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

[ed breya]

Thanks all, for filter info. For reasons that will become evident when I 
describe the LF/DC situation, I plan to use an all-passive LC LPF. I 
assume I'll be needing a fairly high-order (like 9 or so) Butterworth 
type response for good flatness, and enough stop-band rejection for the 
higher frequency junk.

The filter that I'm using for now is an old (1970s - 80s?) K&L brand, 
marked 4L52-20-0/0-100. I've never found specific info on it over all 
the years, but I did just find some data on current products that look 
similar:

https://klmicrowave.nyc3.digitaloceanspaces.com/products/attachments/_plk147_1_LLSeries.pdf

The descriptions look familiar enough to get some idea of what it should 
do, except the passband ripple is nowhere near these specs (0.05 dB). 
The modern part numbering scheme is different, but the "4" and the "20" 
seem to jive, for 4-sections, and 20 MHz fc. It looks like K&L describes 
the number of sections as the number of choke elements, and that's 
what's in this filter. It has 4 chokes and 5 capacitors, so 9th order, 
as I understand.

I picked this up in some junk long ago, and it was in bad shape - 
someone had opened it up, and the cover was left hanging by a thread - 
literally - a single 0-80 screw managed to keep the lid associated with 
the rest. I thought the other corners were drilled out, so I just taped 
it shut, and since it seemed to work, I started using it for experiments 
over the years.

A couple weeks back, I began looking at it closely. I was going to 
replace the original SMAs with SMBs for this project, so figured I'd 
pull the guts out so I could try to ID the part values. The caps were 
easy, just regular mica types with markings. The chokes are small 
ferrite toroids, apparently identical cores. I counted the turns, and 
found 20 on the outer pair, and 22 on the inner. I didn't want to risk 
damage by removing any parts (the chokes are silicone-gooped to the 
shielding), but I did manage to get ballpark in-circuit values using the 
HP4276A LCZ meter at 20 kHz, so the error from the caps isn't too bad.

I also had a bit of good luck in finding that a screw was loose - one 
that anchors the assembly to the floor of the machined Al box, and is 
critical for grounding the circuit board. I had assumed this was not all 
that great of a filter, barely keeping the stop-band 40 dB down, or that 
maybe it would get better if the lid was properly attached. I also found 
that there was enough intact thread left in the corner holes, that 
digging up some 0-80 screws of just the right lengths fixed the lid 
mounting.

So, I got my values, got my SMBs, and the filter is rebuilt almost like 
new. It still leaks some at the higher frequencies, but it's now better 
than 70 dB down, which is on par with the modern spec, which only shows 
to that level. It is a fairly sharp cutoff filter, dropping about 80 dB 
at 50 MHz

Here's the parts info:

Caps (pF labeled, unknown tolerance) 68, 150, 150, 150, 68
Chokes (uH +/- 20% possible measurement error) 2.25, 2.43, 2.5, 2.28
Choke ratio inner/outer 1.21 according to turns count

So, anyway, I know it's symmetric, supposedly 50 ohms, and 20 MHz fc. 
Since then, I've been looking at filter design tools, trying to match 
what's in there to any kind of "standard" filter response, and tweaking 
fc and impedance too. So far, I've found nothing that's close.

The actual amplitude response looks very much like the Chebyshev example 
that Gerhard posted, and the datasheet says that's what this product 
line is, so it's probably in there somewhere. It's just that the part 
values don't make sense.

Ed