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Re: VNA advice wanted

 

what do you mean 8560A is not sensitive enough below 300KHz?
The frequency range is 50Hz to 2.9GHz but you MUST enable DC coupled input (by default it is always AC coupled and freq response is >100KHz)
It really meets the specs down to 50Hz. Of course you will need to choose a small RBW and it will be pretty slow but it works.
I have a 8560E which goes to 30Hz and I can measure? a 10KHz resonator with it however at 10Hz RBW it is slow. Your VNA will also be very slow
for the same reasons your SA is.

for low frequency filters such as EMI filters you really only need transmission measurement to see where the resonance is. "Reflection" is not really an issue at frequencies like 30KHz?
but still can be measured using a good directional bridge anyway.
yes, a balun can take care of the matching but I am not exactly sure if those filters are designed for a specific purely resistive load/source impedance.
In RF filters, this is always the case and the impedance is usually 50 (or 75) or rarely things like 100 or 200 or 300 ohms (resistive) so a transformer easily takes care of the matching
but i don;t know about EMI filters. For example it might have been designed to have its specified frequency response for a specific inductive or capacitive load.
In any case you will need to know that optimum impedance the filter was designed for (if that is indeed how they are designed)
However, just to see where the resonance is or to compare two filters, most of the time you dont even need to care about impedance matching. Just the SA with TG is enough
The actual insertion loss will be wrong though.

Investing on a good EMI probe and even a high frequency active probe (for in-circuit probing) to use with the SA would be my recommendation.


On Sun, Feb 10, 2019 at 08:27 PM, Reginald Beardsley wrote:
The EMI filters are just a reason to want 30 KHz rather than 300 KHz. I've been doing a lot of probing with the SA function of the Instek MDO-2000E line. My 8560A just isn't sensitive enough below about 300 KHz which is where the majority of the noise is. Sadly I discovered that GW botched the calculation of the modulus so the series resonant notch of a crystal shows up as a peak.

So how would you evaluate the performance of an EMI filter? It surely looks like a transmission/reflection problem to me. I'm a retired reflection seismologist, so I'm very used to the time domain and impulse responses and to a large degree can look at either a time or a frequency domain plot and cartoon the other domain (thanks to Ronald Bracewell).

For that matter, how would you evaluate a crossover network? That's not 50 ohms. I would expect one would need a balun at each end, but I can't see why it matters that it's not an RF line. The conversion between balanced and unbalanced and the impedance matched would be required and the latter probably gets "interesting" if it's a 4 ohm speaker.
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