Spectrum Analyzer Probe & Power line communication

[Ashraf Almubarak]

This is a comprehensive question and I don’t where it will end.

I am new to deal with spectrum analyzer but worked for 3 years with the oscilloscope.
I have a new Rigol DSA815 spectrum Analyzer and it didn’t come with any probe. I searched upon probing for the spectrum analyzer. It is definitely not the oscilloscope probe.

One my search I come with this conclusion:
1-   Probing in spectrum analyzer requires the impedance of the probe to be equal to 50? to match the spectrum analyzer input impedance.
2-   The RF input is N male, many adapters (like N male to BNC) have a 50? resistor too.
3-   When probing to the spectrum analyzer you have to take care of the output resistance of circuit under test as not to load it.
4-   Co-axial cables are used for probing purposes in Spectrum analyzers.


What I want to measure:
We have a power line communication energy meters that have suffered from poor communication. It works in a range of 122 KHz.
As a beginning, we have noticed that if there are many Fluorescent lamps have a PFC capacitor in the input which will attenuate the PLC signal for sure and it will affect the communication. But this has to be measured accurately.

 
I am intending to make a PLC decoupling and filter circuit to measure the amplitude of the transmitted signal with the lamp switched ON and OFF with the spectrum analyzer.

What is the consideration that should be taken to probe the signal under study?
Is my approach is reasonable or it is not applicable at all?!

 

[PA4TIM]

I'm not into powerlines etc but I have several SA's (and also repaired a few) and probes
There is a thing called LISN. You can make them yourself and they are used to connect 50 ohm instruments like a SA to powerlines.
https://en.wikipedia.org/wiki/Line_Impedance_Stabilization_Network. Be carefull, a SA is ground referenced.

If measuring absolute amplitude is not an issue you can build contactless probes.
There probably are HV probes for 50 ohm systems for sale but I'm afraid they cost more as the Rigol. You can make a sniffer probe (search for EH probes). Make a coil, solder it to a piece of coax and hold it close to the signal you want to see. I made some I use on my scope to track a signal on pcb's. Made it from an old ferrit cored inductor.  An other sensitive low frequency probe is the head of a cassette player. But I think 122kHz will be to much.
Always add some attenuators until you are 100% sure the power is not to high. You can kill a SA very easy.

[Ashraf Almubarak]

Hi,
I will try your suggestion by using the LISN instead of reading the filter output.

For your probe, I have to measure the amplitude accurately.
But I will try your solution to learn more about SA.
thank you,

[Mjolinor]

An MOV (or several in series) make a a very safe capacitor for signal injection / recovery but I am not sure that it will work at such low frequencies. I have only used it for 1MHz and above.

[Ashraf Almubarak]

I will check the datasheet of the MOV

[Mjolinor]

As this is in the beginners forum then I should add that you do need to fasten your test side of the MOV through a resistor to a safety ground. An inductor is better but at your frequencies it will be BIG.

Whatever you do be careful doing it if you are not experienced in working with live circuits. Better to ask if you are unsure, there is no such thing as a stupid question and it is better than being black and crisp.

[Ashraf Almubarak]

You mean an Inrush resistor like a 150 2W wirewound resistor ?

[Mjolinor]

No, I mean a safety resistor so you do not get your mains voltage on the measurement side as you will if you leave it open circuit and connect the other end to the mains.

It is far better to actually use a small transformer for isolation so the "live" side is:

Live conductor---- MOV --- fuse ------ 5 or so turns of thickish wire on a core --- ground

The fuse needs to be a low value but you will find that anything under around 5 amps has too much attenuation to work correctly. Fuse type should be HRC.

The measurement side is:
connection point ---- 5 or so turns on the same core ----- ground.

You can vary the number of turns on that transformer in order to impedance match your 50 measurement equipment to the network Z0. The house wiring / UG network will be nominally around 25 ohms Z0. If you are using it on an overhead network then Z0 is nominally around 100 ohms.

[Ashraf Almubarak]

Ok, I will consider the safety requirements.

In the coupling/decoupling circuit, I was intending to use there is an isolation transformer.
But I am not sure it works with the LISN network.

I used the oscilloscope to measure the signal with reference to main. if the oscilloscope cord is grounded it will make a short. But if it is not grounded It doesn't make a short.

Can I use the same approach?

[Mjolinor]

I don't know. This is a really "safety first" area and as I don''t really understand what you mean with that last paragraph I won't comment on it.

[TK]

I recommend you to watch this video before plugging anything to the input of a spectrum analyzer:

[Ashraf Almubarak]

What I did, is to connect a probe to an energy meter that has capacitive power supply. The capacitive power supply is not an isolated power supply. knowing that the capacitive power supply reference is the mains LINE. I directly connect the oscilloscope probe return to that reference and nothing occur unless the power cord to that oscilloscope is earthed. I am expecting that the oscilloscope power supply to be a flyback topology or any transformer isolated topology. So no short between mains line and earth will occur through the oscilloscope return unless the oscilloscope is earthed.

But I take safety first any time and measure voltage between the devices before connecting it and also use a 2A circuit breaker.

[Ashraf Almubarak]

I did and I will watch it again