Agilent Active Probe


I recently picked up a broken Agilent 1156A 1.5GHz active oscilloscope probe on eBay for cheap. This was a great opportunity to discover what is inside one of these probes.


The above picture is inside the main probe body that connects to the oscilloscope. It provides the power for the probe head and identification circuitry for when it is plugged into the scope. There is no signal conditioning done inside this case. The coax connector passes straight through and into BNC connector which normally sits in the recessed cutout on the left.


This next picture is the actual probe head. The amplifier is a bare die sitting just behind where the taper stops. It is wire bonded to the surrounding circuitry and is pretty much impossible to repair.

If the probe head is damaged consider the whole probe to be dead. It might be useful for parts on another older-but-working probe, but there isn’t much to be saved. However, if the probe body is damaged there is a good chance it is repairable!

There isn’t much more to explore on these probes without having a good microscope! If I have the chance to get better pictures of the front end I will add them to a new post. To close, here is another picture of the wire bonded probe head.


Multimeter testing at higher voltages

I needed a way to test out my HP 3478A multimeter at higher voltages. I ended up driving a transformer with a signal generator, then running the output directly into a voltage doubler. IMG_0772

Unfortunately the reverse breakdown of 1N4148 diodes is only a little bit above 100V. The solution (as seen above) is to simply run a few in series. I used a ceramic capacitor on the input, and an 0.47uF X2 rated filter capacitor on the output. Not really designed for high voltage DC, but it did the job! I was able to squeeze 426Vdc out with effectively no load (10M ohms).


Lowering the amplitude from the signal generator gave me the 300V max the 3478A will take:



Conclusion: It isn’t too hard to generate high voltages with readily available parts, and my 3478A works as expected. I didn’t even electrocute myself! Seriously though.. a little caution goes a long way.