Hi Ed, One thing I would test, that might not help, but should be easy to test, is to put some RF-absorber in the lid of the small shielded filter box.
Regards, Askild On Sat, Jun 4, 2022 at 2:37 AM ed breya via time-nuts < [email protected]> wrote: > I've been working on final design cleanup, mainly in the RF. I found > quite a bit of spurious LO harmonic content up to almost 2 GHz, with > some quite strong (-75 dBm). It was time to clean up the experimental > wiring layout, so I simplified the cabling and consolidated the RF stuff > onto the LPF board. This improved things a bit, but some spurs were > still pretty big. I presumed most of it was going right through or > around the LPF, and some due to common-mode and cavity resonances inside > the box, which can have many modes. > > I added a small LPF about 300 MHz (10 pF/50 nH/10 pF), inside its own > tiny shield box, forming the last bastion of filtering, right at the > inlet of the pigtail cable that goes to the isolated SMA bulkhead > fitting, and including another CM choke (only 1 pass of cable). This > filter is high enough up (over ten times the fc of the main LPF) that > they shouldn't interact very much - they are isolated only by the 3 dB > pad in between. > > All along, I've wondered what to do about the reflected power from the > main LPF, that mostly has to go back to the mixer. They are separated by > maybe 300 pSec of cable, which could be in the range for resonances at > the upper end. But, various experiments during development, including > padding the LPF input, and even making a diplexer with a 50 MHz HPF to > take the HF content into a terminator, showed no difference in the noise > output flatness, although the spurious levels likely would have changed > a little - some up, some down. So, I decided to keep it simple and just > let 'er rip, with nothing extra at the LPF input. > > Things are now at levels where the fine (and subtle) details show, > mostly cable dress, and grounding. I'll probably be adding bits of > shielding here and there, and maybe fooling with some RF absorbing foam > to see if any box resonances are a problem. > > Speaking of subtle effects, here's something interesting. The little > shield box for the 300 MHz LPF is a type with a fold-down lid, on a > hinge formed by thinning the sheet steel. It's only good for a few open > and close operations before the hinge breaks apart, so I kept it open > while building and testing the filter. It looked great, and the time > came to close everything up and look at the spurs again. I closed the > lid, and bent the retainer tangs a little, for good closure. Virtually > all the higher frequency spurs got a few dB worse. So, was it that the > lid isn't really grounded thoroughly, and acting as an antenna to bypass > the filter, or did it affect the choke Q or part values enough, or is it > that I also changed the cable dress a bit while putting it all back > together? I'll have to figure it out. > > Anyway, it's looking pretty good right now. With everything closed up, > including the box lids, as it would be when completed, all the spurs > show around -90 dBm or less. There were maybe two dozen noticeable spurs > identified earlier. Some are now in the noise floor (around -105 dBm, > but some remain, sticking out. I think most will disappear if I figure > out that 300 MHz filter box lid, which would leave the 70 MHz as the > main offender. This isn't surprising, since it's the biggest signal of > all, and it's not filtered all that much - it's too close to the main > LPF fc, and below the 300 MHz LPF. I should be able to knock it down > enough with detail work mentioned above, and I'm also pondering ways to > make a 70 MHz trap, if it won't go away. I have a couple of 70 MHz > crystals, so I could try this kind fairly easily. Does anyone have any > handy design info for crystal notch filters in this frequency range? For > an LC trap, it looks like a single L and C would be enough to get the > job done, without interacting too much with the other filters. > > Ed > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe send an email to [email protected] > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe send an email to [email protected]
