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

[Robert LaJeunesse]

FYI there are some rather flat video filter ICs that have been made in the past. The 6th order HMC1023LP5E is tunable, at its 28MHz setting its flat then down 0.1dB in the teens, down 0.35dB at 20 MHz. That same setting is 60dB down at about 90 MHz. It is also a dual part, designed for matched I-Q filtering.

Bob LaJeunesse

> Sent: Sunday, May 15, 2022 at 5:29 PM
> From: "ed breya" <eb at telight.com>
> To: time-nuts at lists.febo.com
> Subject: [time-nuts] Noise down-converter project
>
> 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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