I’ve been poking around digital circuits with my oscilloscope’s analog probes for over 2 years now and basically it works just fine.
But. (And this is a big but.)
As soon as you need to have a look at more than one or two signal lines (say you’d like to look at a bus) this setup gets messy real quick. The analog probes are big, long and unwieldy; and due to their weight they tend to shift your PCB around as soon as you look at them the wrong way. Not very ergonomic. On top of that my scope only has 4 analog inputs, making debugging of more complex digital circuits impossible.
But that’s not the only problem with analog probes.
Capacitive loading of the circuit under test can be much worse than what you’d think by blindly trusting the input capacitance printed on your analog probe. It might not seem like much, but in order to clip your probe on the pin you’d like to test you have to stick a clip on the probe, making it longer, thus adding to the capacitance. Then there’s the ground clip, which usually gets attached to some far away pin, adding even more capacitance. All this capacitance quickly adds up and what should have been a nice rising edge ends up as a mushy blob of something on your scope screen.
This is where logic probes come in. They sport micrograbber terminated input lines that allow you to get super close to the chip or pin you’d like to test. Since they’re so small there’s no problem attaching a bunch of them on side-by-side pins of an IC, something that’s nearly impossible to do with analog probes. On top of that, short cable lengths and lowered input capacitance take care of the input capacitance problem. Of course there’s no such thing as a free lunch; you buy these benefits with worse input resistance (100 kΩ for the HO3508, down from 10 MΩ with the analog probe) and lower maximum input voltage (40 V for the HO3508, down from 400 V with the analog probe).
Anyway. That’s what I want. Nay, NEED.
Enter the HO3508 logic probe.
|Input impedance||100 kOhm || < 4 pF|
|Maximum input frequency||350 MHz|
|Maximum input voltage||40 V (DC and peak AC)|
I could have put this purchase off for a bit. But I noticed that Rohde & Schwarz is phasing out the HMO scope line, and quite frankly I was a bit afraid that I wouldn’t be able to acquire this probe a year or two down the road.
Unboxing and first impressions
There’s really not much to talk about here. The probe comes in a plastic bag, the plastic bag comes in a cardboard box. That’s that.
Then there’s a second plastic bag with jumper cables and micrograbber test clips.
You also get a leaflet telling you how to connect the clips and to RTFM of the scope if you want to learn how to use this thing.
Since Rohde & Schwarz sells the HO3508 for different models of oscilloscopes they don’t know if you’re going to use one or two logic probes: my HMO2024 can only use one (I get 8 digital inputs), but its bigger brother HMO3000 can use two (giving it 16 digital inputs, provided you shell out for two HO3508s). This is why they let you label your inputs yourself, using a provided sheet of tiny adhesive labels. Getting these things on in a semi-decent way is a pain, believe me.
All in all there’s not much value in the packaging, and opening this thing up does not feel like handling a piece of kit that goes for 360 €. Ah well, as long as it delivers.
The logic probe in action
Using this thing is pretty easy. Once you plug it into the scope a red LED on the probe indicates that it is working. As long as it is connected, the scope’s CH3 input is used in logic mode (yes, you lose one analog input!) and you can analyze your digital signals the same way you tread your analog signals: you can display them, trigger on them, move them around, zoom in on them; you can do pretty much everything with them you can do with analog traces.
What’s real sweet though is that you can use them as input signals for the bus decoding algorithm. This allows you to decode a bus AND look at analog circuits on your board. Or you could check the analog attributes of a digital signal: pulse width jitter, pulse amplitude aberrations and analog noise can be displayed and measured using the analog input while the DSO is happily triggering on the digital input or bus condition.
Please note that this is an unsolicited review and that I am not affiliated in any way with Rohde & Schwarz, HAMEG, their subsidiaries, or any distributors stocking this item. I have paid for this item with my own dime and it is being used in my private lab. The views expressed are my own.