I measured the open loop transfer function of the 80MHz VCO's PLL while locking it to Marconi.
This measurement is for a health check and a characterization of the PLL
The transfer function looks good, it agrees with the designed filter shape.
The frequency of Marconi is set to 79.5MHz which is the center frequency of the VCO.
The signal from Marconi is mixed down with the VCO signal at a mixer ZLW-3SH.
Then the demodulated signal goes to a 80MHz LPF to cut off high frequency components.
And it goes through a control filter which has 1Hz pole and 40Hz zero (see this entry).
The 80MHz LPF, the controls filter, the VCO and the RF amplifier are all built in the box.
In order to measure the open loop transfer function I inserted SR560 before the 80MHz LPF.
Using T-splitters the input and the output of SR560 are connected to a spectrum analyzer SR785.
Exciting the system using a source channel of SR785, I measured the open loop transfer function.
The unity gain frequency was measured to about 20 kHz.
It agrees with the designed filter shape (though the gain factor is a little bit underestimated).
Apparently there is a phase delay at high frequency above 10kHz, but it is okay because the phase margin is quite acceptable up to 100kHz.
However I found that the control range was quite narrow.
The PLL was able to be kept in only +/- 1MHz range, this fact was confirmed by shifting the frequency of Marconi during it's locked.
I will post another elog entry about this issue.
Marconi power = 6dBm
VCO power after RF amp. = -0.6 dBm
Marconi frequency = 79.5 MHz
Phase detection coefficient = 0.4 V/rad (measured by using an oscillo scope)