I have heard about PTS frequency synthesizers for a long time and they are still being produced. They are known for high spectral purity and fast frequency switching. Finally, I got my hands on PTS-160 with nice features for a very reasonable price. It has 0.1 to 160 MHz frequency range, 0.1 Hz resolution, internal OCXO and GPIB control.
Here are some quick shots, showing the frequency resolution (with the 10 MHz clock locked to the oscillator of the HP 53310 Modulation domain analyzer).
The switching speed for a 1 MHz step
And control with GPIB using AR488 USB-GPIB macro to program 1 kHz steps.
It should be able to switch frequency in 20-30 mu sec according to the manual. So 1 msec GPIB switching speed can probably be improved.
Overall, a nice instrument to play with.