John Miles wrote: >> I am continuing my phase noise measurement quest. I gathered >> equipment (HP 8662a/11729C/8568B/multipliers) to measure >> 100Hz+ from the carrier. I now need to get a grip on the >> 0.1-100Hz range, which is where most of my applications are. >> >> What is the suggested measurement methodology for this range? >> >> My first idea would be to squeeze out the most of the above >> equipment...maybe add a dynamic signal analyzer like the >> 35660/3561/3562? What is a good HP LF analyzer? >> This combined with multiplication (to 1GHz) and EFC >> locking, could take care of the 1-100HZ range? >> Anythings else needed? >> No ideas for 0.1-1... maybe by counting with the 5370B? >> >> I am sure this has been answered before but the archives are >> difficult to search... >> > > So, I've started to look into the sub-100 Hz regime lately, but haven't had > time to get very serious about it. Here's my take on the question: > > 1) The TSC-51xx analyzers are sure nifty if you have the budget and can live > with being limited to HF measurements. They offer high performance with > less fiddling around than any other PN-measurement platform I've seen. > > The range can be extended with a couple of external mixers (plus low pass IF filters) and a low noise offset source. The offset source is mixed with each of the signals to be compared to produce a pair of IF frequencies within the 0-30MHz range. > 2) The next best thing would be a sound-card FFT option that works in > conjunction with an HF analyzer. PN.EXE will do this at some point, using > the 11729's front-panel LF analyzer output. It's really just a matter of > writing the UI code to support it. > > Most, but not all, sound cards have a low frequency cutoff of 20Hz or so. Some (but not all) sound card ADCs can dc coupled. A high resolution dc coupled ADC may be more effective for frequencies below 20Hz. > 3) The next best thing, after that, is a dedicated FFT analyzer, perhaps in > conjunction with an HP 3047A or 3048A system (i.e., an FFT analyzer plus a > 35601A or 11848A interface). Boxes like the HP 3561A and 3562A are neat, > but they are complete technological relics. They are limited to about 80 dB > of dynamic range in a 100-kHz bandwidth, they take up quite a bit of space, > and they require either a lot of custom coding or some obsolete HP software. > The 3582A is in the same boat, only more so. An 11729B/C plus a simple > sound-card interface will be the clear winner once the software support > issue is resolved. > > Sound card support appears to be something of a minefield, baudline thinks my 16 bit 48kHz motherboard sound system is a 24 bit 192kHz system. This probably means that the frequency scale and consequently FFT filter noise bandwidths are unreliable. However with a low frequency noise calibration source and set of marker harmonics derived from a crystal these calibration issues can be resolved. Windows software fares little better and some crashes when set to sample at 192kHz (the windows machine has a sound system with a 192kHz 20 bit ADC system). > Disclaimer: I do have a 3561A and a 3562A. They are really cool boxes, but > if I ever write any code to support them in PN.EXE, it'll be because I think > they're cool, not because I think they're the right answer for any > FFT-analysis applications today. Technology has left them behind in a > drastic way that hasn't happened with RF spectrum analyzers. > > An FFT analyzer can still be used at offsets below 1 Hz, but at that point > people usually want to see Allan-deviation graphs rather than PN graphs. It > becomes a different problem, since you most likely want to use a counter > rather than a spectrum analyzer. > > -- john, KE5FX > A finite beat frequency is required when using a counter. With low beat frequencies a lower noise zero crossing detector than the counter input trigger circuitry is required to minimise system noise.
A dual mixer time difference system can have a lower noise floor than a single mixer system. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
