[Matteo, Eleonora]

In order to lock the PLL, the beating note detected by the fibered PD is sent to the PFD (Phase Frequency Detector) ADF 4002 which provides the error signal for the loop. 

The DC component of the PD signal should be removed and for this purpose we used a transformer. 

The transformer used for the first PLL was soldered by Marco Vardaro and Matteo (pic 1). Yesterday we soldered 3 more trasformer (identical to the first one) to be used for the second PLL and for the two monitor channels. (We want to use the other output of the fiber BS as a monitor and avoid the 90:10 splitter used up to now)

We tried also to use a minicircuit DC block (pic 2) but for some reasons we could not see the beating note on the spectrum analyzer.

We have characterized the TF of the transformer (pic 3), and compared it with that of a minicircuit DC block ( pic 4). 

The behavior is similar but the cut-off frequency seems lower for the DC block. ( Is this the reason why we cannot use it?)

In the two cases the attenuation is negligible but the phase loss is about 40 deg at 20 MHz (Is this relevant for the PPL loop?)

Issues about the characterization

During the characterization we had some problems (probably due to impedance mismatch) which brought to very strange TFs with large peaks and notches at high frequency. At the beginning we were using an oscilloscope and a function generator for the caraterization,  then we swiched to a network analyzer (50 ohm impedence) which gave us more reasonable TFs (shown in pic 3 and 4).