It seems from the list archives that people are familiar with the 3561 and its use in the 3048 PN measuring setup. The PN calibration depends a lot on sweeping out paths with periodic white noise and making a single FFT measurement (the noise comes from 3561 source into the 11848A interface, a path is set up with the various internal switches and the noise measured at the 3561 output). With the source being periodic there are no issues of windowing, or need for averaging to get usable results. I happened to have a 3563 to hand and thought it would be interesting to make it do the same job (as it has two channels it also can do cross correlation). In doing this I ran into a problem with the calibration e.g. for CALLO.The 3562/3 seem to have dropped the periodic random noise, leaving only long period pseudo random noise which is much less suitable. I vaguely thought it should be possible to emulate this using the arbitrary source and indeed it is at full bandwidth uploading some suitable white noise data, BUT for reduced spans the 3562/3 continues to sample at 256kHz but repeats each point from the DAC multiple times; so in effect this is upsampling followed by a FIR with all coeffs 1. Trivial calculation shows this extra low pass filtering gives rise to about a 2db loss in a 4kHz sweep which is what I observed (two separate sweeps, one 100kHz span, one 4kHz with same Y-axis). https://imgur.com/QWGResS Now I can work around this by applying suitable HF boost to the frequencies in my noise sample (which is just an inverse fft of a load of magnitude one coefficients with random phase, with suitable conjugate symmetry to ensure a real output) but I need to use a different noise sample for each span which is painful, or maybe simpler - correct in software. But I get a distinct feeling I'm missing something as I cannot see why HP would drop their favourite measurement technique unless it had been replaced by something better. Looking in the 3561 the pseudo random noise is just obtained by dumping an EPROM looks like a 27128 or similar into the DAC, with a counter clocked at ESPx4 (no precise ESP definition other than at 100kHz bandwidth it is 256kHz, so I presume it drops as the bandwidth drops). Without a dump of U701 I cannot be sure, but I doubt it repeats each value 4 times, so it is likely oversampling to move the knee up out the way. Does anyone know what happened to the PERN "periodic random noise" source in the 3561, since it clearly wasn't the cost of an EPROM and counter. Have HP assumed that with two channels it is easier to use a swept sine chirp to measure transfer functions? Alan