Is there any reason, other than a defective product, why the logarithmic sweep on the 33521 would sweep at way less than logarithmically for periods over 20 seconds, and then go to the Stop Frequency in the last second. I just used the unit out of the box. It is particularly egregious for long sweeps. When doing a 20o second sweep from 10 Hz to 10kHz it didnt even make it to 100 Hz before the last second, and then it goes right to the Stop Frequency.
I had one of my colleagues test this out since I'm out of the office and they did not have any issues.
What waveshape is being used? If it isn't a sine wave, please switch to a sine wave and repeat the experiment.
Are the hold/return times set to 0 seconds?
How is it being measured? Best to use DC coupling and a fixed trigger level, because at slow frequencies (<100 Hz) you can get some strange behaviors with AC coupling or auto triggering. This is especially true if using a counter to make the measurement.
I am using the 33521A (yes, set on sin) to be the external reference on a brand new Stanford Research Systems 2-phase lockin amplifier and simultaneously drive a circa 1k-ohm load. The reference input on the SRS is high impedance. The lockin displays the reference frequency as it is sweeping. There are absolutely no measurement issues here. I had the lockin measure the signal on a calibrated 1mH 16-gauge air core inductor in series with a 1 k-ohm resistor, looking at the signal across the inductor, and I can absolutely see that the frequency is not sweeping the way it should. I can pull out the omega-L impedance and the phase shift and know that the problem is not an artifact of measurement. I substituted my older HP3325B as the frequency source, and it works just fine for everything I'm doing, including all calibration tests, using identical parameters as I had the Agilent 33521A set to. The unit is simply not sweeping frequencies on a logarithmic scale determined by the start frequency, stop frequency, and time (yes, under 500 seconds). I spent a day quadruple-checking this the proverbial six-ways-to-Sunday (mainly because I couldn't believe you'd let something go through with this kind of problem). If there is no firmware update or known problem of this type, then it is clear the unit is defective and will go through the return process with the supplier I bought it from. The quality control department appears to have screwed up on this one. Not what I expected when I bought it.
If you still have the unit maybe you could send me a state file, so I can try and repeat this? So far I am not seeing these problems, so it definitely could be your unit, but maybe I am just missing a setting somewhere.