wmhpowell wrote: > I'm looking for some Quantar "engineering level" help re: an > interesting simulcast issue. > I live in an area where I can hear several of high band our simulcast > Quantars. The whole thing was installed and set up by Motorola > including GPS stabilized time bases. > I'm monitoring with a true "monitor": wide band IF and little > limiting. > When the dispatcher drops a dead carrier I hear little in the way of > hetrodyne or "grunge" as it should be. > That means that the transmitters are on the same frequency... that's good. Do they start out this way, or do they start out a little off and then settle to perfectly quiet? Systems which don't keep the transmit oscillator running can have the former effect. > However, when the dispatcher drops alert tones I hear a hetrodyne > that decreases in frequency over the duration of the tone. > What's the frequency of the hetrodyne? Subaudible flutter? Audible beat note?
If this was an audible artifact that only was really noticeable with pure tones, I'd suggest that the audio phasing isn't set up correctly... but since it changes over the duration of a single tone, it is more likely something pulling the frequency of one or more of the transmitters rather than audio phase. > My guess is that the tone is somehow pulling one of the VCOs in a > Quantar exciter because of a lack of DC restoration in the modulator: > a capacitor is charging and slightly shifting frequency. > That's a good guess, as is the other guess posted that one of the PLLs is coming unlocked due to a harmonic of the modulating tone. Another possibility: are you using tone remote keying? Perhaps the keying tone is leaking through to one of them and unlocking its PLL briefly and then you're hearing it come back into lock. > I consider this to be abnormal and undesirable behavior - especially > in a $y$tem of thi$ caliber. > Agreed, but you're 99% of the way there it sounds like, so a "minor" problem in the scheme of what can go wrong with simulcast. > I haven't done any field tests yet. I suppose I can set up 2 service > monitors: one to receive in the AM mode and the other to provide a > reference carrier and then send tone to each transmitter, in turn. > That, at least would let me isolate the problem to one, two, ?? > radios. > If you can key one at a time with the tone, you should be able to see if the transmit frequency wanders on one of them differently than it does on the others. If you don't have a monitor that is sufficient to measure the error, then yes, using a second service monitor as a low-power generator to heterodyne against is a good idea. But you'll want to receive in FM mode. > Has anyone else experienced a problem like this? > I haven't... usually I hear systems that simply start off frequency when they key and take a fraction of a second for everything to be on-frequency. In the system I built for my ham repeaters, I run the transmit PLL 100% of the time for just this reason. Matthew Kaufman
