Somehow, I knew that when I came to this thread, people would be dying to show off their analog filter design-fu. I'm so excited!
I built a version of Dave's filter, 200 pF shunt C and each end and 1.06uH series inductance, with decent results well up into the UHF range.
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However, it's not that bad a filter, providing at least 50 dB attenuation up to 1.7 GHz. And up to 976 MHz, at least 70 dB attenuation is seen.
I'll put it in the mail to Dave next week for his amusement.
Oh my goodness, please do this. Are you related to Jim Williams per-chance?
Could you write up a small tutorial on how you made those design decisions? I'd love to pick your brain.
My only connection with Jim Williams is that we both attended Wayne State University -- he dropped out after a year of non-technical study and then went later to MIT, whist I completed my EE degree at WSU. Never met him, though.
As far as the filter goes, if you ever take a lumped component VHF/UHF low pass filter apart, you will find that it is built with multiple bulkhead shields strung out through the filter length. (The filter is housed in a long cylindrical or rectangular RF tight box. Between each bulkhead is the inductor and on each bulkhead are the shunt capacitors.)
The shunt capacitors are often made from disks of brass or copper and Teflon or mica dielectric. The bulkhead forms the ground side and there's a hole through the bulkhead for the inductor wire to pass and to be soldered or mechanically attached to the shunt capacitor disk.
The series inductors are air wound from heavy gauge wire, preferably silver plated.
The idea is that the bulkheads shield each filter section (inductor) from interacting with each other and provide a low impedance path for the ground return path from the shunt current.
In adapting this configuration for a 15 MHz filter, some changes have to be made.
1. The shunt capacitors are much easier to make for a one-off filter by stock surface mount parts. 0805 might be better from an electrical performance prospective, but are a bit too small for a quick kludge. 1206, C0G types are about the right size mechanically.
In order to more uniformly distribute the ground return current through the bulkhead, it's better to use several capacitors in parallel, radially spaced around the pad. But there's not a lot of room for the BNC connector posts so 2 or maybe 3 at most capacitors can be used at each bulkhead.
It might have been better to drill holes for the BNC posts and mount it on the non-copper side so that more flexibility in capacitor placement and number was possible. That didn't occur to me until after I had built the prototype.
2. An air wound solenoid style 1uH inductor is starting to become a bit large - so this suggests a toroid, either with a ur=1 phenolic core or a powdered iron core with a VHF rated material. MicroMetals Mix 10 is a reasonable choice.
A toroid is not perfectly self-shielding, but it's noticeably better in that regard than a solenoid form.
Regardless, at some frequency the distributed capacitance of the inductor is going to be sufficient that the self-resonant frequency will be exceeded. At that point, the filter looks like a capacitive voltage divider. You may still get some filtering action but it won't be what is expected. And once you are up in the GHz range, the inductor may even start looking like an antenna and will radiate. Those factors suggest you should use the minimum wire length possible. But that is associated with a higher permeability core which will usually have other problems as the frequency increases. So, it's a matter of where you decide to take the hit. And, the shunt capacitors start to look like tuned LC circuits when the inductance of the bulkheads is considered.
Regardless, at some frequency the lumped element filter will no longer perform like a filter. If you really have to have effective filtering from DC to light, it probably will be necessary to use more than one filter in series - perhaps in the same physical package - each optimized for a different frequency range.
Still, this one turned out better than I thought it would.
Jack K8ZOA