# Monitoring the waveform of a low frequency squarewave, built into unit, with bnc output to scope

I want to monitor the waveform of a high voltage (8kV) 3Hz to 1000Hz DC pulse form. It has been suggested that I use 1pf capacitor in series with a 1000pf capacitor, then put the scope across the 1000pf capacitor for a 1000 to 1 voltage divider.

I have a few questions:

• Will a 3 Hz DC pulse with a 50% duty cycle, look true, (i.e. flat tops without droop) with a capacitive divider?
• How will the 10X scope probe load such a circuit and alter the waveform and amplitude?I will alter values as needed.
• Would I be better off using a resistive divider?

I want to keep the current low, under 0.5 uA.

• Do you actually need to see the true shape of the waveform? Just detecting the edges will likely be far easier.
– pipe
Jun 3 at 21:39
• I wanted to see the duty cycle, and maybe down the road an arbitrary wave for generator would be used, maybe want to see that HV waveform.. Jun 3 at 22:20

Don't mess with Mickey Mouse solutions because you'll never know whether the divided waveform is a true representation of the actual high-voltage waveform. If the 1 pF capacitor breaks down, it will destroy your scope. Also, it could be dangerous.

EDIT:

OP added "build it into the unit".

In that case, the correct way to do this is with 21 resistor and 21 capacitors:

1. Build a parallel set of 1 MOhm 1/2 W resistor and 1 nF 1 kV capacitor.
2. Make 19 more of them
3. Connect them in series in a string
4. Place the whole string across the voltage to be sampled
5. Place the monitor port across the last set, the one connected to ground.

That will give you a 20:1 divider that is faithful from DC to high frequencies.

simulate this circuit – Schematic created using CircuitLab

• Actually planning on five- 5 pf in series to help with the breakdown problem. I would like this monitor point as a bnc connector on the unit to connect the scope to. I actually have a 100MΩ HV probe on it's way. but I had no plans to build it into the unit this was just to verify things. Jun 3 at 22:21
• "build it into the unit". That's an important requirement. Edit your question and that crucial point. Jun 3 at 22:45
• Sorry I had a mistype, I want the current under 0.5 ma. Jun 3 at 22:56
• "I want the current under 0.5 ma". The circuit I cave you draws 400 uA at 8 kV. Jun 3 at 22:59
• It should be noted that your monitor point will be loaded significantly by a standard 10x oscilloscope probe (10 MΩ//5~10 pF in parallel with your 1 MΩ//1 nF), and even worse by a high-impedance oscilloscope input (1 MΩ//20~30 pF in parallel with the 1 MΩ//1 nF). It may be a good idea to compensate for that with a different resistance for R36 (the capacitance won't be significantly affected though). Or use a 100x probe, with an input impedance more like 100 MΩ//5~10 pF. Jun 3 at 23:34