Hantek T3100 passive probe teardown

[hanakp]

I bought this super-cheap 1:100 high-voltage passive probe for some one-time experiment.

http://www.hantek.com/en/productdetail_15_74.html

Then I got curious how Chinese comrades manage to make them so cheap, so I looked inside. The results are... well... amusing. There are two free-hanging THT resistors whose leads are pushed into two coaxial clamps - one in the tip, the other in cable ferrule. The larger resistor is supposed to have 99M, but has around 105M in reality (over 5% amplitude error). The smaller resistor is 680R and I'm actually not sure why it's there. There are two compensation capacitors with 2R0 marking, which is probably 2 pF. They haven't even bothered to heat-shrink the bigger piece of tubing around the resistors, it was moving freely inside the probe.

The probe is supposed to have 100 MHz bandwidth, but I'm a bit sceptical about that, because the resistors aren't exactly RF types. I don't have RF generator at the moment, so I will test it later. Also, I couldn't find any non-destructive way to open the BNC/adjustment box at the end of the cable.

[hanakp]

I tested the probe today and it seems my scepticism in the original post was unwarranted - its frequency response is not bad at all, considering it sells for about 35 USD. I used 1 kHz sinewave as base and I got this frequency response:

1k          0 dB
100k      0,08 dB
1M         -0,15 dB
10M       -0,51 dB
20M       -0,69 dB
40M       -0,82 dB
60M       -0,52 dB
80M       0,14 dB
100M     -0,49 dB
120M     0,31 dB

There is a small ripple around 100 MHz, but it's within 1 dB, which is negligible for most high voltage measurements. Then I pulled apart the BNC box and it was immediately clear how they managed to get such good results: they tune components for every probe by hand. There are two resistors and one trimmer cap. The first resistor is in series with coax center conductor and tunes the overall voltage ratio of the probe. This is why the probe has exact 1:100 ratio even though the large resistor in the tip has 105M instead of expected 99M. The second resistor is in series with the trimmer cap and is tuned for best response at higher frequencies. In my case, the resistors have obscure values of 113R and 42R2. My colleague noted the PCB looks like it was soldered on several times, so it's possible they tried a few different values before settling for these two. In any case, the metal base under the PCB looks surprisingly robust, so it shouldn't break too soon.

[Hydron]

Is the probe co-ax proper resistive stuff? Normally a few hundred ohms over it's length. I have seen some cheap probes that just use normal co-ax with horrible results with fast edges.

[hanakp]

Yes, it is, the center conductor has around 170 ohms. Moreover, there is some sort of resistive coating between white center insulation and shielding wires, see the photo. I've not seen anything like it before, do you know why it's there?