One of the optional extra supplied with the 6424E was the probe positioning system. This is a serious attempt to address the issue of multi channel board probing. I had a quick look on the web and found several companies offering probe props (ranging from simple plastic things to more clothes-peg like efforts), articulated probe holding arms and even full on powered XY platforms. I found just one system quite like the Pico PPS but it uses truly horrible open frame probes.
Pico's PPS is a way of getting a number of standard probes to dig their points in at the right places on a board. (Perhaps "standard" is putting it a bit too strongly, currently the system only supports Pico's own x10 2.5mm diameter probes, but these are the probes supplied with the 6000 series as standard.)
The kit consists of a shiny steel base plate, 4 probe holders, 4 board support posts and special coloured rubber bands to secure the probe cables to the probe holder goose-necks. You can buy the kit with or without probes for £259 or with 4 x 500MHz x10 probes for £739. (That's pretty cheap for a 500MHz probe - only 4 on Farnell for less, 26 for more).
PPS components
The board you are testing can be secured by the 4 magnetic board holders (the pillars in the background of the picture.) Little self adhesive insulators are supplied and you have to stick two two each pillar. (You get some spares which is just as well because it's quite a fiddly task. Once you've done 4 you'll be about getting the hang of it !)
Board held in place by the 4 PCB holders.
The PCB holders have a sprung collar which you push down to allow the holder to grip the board. Once all 4 are in place the PCB is very well attached and seems pretty secure. The insulating washers ensure that the holders won't short tracks but there is a chance that PCB copper on the very edge of the board might short to the inner metal core of the holder. Normal PCBs don't have copper there but prototypes, especially on perf board, might. It would have been nicer if the parts of the PCB holder that touch the board were made of inherently insulating material. I tried lifting the whole assembly by the board and nothing slipped or fell off. (I did the same thing with each different board I tried with the PPS and it held them all very securely.)
The scope probes have to fit the holders and need a fair bit of force to push in. The holders almost certainly won't work with any probes other than 2.5mm diameter probes supplied by Pico. Currently there are no x1 probes available, but Pico told me that they are looking at making holders for other probe types.
Prototype CO2 sensor probing.
This is the first board I did any actual measuring on. The device being tested is a CO2 sensor (all pictures with CO2 sensors in, by kind permission of Gas Sensing Solutions Ltd) . This is a prototype plugged into a perf board adapter which is glued to a test board. I'm probing 2 signals on the perf board. The PCB holders did their job just fine, and the cantilever effect of the board extended didn't cause any problems.
Directly probing a CO2 sensor.
Here I'm probing directly onto the pcb of the CO2 sensor. One probe is on the edge of the pad of an 0402 resistor, the other on a via. The probes tips are sprung loaded and very sharp, it wasn't that hard to get them to make contact. The probe holders work by having a fairly floppy goose-necks and using the weight of the probe holder to push the tip down. The holders are deliberately heavy to achieve this. This is a good result, normally I need to solder tiny wires to these things unless I'm just looking at only one point and can hold the probe in my hand.
Probing 16 channel lock in amplifier (kind permission of University of St Andrews)
Probing 16 channel lock in amplifier (kind permission of University of St Andrews)
The digital signals are connected to the Pico Digital Pods by two adapter cables from the debug connectors.
The hard bit is getting the analogue channels onto the FPGA clocks, shown here.
Eventually I did manage to probe a crucial signal on the bottom board (no picture - I needed a torch to see where the probe tip was going !) Without the PPS I would have needed to solder to the board.
My final probing example is the power supply board I mentioned in the ERM (Earth Resistance Meter) thread. This board takes a 5V DC input and produces +/- 25V, +/- 15V and +/- 5V, the total power is only about 3W maximum but it's intended to be very low noise.
Power supply testing
I did some initial testing and then I had to add the zener diodes (too impatient to wait for them to arrive).
The PPS was handy for holding the board for soldering !
As with the other boards the power supply was held firmly and there were no issues with shorting.
Conclusions
The PPS isn't cheap, but it is useful. If you have to buy a new set of probes to use it then it might be a bit too pricey, but as an add on the the 6424E it only increases the price by about 3%, and that seems reasonable.
It could be much improved by using insulating material on the PCB holders (so it will definitely never short to the board). An earth tag on the base plate would be nice for low noise work (and essential for safety if using high voltages).
I'm not sure about the metallic mirror finish on the base plate, it's easily scratched and a shorting risk - I think I'd rather it was insulated.
On the 3 different boards I tried it with it made a very positive contribution.
(In blogs to come I'll describe what the Picoscope revealed about the CO2 sensor, the Lock-In Amp and the power supply, and what they revealed about it.)
MK
