Hi Darrell, I've done it with an oscilloscope (slower than yours) and a Sweep-Marker generator; but not with just the oscilloscope, signal generator and frequency counter.
If I recall, the HP 8640B is also an RF Signal Generator; so if combined with an RF-detector and your oscilloscope and frequency counter, you could "theoretically" do it. I have tuned simple filters with a signal-generator, frequency counter and an oscilloscope; but haven't had any luck with tuning full duplexers this way. The problem with this approach is that it is like trying to understand what's happening in the forest by only looking at one tree at a time. What I mean by this is that you will see the apparent changes at one frequency, without seeing what's happening to the spectrum around it. If you were only looking to tune a single notch band-pass circuit, then it might be Ok to just use the equipment you listed; but today's duplexers are made up of several band-pass and several notch stages, all working on a common signal. You can easily tweak one piece and completely destroy your ability to meet the overall goal (because you aren't also looking at the spectrum around the one frequency). The other (not-so-intuitive) part to this is that it is often difficult to do the tuning of a duplexer piece-meal; not impossible, just difficult. What I mean by this is trying to tune one stage (cavity), then another, then another; and combining it all together into the overall duplexer system. The problem is that the interconnecting pieces of coax become part of the tuned circuit. Once combined together, one cavity's tuning can impact the adjacent cavities. When I pre-cut the interconnecting cables to the specific resonant lengths, I could get much closer to having the combined system pretty close; but I've always had to adjust things just a little after it was all connected together as a duplexer system. I am also told that a service monitor or spectrum analyzer with a "tracking generator" built-in is also a preferred method; but I have never been able to afford either. A Sweep Generator effectively turns the oscilloscope into a spectrum analyzer (so that you can see the forest); and a Sweep-Marker Generator also provides you references to use to easily make your adjustments (letting you see which trees are important to you). To my perception, a decent Sweep-Marker Generator and even a home-made RF-detector can promote even a relatively low bandwidth oscilloscope into something nearly equivalent to an expensive spectrum analyzer with a tracking generator. By the way, you can often find used Sweep Generators around that will work on Amateur Radio frequencies (especially VHF/UHF); many that were made to help align TVs are even applicable. If it doesn't have the Marker Generator built-in, you could substitute the signal generator and frequency counter to inject a reference marker at a known frequency; but it will take you a bit longer to continually adjust the setup. Anyway, that was my 2-cents; hopefully it was useful. de W6NCT (Vern) --- In Repeater-Builder@yahoogroups.com, "jistabout" <[EMAIL PROTECTED]> wrote: > Just curious if anyone here has used a wide-band oscilloscope (along > with a signal source, of course) for duplexer and/or filter tuning? > > I use an older Tektronix 7904 500mhz scope along with an HP 8640B > Signal Generator and it works great. ... > - Darrell/KA7BTV
