver preselector, then I will most likely install a transmit antenna- collinear and below the receive antenna- and bypass the postfilter (placing a less lossy low pass filter in it's place).
I've been reading the articles on repeater-builder about measuring effective sensitivity and the like... good reading.

Thoughts?

JJ

[KI4M]

I believe if it were me I would mount the TX antenna above the RX antenna. Reason being in our applications we have had some tower crews mount the darned RX antenna on our 800 TRS out the top of the tower. Well that equates to lots of repair trips up the tower for Polyphaser change outs in the TTA box. Had we put our TX antenna out the top that problem would have been null and void. A RX pre-amp can be very susceptible to any type of static discharge. Given the amount of pre-amplification you will be running, and the inherent risks of the GaAsFET device to static mount it lower than the TX... I would bet there will be little if any difference in coverage on talk in v/s talk out. Be sure to use a true DC ground antenna on both for extra protection..