If you've seen the previously posted video, you might wonder what happened. What was shown, was a spectrometer displaying a 100 MHz wide spectrum of the output of the PLL. The Y-axis displays the intensity. The center was set to 400 MHz. The arc is noise, and you can see a spike travel from right to left, see below:
The travelling is caused by changing the FFO (flux flow oscillator) control line. When we find the FFO control line setting where the power of the PLL signal is at the maximum, we remember that setting.
As I explained in 2006-04-25 SIR acronym, the FFO is a voltage-controlled oscillator. If you keep the complete schema in your mind (hm ffo pll schema.png), then you'll see that the signal that the FFO sends, is mixed by the harmonic mixer (HM) and ends up in the PLL again.
Below, a little schematic is shown which basically highlights the upper part of the complete schema hm ffo pll schema.png. The LSU (Local Source Unit) sends a signal of about 20 GHz into the harmonic mixer and it's possible to regulate with which power this is done. This influences the harmonic frequencies at the right. The length of the spikes says something about the power; altering the LSU power influences the length of particular spikes.
Another knob which we can fiddle with, is the bias of the HM. The bias is important, because this controls the way how the HM mixes the signals from the LSU and FFO. If you'd draw an I/V curve of the HM, it would be something like this:
You don't want to set the bias voltage so high that the line is linear, because then it would act as a resistor and resistors don't mix signals. Instead, the bend in the lower-right is the best setting (at around 2.5 mV).
The schematic below highlights the lowerleft part of the schema hm ffo pll schema.png. We get the phase difference as a voltage which the electronics can read out. We need this because we want it to be 0 volts before we turn the PLL gain higher.
