[time-nuts] Re: Noise down-converter project

[ed breya]

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