I have been trying to find some inductors, but I have been having trouble. I am looking at Bob Moog’s design for the EM Theremin (https://www.cs.nmsu.edu/~rth/EMTheremin.pdf), but some of the parts no longer seem to be manufactured. In particular, I am talking about L7 – L10 from this parts list:
These inductors are connected in series with the antennae. I am not very familiar with RF design, but I believe they are acting as an “antenna coil.”
I managed to find the 10 mH AIAP-02-103K, but I’m having trouble determining if any 2.5 or 5 mH inductors would be equivalent to the discontinued J.W. Miller products. I see a few on the site with equivalent inductance, but they seem to be large devices with high current ratings that cost upwards of $30.00 each. I was hoping to find something cheaper.
I have two questions:
- Is there a modern equivalent to these inductors?
- I know that in theory parallel 10 mH inductors could create the required inductance, but would it be practical to use several closely-situated inductors in an RF circuit?
Thank you for taking the time to read my questions.
Hi MC
I took a look through our product offerings and did not find a direct cross with those values. The closest cross references I found were 2.2mH an 4.7mH. Please see:
Part Number - M8305-ND
Part Number - M8335-ND
Please take a look at these options if they may work for your application.
Best regards,
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JW Miller was bought out by Bourns a while back, and the requested items are still listed for reference sake on the DK website under part numbers 6302-ND and 6304-ND .
Given that we’re talking about an instrument that functions on RF, the inductance value of a possible alternative is perhaps not so important as the secondary properties; making quarter- and half-henry inductors takes lots of turns of wire… Just wrapping them all up in one big coil provides lots of opportunity for parasitic capacitive coupling between winding layers, and can easily leave a person with an inductor that thinks it’s a capacitor at frequencies beyond a few tens of kHz. Hence the 3-donut construction of the requested parts–it partitions the windings to reduce parasitic capacitance and thus high-frequency performance, and was an item made specifically to retain its inductive characteristic at higher frequencies.
In seeking an alternative, I’d allow some margin on inductance values, and look for something with comparable Q and similar or higher self-resonant frequencies. On that basis, I’m coming up with RL875S-272K-RC-ND as an option I like best. It’s Q and SRF values are pretty close to those of the 6302, and putting two in series might make a passable approximation of the 6304. I haven’t looked closely enough to see what the operating frequencies in question are, but some of the other series mentioned have SRFs around a quarter to half that of the requested items. Generally, a person can tweak things to accommodate small differences in nominal component value when making substitutions. Trying to substitute a part that behaves as a capacitor at frequencies of interest in place of an inductor? That’s a lot harder to compensate for…
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You guys have been a tremendous help, thank you for your advice!
