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

[ed breya]

Continuing with experiments and spur measurements, I found that closing 
the lid on the little filter box does seem to reduce the LPF's 
effectiveness at the higher frequencies, but leaving it open reduces 
effectiveness at the lower. I can sculpt it to a taller structure if 
necessary, which would give more clearance to the choke, and allow for 
some microwave absorbing material. Yes Askild, I did manage to squeeze a 
little strip into the existing can, but only near the output end - 
there's no room above the parts, especially the choke, so pinching 
anything there would likely spoil the whole thing. For now, I have the 
lid closed, and the absorber strip at the output area. The hinge of 
course has already broken, but I just tack some solder gobs over it as 
needed.

What I discovered next, however, may mean I won't have to improve this 
box anyway. Doing a spur review, I found that the remaining significant 
ones were the 70 MHz, and all but one of the ones between 1260 and 1820 
MHz. Above and below, everything else was in the noise floor. I had 
gradually worked the 70 MHz down some with shielding and such, but a 
little remained even all closed up.

I started thinking again about possible resonance of the cable from the 
mixer to the LPF. The length is in the right ballpark to aggravate the 
problem spur range. The only reason for the length was to get the 
desired three turns on the CM choke, so one option was to give up most 
of the choke value and go one turn, with a short, straight connect to 
the LPF, which would force any resonances way upward (but maybe they'd 
just show up elsewhere, if there's not enough loss at the higher 
frequencies). The other option was to revisit padding the input of the 
FPF, or the diplexer again.

After thinking back over previous experiments with these, I recalled 
that I was really only looking for noise flatness then, and hadn't even 
gotten to detailed spur measurements. I also recalled that the original 
diplexer setup did interact some at the top of the noise band - I chose 
a 5th order 50 MHz .05 dB Chebyshev response, but the real parts made it 
something a little different. I realize now that I should have just 
disconnected it and left it in place - unfortunately, I took it all out 
during my last cleanup and consolidation round.

So, not wanting to change too much around, and only guessing about the 
cable situation, I figured on trying something simple and quick to 
diplex out the upper stuff, to suppress possible resonances. I chose 
somewhat arbitrarily a 3rd order Chebyshev around 140 MHz, which is 
where the upper image lies, and the choke is about 50 nH, and I just 
happened to have another of the same part I had measured around this 
value and used in the 300 MHz LPF. So, in went a 22 pF/50 nH/22 pF/51 R 
HPF, and out went the reflections.  All the bad spurs are in the noise 
floor in a broadband view, but can still be found with narrow band spot 
checks, around -95 to -100 dBm. The net reduction from previous "feels" 
like maybe 6 dB, which I think corresponds with cutting VSWR in half. 
Interestingly, the 70 MHz is now virtually gone too.

So, it looks like my first instinct to have a diplexer was right, but I 
didn't study it deeply enough, and my assumption about its effect on the 
spurs being small was wrong. Now that I can see some results, I can set 
the HPF a little lower to at least terminate the entire upper image 
(about 115-165 MHz), but not so low that it interferes the LPF response. 
Another interesting thing is although the upper image is the second 
biggest after the desired output signal, it has never shown above the 
noise floor since I installed my new LPF to replace the commercial one.

Ed