Decades back there were at least half a dozen instrumentation companies offering Dynamic Signal Analyzers (DSA's). Not Spectrum Analyzers, but rather Dynamic Signal Analyzers. The main differences being dual or multiple channel input and frequency ranges that went down to milliHertz. The DSA's were complete systems that would allow one to measure the transfer function of a servo system and analyze its stability margins in a matter of minutes.
I recall HP (Agilent), Onno Sokki, Bruel & Kjaer, Anritsu, and there were at least three others. But today there is only Keysight (Agilent spinoff) and Stanford Research System's 780 & 785, but the Keysight analyzers are the only viable ones for any serious servo work.
So where did this market go? Surely the demand has only gotten bigger with the rise of robotics work. I'm somewhat dismayed that development of new analyzers has not advanced beyond the HP35670A, which I consider the best DSA ever designed. Keysight still offers that model but the inputs are still analog. And no USB to get the data off.
Best Answer
Ah yes, the HP3561 100kHz DSA, I remember it well. The first 'difficult' problem we used it on was the stabilisation of a thermal servo loop. With a characteristic times of minutes, those mHz were needed.
The key to its time-limited usefulness is that that was back in the 1980s, when my thermal control loop was analogue, and ADC/MCU/DSP/DAC systems were uber expensive or non existant, certainly unheard of for low cost servo work.
Now, 99% of servo designers would start with an ADC/MCU/DAC system, simulate the system with LTSpice or MATLAB before they built real hardware, and be able to capture live data from the system and analyse it. The test gear is built into the system, or is only a LAN port away!
That's a view from the application side. Looking the technology behind the front panel ... what those instruments were doing was building on some serious state of the art hardware. The HP3561 used the first 250ks/s ADCs with decent dynamic performance, long before they were readily available on the open market, with TI 30xx DSPs offering 10M ops/s behind them for signal processing, this was bleeding edge in those days.
Instruments that do that still exist today. The ADCs and DSPs have moved up to Gs/s, so they are aimed at different applications/markets, and called something different.