Agilent 1152A active probe "smartprobe" conversion for DSOX3000 series scopes

[TheSteve]

I recently bought a pair of HP/Agilent/Keysight 1152 2.5 GHz active probes for a great price. I wanted to use the probes with my Keysight DSOX3000 series scope however the scope doesn't support the probe by default. I believe the reason it isn't supported is that the probe wants 4 volts from the probe interface and unlike some other/older scopes which provide 3-6 volts the DSOX3000 series has the voltage fixed at 5 volts(read - cost reduction).

I am not breaking new ground here. Howardlong has shown a way to get them to work by adding one resistor here: https://www.eevblog.com/forum/testgear/agilent-mso-500060007000-anyone-hacked-these-scopes/msg868073/#msg868073
However this doesn't address the 4 vs 5 volt operation and there is a 400-500mV offset on the DSOX3000 series(I verified I saw the same, even with a 4 volt regulator added).

A fancier mod was suggested by tnt. In his mod you add a 4 volt regulator, change two resistors and add a 24C02 eeprom for the smartprobe interface. His mod was mentioned here:
https://www.eevblog.com/forum/projects/agilent-probe-interface-(-autoprobe-and-probe-id)/msg43788/#msg43788

I decided to modify both probes with the tnt mod.

To do the mod you'll need:
-small 4 volt LDO regulator with associated filter caps(I used a LP2985IM5-4.0)
-10K and a 46.4K resistors(I used 0603 size)
-24C02 eeprom(I used an SOIC package)

The 10K resistor goes in place of a 40.2 K resistor which tells the scope what voltage the probe wants(5 instead of 4 volts).
The 46.4K resistor goes in place of the 56.4K which tells the scope a smartprobe has been connected.

I used a small PCB cut from a larger one to mount the regulator and caps. The 24C02 sits right on the bottom of the original probe PCB insulated by kapton tape with its ground connection(pin 4) directly soldered to the probe PCB.

The 24C02 eeprom gets programmed with a image from a 1158A smartprobe. Thank you to tnt for providing a copy of the eeprom image. You can see the probes serial # in the binary file which I changed in the second probe so the scope will store individual calibration values for each probe on a per channel basis.
(EDIT - Sept 21 - Did some experimenting and you can change the probe part # in the eeprom to any supported part from the looks of it. You can also change the serial number to anything you like. I changed my probes to indicate they are 1157A as the 1157A is also a 2.5 GHz probe, same as the 1152A.)


After the mod the scope will read the eeprom when the probe is connected and let you calibrate the probe which removes any offset it has. Both of my probes behave exactly as expected after calibration and give performance the same as my N2795A active probe. My scope only goes to 500 MHz so I can't test the full probe bandwidth ratings.

To open the probe casing start at the cable end, once you have the halves separated a little bit you can work on the two front clips. The front clips are more delicate.

Any questions please ask.

Here are some pictures detailing how I did the mod and eeprom images with two different serial numbers.

Top of the PCB showing my regulator board installed and the location of the 46.4K resistor:


Here is where to cut the PCB and connect the 4 volt regulator:


Bottom of the PCB showing the 24C02(wired at address A0) connections:


PCB reinstalled after the mod:


1152A probe tip:


3dB point as measured on my DSOX3014A scope which has been modified to 500 MHz operation:
For those interested in modifying the scopes bandwidth the info begins here:
https://www.eevblog.com/forum/testgear/dsox2000-and-3000-series-licence-have-anyone-tried-to-hack-that-scope/msg987126/#msg987126

[Gandalf_Sr]

Nice job Steve, thanks for sharing

[jjoonathan]

Baader-Meinhof! I just got done powering up my cheap ebay HP54701A 2.5GHz active probe -- probably the predecessor model to this one based on the similar appearance. I used a bench supply, but the next step is of course to make a proper one. It won't be as elegant as what you have here though :-)

Out of curiosity, what's the current draw? The 54701 takes 100mA at +/- 17.3V and gets quite warm to the touch (both the tip and the supply box)! Are the newer ones more efficient?



One other thing: I'm getting a pretty bumpy frequency response, and while I haven't entirely decided whether or not I think it's due to the power supply I have my suspicions. Your model might be more difficult to test in the frequency domain, but the bumpy response on mine can be seen just by sweeping a sine wave out to 500MHz. What does yours look like? Do any of you probe aficionados know if this sort of thing is typical for old active probes?

[HighVoltage]

What a great mod.
Thanks for sharing this.

When you push "probe" on the scope, what does it say as to identification?
 

[TheSteve]

It identifies as a 1158a(model number is in ASCII in the eeprom binary).

[TheSteve]

Baader-Meinhof! I just got done powering up my cheap ebay HP54701A 2.5GHz active probe -- probably the predecessor model to this one based on the similar appearance. I used a bench supply, but the next step is of course to make a proper one. It won't be as elegant as what you have here though :-)

Out of curiosity, what's the current draw? The 54701 takes 100mA at +/- 17.3V and gets quite warm to the touch (both the tip and the supply box)! Are the newer ones more efficient?

