Jeff, Thanks for the detailed instructions. I understand everything, but I'm confused about one detail. Using this method will produce the largest RL and consequently the lowest IL. But I don't want the lowest IL; I want a specific value, i.e. 1 db per cavity. How do I use RLB to set a specific IL?
On Tue, Apr 20, 2010 at 6:46 PM, Jeff DePolo <j...@broadsci.com> wrote: > > > > I adjusted the loop positions, trying to maintain symmetry of > > the curve, aiming for 1 db > > on the analyzer. I didn't adjust the loops while looking at > > the RL. How would I translate RL > > into IL? > > You can't directly translate from RL to IL or vice-versa. Here's how to > tune a pass cavity: > > 1. Ballpark the insertion loss using the stickers on the loops and/or by > measuring the insertion loss at whatever frequency the cavity is presently > tuned to. > > 2. Rough-tune the cavity to something near your desired frequency. Don't > bother being too critical here - the resonant frequency is going to wander > a > bit as you adjust the loops in the following steps. > > 3. Terminate one cavity port with a high-quality 50 ohm load (high quality: > >= 30 dB return loss). Connect your RLB to your SA/TG, with the DUT port > connected to the other port on the cavity. You *must* use a cable between > the DUT port and the cavity that is known to have excellent return loss! > The cables between the SA/TG and RLB should be good quality, but are > nowhere > near as critical as the cable between the RLB and the device under test. > > 4. While measuring the return loss, make minor adjustments to one of the > loops to maximize the return loss. Again, ignore the frequency of the > return loss "dip", it's going to vary slightly as you adjust the loop, just > go for maximum return loss at whatever frequency the dip happens to fall > at. > Keep the screws snugged down well on the loop assembly; if it's not sitting > tight and flush in the top of the cavity the tuning will change when you go > to tighten the screws later. There's a little chicken-and-egg here; you > have to loosen the screws to adjust the loop, but when you tighten them > it's > going to change it a bit, so you have to emperically find the sweet spot. > With most cavities, you should have no problem getting well in excess of 20 > dB return loss - shoot for 30 dB if you can, even though at that point > uncertainty due to the test equipment's limitations will be dominating the > measurement accuracy. > > 5. Reverse the connections you set up in #2 above. Check to make sure the > return loss is still high looking into the other port (it should be). > > 6. NOW, adjust the resonant frequency using the rod to put the return loss > maxima it where you want it (i.e. at your pass frequency). Assuming the > cavity was rough-tuned in step #2 above, the return loss should not change > as you fine-tune the resonant frequency. > > 7. THEN, check the insertion loss through the cavity using the SA/TG. It > should be fairly close to what you set it to in #1 above; if it's more/less > than what you'd like, adjust ONE loop for more/less insertion loss, and > then > repeat again from step #3. DO NOT adjust the resonant frequency via the > tuning rod during this step!!! Unless the cavity was poorly designed, > tuned, or handled, the return loss maximum should align very closely with > the insertion loss minimum. > > Once you've properly tuned the cavities individually, then cable them > together and re-check return loss and insertion loss. Report back how it > goes and what numbers you come up with. > > --- Jeff WN3A > > >
