> -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of w6nct > Sent: Monday, April 02, 2007 6:37 PM > To: [email protected] > Subject: [Repeater-Builder] Re: Tuning duplexers with sweep generator? > > <<< see embedded responses below >>> > > --- In [email protected], "Gary Schafer" > <[EMAIL PROTECTED]> wrote: > > > > Duplexers and cavities have been tuned for many years > > without a tracking generator or spectrum analyzer. It > > is a little easier with a tracking generator or even a > > spectrum analyzer and a signal generator. But you can > > do the job with a signal generator that you can keep > > on frequency and a receiver. > > W6NCT: This sounds encouraging. > > > It helps to have some kind of signal level meter on the > > receiver but you can also just tune for quieting by riding > > the generator level so that the receiver is always noisy. > > That way you can hear if the level goes up or down. > > W6NCT: I think that understand the notion of using a receiver with a > quantifiable indicator of receiver signal strength; but I don't > understand what you are saying in your statement "tune for quieting by > riding the generator level so that the receiver is always noisy". > Could you explain this a bit more? >
If you have a receiver without a signal strength meter on it you can just listen to the noise with a weak signal applied to tell if the signal gets better or worse as you tune the cavity. There is a range of signal strength on all receivers where the receiver is partly quieted with a weak signal. Higher signal strength makes it a little quieter and a little weaker signal makes it noisier. By adjusting the signal strength out of the generator so that the receiver is always partly quieted you will know that the signal is still there and you will be able to hear the difference in noise level as you tune. If it gets noisier as you tune the notch on the cavity you know that it is going the right way. If the receiver gets quieter then you know that the tuning is going the wrong way. The only catch to this is that you have to keep readjusting the signal level of the generator to keep it in that rather narrow range of partially quieted on the receiver. This means constant adjustment of the generator output. As you tune the notch you will need more and more signal out of the generator. You will then hit a point in the tuning where the signal will go the other way. You have then found the peak of the notch. > > > > Using a sweep generator works fine for tuning pass band > > cavities or even the pass band part of a pass/notch type > > duplexer but you can't tune the notches with a sweep > > generator. The reason being is that the detector is not > > sensitive enough for the low signal levels that you need > > to work with. A typical diode detector will only work at > > about -20 to -30 dbm. Even if your sweep generator is able > > to put out +20 dbm that only lets you look down -40 to > > -50 db and the duplexer notch is going to be (hopefully) > > in excess of -70 db. You will never find the peak of the > > notch. > > > > So just a signal generator preferably a synthesized unit > > so you know what frequency it is on. You could use a tunable > > generator and a frequency counter but lots of generators > > tend to shift frequency slightly when you crank up the output > > level (high enough for the counter to see) and you will be > > working with the signal attenuated down to -80 to -100 dbm > > most of the time while tuning the duplexer. You can not afford > > to have the signal shift around or drift while you are trying > > to tune things as it doesn't take much of a frequency shift > > to mess up your tuning. > > W6NCT: Can I use the sweep generator to tune the band-pass; then just > the generator in the IFR-1100 to generate the driving signal into the > cavities to tune the notch? The IFR is synthesized and I'm guesing > that it might be able to put out more than the sweep generator without > becoming unstable. > Yes that should work fine. You will need an accurate frequency marker to show you where you are on the sweep frequency. I can't remember if the IFR-1100 has a duplex mode on it or not. If it does you may be able to use it in the duplex mode but set the offset to zero so it generates and receives on the same frequency. That will give you a generator and a receiver with a signal strength meter all in one! Some monitors you could do that with and it is great to tune duplexers and cavities when it doesn't have a tracking generator. > > > > With a good generator tune the pass bands at the proper > > frequencies first. Then tune the notches at the opposite > > frequencies. On the transmit cavities you would tune the > > pass to the transmit frequency and then tune their notches > > to the receive frequency (etc.). Then tune the pass tuning > > at the proper frequency again now that you have everything > > in the ball park. Then tune the notches again at the proper > > frequencies. > > > > ALWAYS tune the notches last. Do NOT touch the pass tuning > > again when you are done with the notch tuning. Tuning the > > pass just slightly will throw the notch way off. > > W6NCT: Understood. Tx cavities are to band-pass Tx freq and notch Rx > freq; similarly, Rx cavities are to band-pass Rx freq and notch Tx > freq. Right? Yes that's right. > > > > > Be sure and have a dummy load on the port of the duplexer > > you are not working with and preferably 6 to 10 db pads on > > each of the ports that you are working with or you may not > > have the same tuning when you hook it up to your radio. > > W6NCT: This confuses me a little. In addition to the X and Y outputs > to the oscilloscope, the Wavetek 1080 has an "RF output", "demod In", > and a "marker In". Not having a manual to tell me the proper setup, I > was guessing that the sweep generator's "RF output" would connect to > the input port of the cavity, and that maybe I should connect the > output of the cavity to the "demod in" (leaving the marker input on > the sweep generator unconnected). I wasn't sure if I should use some > sort of an external circuit between the cavity output and the 1080's > demod in. Do you know? Alternatively, I thought that maybe I should > put a T connector on the output of the cavity with one side connected > to a (50 ohm) dummy load, and the other side connected to an > RF-detector that feeds into a second channel of the oscilloscope > (leaving both the demod and marker inputs on the sweep generator > unconnected). I tried this setup, since I didn't know what the "demod > In" expected. Am I off-track with these possible setups? > > Either way, I don't understand what the pads are for. Please explain? > First, the pads are used to make sure that the duplexer ports see a true 50 ohm load as signal generator outputs and receiver inputs are not always pure 50 ohms. The unused port on the duplexer, the one that you are not working with at the moment, needs to have a 50 ohm dummy load on it. This is so that there are no reflections back thru the cavities that may upset the impedance of the other ports that you are trying to work with. Your sweep generator output should go to a cavity input, the output of the cavity should go to an rf detector that has a 50 ohm load in it and or a 6 db or 10 db pad should be between the cavity and the detector to make sure the cavity sees 50 ohms. You should also use the same kind of pad between the sweep generator output and the cavity input to also make sure that the input sees 50 ohms. Even though the sweep generator may have a 50 ohm output it may not be a good pure 50 ohms. Sometimes they are and sometimes not. The output of the detector goes to the "demod in" on the sweeper. That is so that the markers are superimposed on the vertical trace of the scope along with the sweep trace. You could just hook the output of the detector to the vertical input of the scope directly but then you would not see any markers on the trace. Just hook the rest of the outputs as they are labeled, to the scope. Horizontal output into the horizontal input on the scope and vertical output into the vertical input on the scope and you are ready to go. Put the time base on the scope to external input and the sweep generator should drive the horizontal. If you hook the rf out of the sweep generator directly to the rf detector input rather than going thru a cavity you should see a straight horizontal line on the scope. It should move up and down as you change the attenuation on the sweep generator output level. You know that it is working. You will see pips along the line when you turn on the markers. Now hook up the cavity and tune the sweeper to the frequency where the cavity should be and you should see the curves. You will have to adjust the sweep width and output levels to get things on screen. Note that when tuning the pass band on some duplexers that tuning is sometimes very broad but notch tuning will be very sharp. In tuning the pass for maximum output it is sometimes hard to tell exactly where the peak is. This is where a linear scale on the scope is better than a log scale as a linear scale will show smaller changes. You will have a linear scale with your sweep setup. Hope this helps. 73 Gary K4FMX > > > > 73 > > Gary K4FMX > > >
