In this episode Shahriar attempts the repair of an Agilent PSG Vector Signal Generator. Aside from excellent RF performance, the PSG series generators are also equipped with built-in baseband generators allowing for a wide range complex RF modulations. The instrument has been loaned to The Signal Path by AllTest:
This particular unit fails the internal I/Q Calibration routines. This routine calibrates for image rejection, LO rejection as well as a wide range of I/Q routing, offsets, gain/phase corrections. The calibration errors points of the I/Q MUX board and the summing nodes inside that assembly. The measurements confirms that the Q signal exhibits large distortion.
The teardown reveals the complexity of the I/Q MUX board which comprises numerous op-amps, selectors, amplifiers & multiplexers. The block diagram of the I/Q mux is presented in details along with various measurements. The fault on the I/Q MUX board is traced to one multiplexer on the Q path. The IC is replaced and the waveforms return to their linear forms. The instrument passes the internal I/Q Calibration routine after the repair. The RF output of the unit is also verified and presented showing the correct operation of the vector modulation path.