I love challenges!
Anyway, here comes my answer to the question what single device can “do both”, i.e. works for very low as well as high frequencies. When I saw the noise floor of the ancient HP8568B, I thought “oh boy, this would be hard to beat” – but I decided to give it a try anyway.
My contender is the Signal Hound SA44, which works in the range of 1Hz to 4.4GHz (Tracking generator TG44 from 10Hz to 4.4GHz).
Now the question is … is it fair to place a modern low cost SA against an ancient high end boat anchor (the datasheet says 45kg!) that has probably cost its first owner an arm and both legs?
The answer can only be a resounding “Yes”!
Both are affordable options today, even though for the HP8568B one has to calculate the additional costs for a crane truck as well as another (reinforced) workbench – but I guess these are minor considerations for a true test gear aficionado…
First I tried to resemble the scene shown by G0HZU as close as possible, just had to use 25Hz RBW because 30Hz isn’t available on a SA44. No input signal, I’ve just terminated the input with 50 ohms.
SA44_DANL_2kHz_F
Voilà, David has scored against Goliath. A DANL of -149dBm/Hz should certainly be good enough for the majority of practical applications in this frequency range.
However, a low noise floor alone does not make us happy – we are more interested in the signal to noise ratio, and for that we need a signal. When I tried this, I immediately realized why G0HZU preferred to show the noise floor without a signal: at frequencies that low, we can get all kinds of interferences by connecting the cable alone, even when the signal source is still off. In fact I’ve verified that the ground connection to any device (that isn’t even powered on) quite obviously causes a ground loop with lots of unwanted signals related to mains hum and its harmonics at levels up to -110dBm below 1kHz, so I had to measure the noise floor at 1500Hz this time:
SA44_DANL_2kHz_1000Hz_-40dBm_F
Now I would love to see how Goliath behaves in this test scenario, and I have the sneaky feeling this could already be a decisive battle in this contest…
The noise floor is a bit higher now by about 4dB and without more tests we cannot be absolutely sure where it comes from – the signal generator, the ground loop or the analyzer itself when it sees a strong signal.
So here’s the exact same measurement with the signal generator output off and powered off on top of that. As can be seen, the noise floor and the interferences are pretty much the same, so it’s all coming from the ground loop:
SA44_DANL_2kHz_GroundLoop_F
In any case the signal to noise ratio is close to 90dB in this scenario, but for sensitive measurements one would really have to find a cure for the mains hum and other interferences caused by ground loops. Shielded audio transformers might be an option, but are usually not made for 50 ohms and the frequency response might become problematic – apart from the fact that this obviously cannot work down to DC anyway.
Now finally I need to prove that this analyzer can not only work at very low frequencies, but also higher ones. Since the HP8568B works up to 1.5GHz, I’ve made another measurement at that frequency. Of course I’ve taken advantage on the built-in preamplifier, that can be engaged for frequencies above 100kHz and that’s also the reason why the noise is now even lower at -164dBm/Hz:
SA44_DANL_2kHz_1500MHz_-100dBm
WARNING: The SA44 does not have an internal DC block – of course not. During normal use, I always have an external one attached to its input and one really needs to be careful when working without. The input cannot tolerate DC at all and any voltage exceeding +/-100mV might damage it permanently.