> 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.
Well, sort of. You want the most return loss AT THE DESIRED INSERTION LOSS.
Maximizing return doesn't mean you have the minimum insertion loss. A 20 dB
pad might have great return loss, but obviously it also has 20 dB of
insertion loss!
> But I don't want the
> lowest IL; I want a specific value, i.e. 1 db per cavity.
Right, and that's what you set in #1 in my instructions/notes. You "rough
in" the insertion loss setting initially, but the actual tuning of the
cavity is done based on return loss. In step 7 you measure the final
insertion loss after you're done tuning. If it's too high or too low, you
increase/decrease the coupling respectively and re-tune from scratch.
If my instuction on changing the coupling again in step #7 and then
re-tuning from scratch confused you, I apologize, I probably should have
been more clear. If you change the coupling of one loop to
increase/decrease the insertion loss, then you should be adjusting the OTHER
loop in the next round of tuning. Obviously if you adjust one loop and then
go back through the same procedure with the test equipment connected to that
same loop you just adjusted, you're just going to end back up where you
started. So, just so we're clear, if you're going to connect the RLB to
port A, you would want to increase/decrease the insertion loss by adjusting
the port B loop in step 7 before re-tuning starting at step 3.
> How
> do I use RLB to set a specific IL?
You don't. An RLB measures return loss (obviously). The SA/TG alone is
used to measure the insertion loss.
--- Jeff WN3A
>
> 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
>
>
>
>
>
>
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