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Linear fit yields 22.52 GHz/mA.

Note that with this sensitivity, which is well within the 23 +/- 1 mA per GHz spec, that at the low end the output is BELOW the 3.8 GHz cavity oscillator.? This plot was taken with the following settings:

1)? PLL off
2)? Source in CW mode
3)? Default pretune constants (test 45) loaded
4)? HP 34401A inserted at 40 ohm sense resistor
5)? HP 5342A counter connected directly to YIG oscillator output

Linearity of YIG oscillator looks great.

How does the firmware deal with the YTO being below the cavity oscillator?

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

I don't know if this will help, but here goes.

First, don't worry about the apparent offset in coil tuning current. YTOs are only specified to operate over a certain range, like an octave or two, but can run a ways beyond at either end. It doesn't help (other than mathematically to get the tuning chart numbers) to extrapolate down to zero coil current - the YTO will stall out somewhere a little below its lowest spec frequency and coil current. Any slope variations or offsets extrapolated at zero just represent the character of the magnetic structure in the YTO.

The proper range, slope, and offset needed for tuning a particular YTO are typically handled by the YTO driver circuit, which can make it so the net result is a nice convenient GHz/V type system, at its input. There are usually sufficient adjustments (pots and/or DAC settings) in the driver circuitry to accommodate the tolerances of the YTO, the 3.8 GHz oscillator in this case, and the rest of the parts, to make everything come out right, during calibration.

There may be circuitry included to improve the tuning linearity (due to magnetic saturation), and compensate for the dynamic response (magnetic delay and hysteresis). Ideally, the YTO frequency is perfectly and instantly proportional to the magnetic field, but in reality the field depends on an iron core, coil inductance, AC core losses, and so forth.

The FM coil may be used in a PLL, for fine tuning. Its effect is very small (but much faster), compared to the main coil. It too has to be properly driven, of course.

In a down-converting generator made for reaching low frequency, it would normally be set up to avoid spectral inversion - that's why there's a minimum output frequency that can be expected. If the 3.8 GHz LO is free-running, its drift may be enough to limit how far down you can go at the low end, regardless of the YTO's tuning precision. For sources that can get way down toward zero (DC even), both the LO and YTO need to be precisely controlled (synthesized).

The coarse and fine tuning, compensation, and calibration all contribute to successful operation, so there may be quite a few possible suspects.

If you have schematics, you should be able to find all of these elements, and track down the proper conditions for diagnostics and repair.

Good luck.

Ed

Hi Peter

I don't know this instrument, but here a couple of things I would check..

I cant remember if you already checked this, but can you connect a coupler to the 3.8GHz oscillator, to tap of a small bit and check that this is always stable?
I guess it will be, but will be good to get it verified.

The YTO unlock instability, you should see on the FM coil drive.

Next I would follow the signal all the way to the PLL circuit. Check that it don't get attenuated to much at lower frequencies so the PLL can't lock to it.

Good luck with this.

Br,
Askild


On Sun, Mar 24, 2019 at 10:29 PM Peter Gottlieb <hpnpilot@... <mailto:hpnpilot@...>> wrote:

I checked the FM coil drive and it is right about at zero so that seems like
it's not the issue.

Nobody seems to know how the firmware works so we are left to guess and try
things.? There is some range which the firmware can correct for and I
heard from
someone who needed to nudge his phase lock board to get his system into that
range.? I tried the same thing and can get some measure of success as well,
although apparently I have another problem causing me grief. At this point I
have this temporary fix in which permits the unit to run on default pretune
values but won't pass test 48 which sets more precise pretune values.

Below about 50 MHz I have a PLL instability which gets worse as the frequency
decreases.? The PLL begins to lose lock below about 25 MHz.? I took two
photos
of the unit output when set to 25 MHz in CW mode showing the instability,
then I
put it into source tune mode which opens the phase lock loop. See attached.
Notice how the output is nice and stable as soon as the loop is opened.

Not sure what to make of this.? The YTO main coil drive doesn't look any
noisier
at 25 MHz than it does at higher frequencies.



On 3/24/2019 3:28 PM, Ed Breya via Groups.Io wrote:
> I don't know if this will help, but here goes.
>
> First, don't worry about the apparent offset in coil tuning current.
YTOs are
> only specified to operate over a certain range, like an octave or two,
but can
> run a ways beyond at either end. It doesn't help (other than
mathematically to
> get the tuning chart numbers) to extrapolate down to zero coil current -
the
> YTO will stall out somewhere a little below its lowest spec frequency
and coil
> current. Any slope variations or offsets extrapolated at zero just
represent
> the character of the magnetic structure in the YTO.
>
> The proper range, slope, and offset needed for tuning a particular YTO are
> typically handled by the YTO driver circuit, which can make it so the net
> result is a nice convenient GHz/V type system, at its input. There are
usually
> sufficient adjustments (pots and/or DAC settings) in the driver
circuitry to
> accommodate the tolerances of the YTO, the 3.8 GHz oscillator in this case,
> and the rest of the parts, to make everything come out right, during
calibration.
>
> There may be circuitry included to improve the tuning linearity (due to
> magnetic saturation), and compensate for the dynamic response (magnetic
delay
> and hysteresis). Ideally, the YTO frequency is perfectly and instantly
> proportional to the magnetic field, but in reality the field depends on an
> iron core, coil inductance, AC core losses, and so forth.
>
> The FM coil may be used in a PLL, for fine tuning. Its effect is very small
> (but much faster), compared to the main coil. It too has to be properly
> driven, of course.
>
> In a down-converting generator made for reaching low frequency, it would
> normally be set up to avoid spectral inversion - that's why there's a
minimum
> output frequency that can be expected. If the 3.8 GHz LO is
free-running, its
> drift may be enough to limit how far down you can go at the low end,
> regardless of the YTO's tuning precision. For sources that can get way down
> toward zero (DC even), both the LO and YTO need to be precisely controlled
> (synthesized).
>
> The coarse and fine tuning, compensation, and calibration all contribute to
> successful operation, so there may be quite a few possible suspects.
>
> If you have schematics, you should be able to find all of these
elements, and
> track down the proper conditions for diagnostics and repair.
>
> Good luck.
>
> Ed
>