How does MCL electronic line stretcher work?

Hi all,

I am using a Mini circuits lab electronic line stretcher from the ELS series.
It works quite good and actually I am using it occasionally to check VCO load pulling for TX'es at about 868 MHz. You can change the phase over 360 degrees by applying a DC control voltage to it. Further is is quite broadband... The MCL device is especially made for load pulling tests.

Anybody that can point me to the way of working of this device ?
I just need to know how they do the phase shifting electronically...
Any article other than the Applic note from MCL that is on the MCL web ?
I want to make something similar for an application I have in mind. It must be controllable by a voltage or current.

Kind regards

Mike

From the key word "reflection" and the octave bandwidths, I suspect that it is a quadrature hybrid with the two ports on the left side connected to the outside world and the two ports on the right side connected to reactances through PIN diodes which act as variable resistors. Or possibly there are PIN diodes only across the right ports to ground acting like a variable capacitor. By changing the two right port terminations, the phase shift between the two left ports can be adjusted.