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

[Askild]

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 <
time-nuts at lists.febo.com> 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
>
>
>
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