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
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