[time-nuts] Noise down-converter project

[ed breya]

Continuing on, the mixer's output looks amazingly good. The filter's, 
not so much. I have the IF now going directly to the SA input - no pads, 
no filters, no nothing, except some SMB cable/adapter stuff, and about 
20 feet of BNC cable. It looks great, letting the SA do the filtering. 
The low end is a beautiful down-converted replica of the 50-90 MHz noise 
signal.

I can't make high precision measurements here - most are eyeball 
estimates from the SA screen, but everything is in the right ballpark, 
and makes sense. The amplitude measurements depend on the SA's IF RBW 
setting, which is 3 MHz maximum. The measured levels agree well with 
different RBW settings. The video BW also affects it some, since extra 
filtering is needed sometimes to smooth the curves.

The spec of the 4647 says the effective noise BW is 48.2 MHz. The IF 
passes through the -3 dB point near 24 MHz, in close agreement. The 
level is very flat (no discernible deviation), to around 20 MHz, where 
it just visibly starts to curve into the band edge. The maximum PSD 
appears to be around -80 to -83 dBm/Hz, estimated from the displayed 
levels at different RBWs.

So, the desired signal is wonderful, if only it didn't include 
everything else above. What I need is a very good LPF to get the job 
done - the usual problem.

The actual filter I've been using does a good job on the higher 
frequencies, but is poor on flatness. It has about 2-3 dB p-p passband 
ripple, with periodicity around 5-7 MHz. I've tried various padding 
arrangements at both ends, all of which tend to flatten it only a little 
bit at best. Looking at it with the TG/SA setup, the character is 
intrinsic to filter, and not due to just its reaction to the mixer and 
cabling and such.

I hate building filters. Designing them in principle is easy, with all 
sorts of available tools online, but actually rounding up the real parts 
(and their parasitics) and physical implementation is a PITA. But, I 
suppose I'll have to do it eventually for this project. I know how nice 
it can be, with the right filter, but for now, I'll have to go with what 
I have.

This particular filter is a packaged module type that I've had for a 
long time, and used in many experimental setups. In fact, I had to 
borrow it from its commitment to another project. Despite its 
limitations, it can be very handy, and it is very simple inside, so I'd 
like to replicate it for other uses. I plan to open a thread about this 
as a separate issue.

In the mean time, it will be for this noise project, and I'll have some 
more to report, so next up will be the low frequency/DC aspects.

Ed