> 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
