HP 3325B Synthesizer Performance

I measured the phase noise of an HP 3325B synthesizer operating at 10 MHZ with both the standard internal oscillator, as well as a very low noise external reference. There's quite a dramatic difference at offsets below about 10 Hz; for low-noise performance, it would be very wise to use an external reference.

Here's a short term Allan Deviation plot of the 3325B using the standard internal oscillator; it shows that the internal oscillator is nothing to write home about in the short term, either: