[Lowfer] B+ choke for my LF/MF converter

[Michael Sapp]

Hi All:  I wanted to improve the B+ choking and bypassing on my LF/MF converter and decided to try

a binocular-type design based on a pair of FT50B-75 cores that are 1/2 inch tall. I'd like to tell you

all that I estimated the distributed capacitance ahead of time to get the peak choking reactance at 

a desired freq range, but that was not the case.  Ended up that I was able to squeeze about 40 turns

of #22 on the bino pair so it was a "wind and see what happens experiment."

I made three of them and they all came in around 22 to 24mH in value  and the choking attenuation in a 50 ohm

 series circuit figure of merit test came in peaking around 57 dB  at 185 kHz using an HP-3586C which I acquired from Jay a few months back.....


Here are the test results   https://www.dropbox.com/s/9zmbauwyrnve9ac/FT50B%20CHOKE.doc?dl=0

So I have three of these chokes, one Each on the B+ line of the OCXO, oscillator driver amplifer, and W1VD design preamplifier of my LF/MF converter. Each

is bypassed with a 47uF / 35V tantalum capacitor.  With each choke having  a .1 ohm DC resistance on about 7~8 feet of #22 wire, there was not much concern

for voltage drop to each active stage of the converter.   I may add an "active capacitor" filter circuit (detailed at http://www.vlf.it/smith1/opticalink.html towards the end of that page)

for the OCXO B+ line as I have enough B+ volts margin there to sacrifice ~.7 volt drop that would occur across the transistor.  I could try the active capacitor circuit 

on the 2N5109 amplifier stages but I need to go to an adjustable LM317 for that experiment---which may something to consider vs running everything off of a single LM7815.

Since I had some "beginner's luck" with the choke attenuation peaking near the watering hole frequency, I thought I would share the results....


One final note, espcially for the microwaver's on this list, is that I have a few HP430C part's units here. If you recall, these microwave power meters used an AC bridge circuit

that ran off of a 100kHz oscillator.  The inductor for that 100kHz oscillator turns out to be in the 132mH range, at least the one I measured.  But it also has a 140 ohm DC resistance which

makes for a rather steep DC V drop unless the current draw is low ( 10ma * 140 ohms = 1.4V, etc.).  Not  too practical for a 2N5109 drawing 70ma but posssibly useful for an active loop

op amp that has a light current draw.... only a pair of 6-32 studs and bolts hold it to the HP430C chasis, so it is an easy thing to remove if you need a high mH inductor....

I can see Dex heading for his part's shed now....~:)


73, Mike wa3tts