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Entry  Thu Dec 16 10:46:36 2010, Zach, Electronics, GYRO, EOM circuit 400_ohms.png25_ohms.png
    Reply  Thu Dec 16 13:13:02 2010, Koji, Electronics, GYRO, EOM circuit 
       Reply  Thu Dec 16 19:46:39 2010, Zach, Electronics, GYRO, EOM circuit refl_signal_with_and_without_R.png
Message ID: 1226     Entry time: Thu Dec 16 19:46:39 2010     In reply to: 1223
Author: Zach 
Type: Electronics 
Category: GYRO 
Subject: EOM circuit 

As I mentioned in my previous sad post, I took your advice and it seemed to work, but then it didn't (hooray!). I chose door #3, namely to revert the first transformer to pull from the half-tap point and then add a resistor of R ~ 75 Ohms to ground at the input of the circuit in order to make the total impedance 50 Ohms. I was not exactly sure of the impedance I measured when using the half-tap point before (I quoted 150 Ohms, but this was a rough memory), so I decided to empirically optimize the coupling by varying the resistance.

Since Frank was using the impedance test kit and I couldn't find him, I decided to do this by measuring the signal reflected from the circuit: I put a directional coupler between the RF source and then took the transfer function from "IN" to "CPL OUT", with "OUT" connected to the circuit and "CPL IN" terminated (50 Ohms). Between "OUT" and the circuit itself I had a T-connector with the third port containing the resistor under test using a BNC-to-clip adaptor. To my surprise, I found that the attenuation was maximum with R = 39 Ohms, which doesn't make sense since the maximum overall impedance of this configuration is 39 Ohms (i.e. R should have been something greater than 50 Ohms). Below is a plot of the frequency response with and without the resistor to ground at the input.



I found that the frequency of the notch was pretty sensitive ( O(+/- 1 MHz) ) to the resistor value and the amplitude was extremely sensitive ( O(+/- 30 dB) ) to not only the resistor value but also the way in which I clipped the resistor or how I had it resting in 3D space when taking the measurement. I am certain this has to do with some spooky RF rule where you can't use a metal film resistor or you can't use clips or you can't wear green underwear while working with resonant circuits or some mumbo jumbo.

By far the worst thing that happened is that even though I got the above beautiful result when half-assedly clipping the resistor in, when I actually soldered it into the Pomona box it did something ENTIRELY different. Unfortunately, I don't have a plot for this and I won't complain if I never see one again.

As I said in the last post, I will try to do the matching using option #1 when I get back (i.e. using a third transformer to match to the true impedance of the LC tank instead of adding parallel resistances to do the job). We'll see how it goes. 


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