I found that the 34401A had excessive internal resistance while in the loop so redid the YTO measurements using instead a 0.102 ohm shunt resistor.? I had redone the chart but flipped it so the trend line would show mA/GHz directly (attached).? This doesn't show the MHz offset directly but the linearity of the YTO is remarkable.? I have to run out and do some errands but later today I will check the output to the FM coil.? However, a NOS replacement phase lock board behaves identically so I don't expect to see this as a board fault.
In the analysis I posted earlier, I calculated the worst case tolerance stack up both positive and negative.? The firmware would have to be able to handle this range.
Peter
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On 3/24/2019 1:34 PM, pianovt via Groups.Io wrote:
Hi Peter,
Your plot shows? y = 0.0444x + 0.0656. So, there is a 65.6 MHz offset at 0 mA main coil current. This could in part be caused by a FM coil current which does not show up across the 40 Ohm resistor. Basically, I think you have confirmed that your YTO works and produces an output well below 3.8 GHz. The only exception to this statement would be a case where the YTO stops working in a narrow range of frequencies, but that almost always happens near the very low end of the oscillator's range (well below 3.8 GHz).
The question now is, how to allocate the 65.6 MHz offset. How much of it is due to some FM coil current and how much is due to some other problem. Keep in mind what I said about the FM coil current, it should not exceed 150 mA, which corresponds to approximately 35 MHz tuning range in either direction. I guess, I would somehow check to make sure the coil isn't being fried with too much current.
I have no idea how the firmware deals with pretune constants in CW mode and/or PLL off.
Vladan
