Video: Tuning a bandpass cavity's center frequency using Return Loss - Page 1
Forum section: electronics / rf-microwave

author: w2aew
author_rank: Super Contributor
timestamp: December 23, 2016, 08:17:36 pm
content: This short video shows why you can more precisely set the center frequency by measuring input return loss rather than using insertion/transmission loss.

author: G0HZU
author_rank: Super Contributor
timestamp: December 23, 2016, 10:59:54 pm
content: For multi section interdigital filters up in the microwave bands I was taught another method that looked at the phase of the test signal on a network analyser. In my youth I once had to tune up a boxful of these things and an old guy showed me how to align the filter by looking at changes of the phase angle at the filter centre frequency as the screw for each finger of the filter was adjusted in turn. Before he showed me this I was tuning back and forth numerous times to try and get the filter response correct. But it had a strict return loss spec for both ports as well as for insertion loss and this became very time consuming to try and achieve. But the phase method meant I could do the filter in a couple of passes and get almost perfect symmetry for S11 and S22 response. My previous attempts couldn't get close to this. The fact that this method worked every time for dozens of filters proved to me how well it worked. It might not work as well on every interdigital filter design out there but the old guy said he had used it on other filter designs many times.

author: w2aew
author_rank: Super Contributor
timestamp: December 23, 2016, 11:21:30 pm
content: Quote from: G0HZU on December 23, 2016, 10:59:54 pmVery interesting. I'll have to play with this method the next time I have a VNA in my lab.

author: G0HZU
author_rank: Super Contributor
timestamp: December 24, 2016, 12:17:31 am
content: Sadly, I don't have a suitable example of a filter here to demo the method and I haven't used this method since. All the interdigital and combline filters I've designed since those days have been on a PCB with nothing to tweak. But the method involved looking for (S11) phase reversals/flips when tuning each resonator away from a shorted screw position. This is from memory, it was a long time ago that I was shown how to do this.

author: G0HZU
author_rank: Super Contributor
timestamp: December 24, 2016, 12:26:18 am
content: I did a quick google and there's some stuff in the link below that looks familiar. Maybe not quite the same but you can see that the idea is to tune for a 180degree phase flip.https://www.rfshop.co.uk/Martin.pdfSearch for this text in the link above and then start reading from a few paragraphs before this.QuoteBack in my youth this method appeared to me to work like magic and I tuned all the filters in a tiny fraction of the time expected of me and I did it better than previous 'tuners' of the same filters. It often worked so well I could lock down each adjuster locknut on the first pass through. I can't remember if I always used the centre frequency for each phase flip. It may be that I used a different 'flip' frequency for each resonator. I can't remember for sure but I think I only used the centre frequency because I could tune the filter very quickly and accurately and I don't remember fiddling with the VNA markers very much.