Hi Dennis,
This job is indeed very busy and complex. The temperature, ice and wind conditions at 3600m altitude forced me to develop a lot of specific equipment for the task. The problem is not only design, construction and repairs. The need for day-long observations depending on a very unpredictable meteo puts chaos on my agenda and consumes most of the time that would be needed for development. Since 2008, I could implement internet remote measurements that were considered impossible by my predecessor. But covid and the impossibility to reach the lab for repairs put it down since november.
I would like here to bring to the group attention the probable cause of this failure. We had two IP KVMs to control a vintage critical computer. So, one KVM was a spare. The two died within three months and my diagnostic is that the two forgot their firmware programmed at around the same time. One has to be aware that at 3600m altitude the cosmic rays are more intense and speed-up memory loss. This to say that I see a lot of people careful enough in the HP and Tek lists to remove (sometimes desolder) the eproms in their nice equipment to save their contents (if possible and safer, reprogram them to refresh their contents). Now, the situation got worse with the embedded EEPROMs in microcontrollers. In that case, the contents are not accessible and the instrument is doomed when the contents vanishes with time. I already have IP cameras in the lab that seem to have been attacked by this modern electronic Alzheimer. For sure, this is a very convenient unavoidable obsolescence for the sellers. Your motherboards will also forget their Bios in soldered EEPROMs btw. Socketed EPROMS are long gone.
Back to Fourier Transform SA. When you are in the domain, you get quickly the feeling that you can easily buy more resolution by taking more samples, which takes more memory and time. This is the same feeling as an analog SA that needs a slow sweep at small resolution bandwidth. But with FT SA you need a stable source. Otherwise you just would mix-up added noise on top of runing your line profiles. In our case, the target resolution is the one needed to well resolve the thin absorption lines of the atmospheric gases. This means a resolution between 500000 and 1 million. Hence the reason why we usually manipulate 2 million points samples (Nyquist). If the frequency domain of interest is reduced, applying a bandpass digital filter to this set would allow the equivalent of a heterodyne frequency change and the possibility to undersample a lot to speed-up FFT. Otherwise, with a big FFT you can get a 0 to Fmax spectrum at max resolution.
The possibility you mention to slow down the clock of the 7530 is indeed to reduce RBW but at the expense of the maximum input frequency at the instrument input. Because the problem is that you can't increase its maximum resolution which is determined by its memory size. Unless an antialiasing low-pass analog filter was put in front of the S/H, Nyquist would hit you hard and mix-up all your frequencies. ;-)
I never had the chance to see a J20/7J20 and I would have been very happy to play with. It for sure was very compact. Much more than my 4 meter long interferometer.
I remember now that the Rockland synthesizers were operating at 1.6MHz, not 8. I repaired one of them once. A lot of 74xx chips in them and one had failed.
So, group members, save all your Eprom contents when possible and be cautious when buying post-2000 equipment containing soldered or embedded EEPROMs. The time bomb is running...
