EMRFD Message Archive 11766
Message Date From Subject 11766 2015-10-12 11:00:47 n1vc QST SA 110MHz bandpass filter nearly complete but I have a few quest I built my filter up using the parts in the Kanga kit including the piston trimmers. I did all testing so far with an antique HP8407a and an HP8601a sweep generator. However, I did verify all frequency measurements with a known good frequency counter.
In summary, the pass band is centered at 110 MHz and about 4 MHz wide. The article suggests it should only be 1 or 2 MHz. Will this be problematic? Also the passband was about 72 db higher than the stop band around 90 MHz. Other parts of the lower stop band do rise to 10 db or more above the floor with 45.2 MHz spur measuring around 15 db. Please note I did try to decouple the .092pf wire capacitors as far away from the inductor as I could to reduce spurs from over-coupling. A full sweep photo is at the link below.
If you want to look at all my waveforms they are at the same link: The photos were taken with my Android phone and I am have glare issues. Unfortunately, if you look hard enough you may see images of me in the reflection off the 8412 display.
Fig 1 is a full sweep using the 8601 from 0.1 to 115Mhz.
Fig 2 is the passband with higher resolution showing the 4Mhz BW
Fig 3 is the same as Fig 2 only showing 105 and 110 mhz markers.
The remaining images show markers every 5 Mhz down to 90 Mhz.
Do you feel I am finished with this and should move on?
Thanks for reading
11767 2015-10-12 11:33:06 John Marshall Re: QST SA 110MHz bandpass filter nearly complete but I have a few q The ripple in the passband shows your filter is still a bit overcoupled but the -3dB bandwidth isn't really that critical. The important things for this filter are:
- Center frequency insertion loss. Excessive loss here reduces the sensitivity of the instrument.
- Rejection at 90 MHz. This is the secondary image frequency (100 MHz 2nd LO minus 10 MHz 2nd IF). Any signals getting through at this frequency produce a spurious response on the scope.
If you can reduce the coupling a bit more you should increase the 90 MHz rejection and get closer to a single humped peak response without increasing the center frequency loss too much.
11770 2015-10-12 14:16:51 n1vc Re: QST SA 110MHz bandpass filter nearly complete but I have a few q Hi, John,
Thanks for the good information.
You said: The ripple in the passband shows your filter is still a bit overcoupled...
I am assuming you read my passband ripple from Figure 2 in my link, correct?
Also I forgot to check insertion loss. What should I read to place me in the ballpark?
11771 2015-10-12 15:46:16 John Marshall Re: QST SA 110MHz bandpass filter nearly complete but I have a few q Vince,
Yes, I see the ripple in Figure 2. Three humps at the top, centered on 110 MHz. As the resonator-to-resonator coupling is reduced, the width will narrow and the shape will morph into a single peak. If coupling is reduced further, the bandwidth gets even narrower but the amplitude at the top goes way down. I scanned the articles again but didn't see the insertion loss mentioned. I think 6 - 8 dB would be good. The first article (August 1998) says they got 90 dB rejection at 90 MHz. That's the difference in response between 110 and 90 MHz.
Tweaking 3 trimmers and 2 coupling wires can be a challenge, but you're taking the right approach by making swept measurements.
Good luck and 73,
11772 2015-10-12 19:16:42 John Lawson Re: QST SA 110MHz bandpass filter nearly complete but I have a few q Hi Vince, see Wes' post on his website for the mixed form filter. The first two filters he noted about a 5 dB insertion loss. He then mention the final filter built with the piston trimmers was a little better. Bob Kopski and I built the filter shortly after Wes had completed the filter in final form. Bob and I both achieved around a 5 dB insertion loss in our filters...Bob and I both swept our filters while making the alignment of the filter to get the proper shape......I replaced the original filter in my SA with the new one........hope this will be of help....73, John K5IRK
Sent from my iPad
11773 2015-10-12 20:16:44 Ashhar Farhan Re: QST SA 110MHz bandpass filter nearly complete but I have a few q Vince,try using the filter that I built for the spectrum analyzer. it is quite repeatable and you can built it in an hour. it is on wes' site as well. it is a drop-in replacement.- f 11774 2015-10-13 04:22:52 Vincent Coppola Re: QST SA 110MHz bandpass filter nearly complete but I have a few q Ashhar,I am aware of your filter and I am contemplating building it next. I went with the pistons approach because all the parts were in my kit. Also I like your approach to eliminating the oscope interface, and replacing it with ArduinoVince N1VCSent from my Verizon Wireless 4G LTE DROID 11776 2015-10-13 09:56:07 n1vc Re: QST SA 110MHz bandpass filter nearly complete but I have a few q Well I am measuring 10 db insertion loss. That sounds quite high in comparison to what you folks are telling me. Possibly if I can correct the over-coupling issue, maybe the IL will improve. I believe I am measuring it accurately and not relying on the antique equipment probably calibrated 30 years into the last century. I bypass the filter and put an accurate attenuator in line. Then I adjust that attenuator to a horizontal line I have picked on the screen and note the att. reading. Then I re-insert the filter and adjust the trace back to the that line and re-read the attenuator and the difference is my insertion loss....10 db.
PS. one more thing...I have no lid soldered to the top of the box, But if there is over-coupling I don't think the RF is jumping over the copper dividers.