> But would I be too simple minded to suggest that maybe some form > of A/D PC/workstation input device with high dynamic range and decent > sample rate (certainly available in high end audio stuff to 192 KHz) > would be the logical vehicle for close in measurement in a quadrature > locked PLL type phase noise setup ?
Correct, that's why I was complaining the other day about the lack of any good high-dynamic-range DAQ dongles in the *medium*-speed market segment. You can buy an ADC outfit for audio work ($) or a complete GNU Radio USRP with >20 MHz acquisition bandwidth ($$$$), but there is nothing economically available in the 1-MHz neighborhood. There are some superb 24-bit sigma-delta chips in the 0.5 MSPS-2.5 MSPS range, but nobody has anything I can plug into my USB port and talk to with a C compiler. > Otherwise why would you care about > performance below 9 KHz ? With quadrature conversion, the carrier is downconverted to 0 Hz, and the analyzer sees the offset frequencies of interest directly. If the analyzer can't look down below 9 kHz due to limited RBW or a noisy LO, the measurement won't be valid down there. Phase noise at >= 10-12 kHz is important for adjacent-channel rejection in narrowband communications work. Below that, it's important for clean recovery of the on-channel signal. And when comparing crystal oscillators, you'd like to be able to look down to 1 Hz, to bridge the gap between phase-noise and Allan deviation measurements. Conversely, you rarely need to look at offsets beyond 1 MHz. Anything the DUT is going to do, noisewise, it probably will do before that point. (Spurs are a different story of course.) > Why exactly does one need a wideband SA that goes down to 100 Hz > (common spec) or 30 Hz (nicer and newer...) for this ? Maybe I > am missing > something here.... (probably am, I often do...) I can only look down to 100 Hz with my 8566B, and I'm fine with that for the most part. Someone testing extremely stable sources, again, may want a closer look at the immediate vicinity of the carrier. The 3047A/3048A outfits are also quadrature downconverters, but they have two parallel outputs, one for an HF spectrum analyzer, and the other for an FFT analyzer. The software merges plots from both analyzers into one wideband plot (offsets from 1 Hz-10 MHz). > And should one want an actual SA for these measurements instead > of a soundcardish thing and FFT software, I know that LF/HF boxes with > better specs than any of the affordable general coverage SA families on > Ebay show up regularly for not too much money. Yes, noting that the FFT analyzers generally are limited to DC-100 kHz. They also work better with switchable lowpass/highpass filters that eliminate signal components outside the decade currently being measured. (That's one of the things you get with a 3047A or 3048A outfit that you don't get with the 11729B/C by itself.) Chances are you already have an RF/microwave SA on your bench already if you're doing this kind of work, so it makes sense to use it, as long as its minimum frequency limit isn't a problem with the measurements you want to make. -- john, KE5FX _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.