One other thing: I'm getting a pretty bumpy frequency response, and while I haven't entirely decided whether or not I think it's due to the power supply I have my suspicions. Your model might be more difficult to test in the frequency domain, but the bumpy response on mine can be seen just by sweeping a sine wave out to 500MHz. What does yours look like? Do any of you probe aficionados know if this sort of thing is typical for old active probes?

It does seem most of the probes are likely the same. I only measured the current draw on the 5 volt input which was 56mAh, the scope also supplies +/- 12 volts which is required. According to the datasheet the current draw maximums are: 4 volts @ 90mAh, +12 @ 5 mAh and -12 @ 95 mAh max. I did a manual sweep to 600+ MHz and it looks quite flat on my scope. I was scoping a 50 termination made of 0603 resistors which is quite flat into the low GHz. I do have a spectrum analyzer I could connect one too but I'd need to try running 3 power supplies to the probe to power it.

The probe tips themselves do run nice and toasty.

I did an experiment a while back comparing passive to an active probe(N2795A) and have run the same on the 1152A with great results - the probe looks to have less loading then my N2795A. A great deal for the price I think.
https://www.eevblog.com/forum/testgear/comparing-an-active-probe-to-passive-probes-for-fun/

[KE5FX]

Looks like a reflection from the probe body is traveling back to the tip and returning to the body.  The ringing artifact is about 60 Mhz wide, suggesting a period close to 3 meters at Vf=0.66.  Since it's an active probe, the cable they're using might be 50 ohm coax, so check the terminating resistance where the cable enters the body, maybe?

[TheSteve]

Sweeping from 10 MHz to 700 MHz in small steps over a long period the amplitude looks pretty good, it only varies by a couple percent as measured by my scope(not counting the 3dB point of my scope). I am probing a termination directly connected to the output of an Anritsu 69017B gen.
As soon as I get more time I'll see if I can wire power up to the probe and connect it to my spectrum analyzer.

[TheSteve]

Connected +5, +12 and -12 to the 1152A today, before connecting it my spectrum analyzer or network analyzer I tried it with the scope and it had zero offset voltage on its own. I then connected it to my network analyzer. Here are two shots the first showing the return loss of my home brew SMA termination and the second shot showing return loss and transmission signal with the probe connected to the SMA termination. Based on the specs of the 1152A and the probing technique I think it is reasonable, especially considering I have to use a N to BNC and then a BNC through to connect the probe to the input.



Probe connected to the termination.



Added one more pic, this time using the stimulus/response suite in my spectrum analyzer. Pretty much the same measurement as the path was normalized then I installed the same SMA terminator and attached the probe to the spectrum analyzers input.

[Bud]

Looks like a reflection from the probe body is traveling back to the tip and returning to the body.  The ringing artifact is about 60 Mhz wide, suggesting a period close to 3 meters at Vf=0.66.  Since it's an active probe, the cable they're using might be 50 ohm coax, so check the terminating resistance where the cable enters the body, maybe?

It says it right on the body


Also in the manual:

The probe output is through a Type-N connector and the probe is designed to be terminated with 50 ? 1%.

[jjoonathan]

Yep, reflections between SA and probe were the issue, classic trap for new players. I wasn't previously aware of how poor the 50 ohm termination on my signal hound was. Here's the response of the probe on a SA with slightly better termination, note that the ripple easily halved and now agrees much better with TheSteve's measurements. Adding a 3dB attenuator between probe and SA (not pictured) didn't reduce the ripple further. I didn't try multiple lengths of cable to see if the ripple width changed, so I can't (yet) properly assign the remaining ripple to probe vs attenuator.

The remaining distinction between TheSteve's probe and my probe seems to be bandwidth: my 54701A looks more like a 2GHz probe, and TheSteve's 1152A looks more like a 3GHz probe, even though they're both nominally 2.5GHz. I still don't entirely trust the 50 ohm terminators I'm probing, one is no-brand and one is home-brew, so maybe they're a bit heavier at 2.5GHz? If so, they both give the same results.

[Bud]

I wasn't previously aware of how poor the 50 ohm termination on my signal hound was.

Thanks for acknowledging that. I was asking for years why signal hound did not specify input return loss for their doggy product since it came out, though I think I knew the answer.

[TheSteve]

The termination I used/probed is a SMA jack with three 0603 150 ohm .1% resistors soldered to it. It tests out pretty decent to 6 GHz and is easy to probe as the resistors are exposed.

[rx8pilot]

This is an interesting thread.... looking for some project probes now :-)

Sent from my horrible mobile....

[TheSteve]

This is an interesting thread.... looking for some project probes now :-)

Sent from my horrible mobile....

Active probes rock - especially when they are cheap.

[rachaelp]

This is an interesting thread.... looking for some project probes now :-)

Sent from my horrible mobile....

Active probes rock - especially when they are cheap.

Thanks for posting up about your mods and the results you're seeing I'm currently looking for some cheap active probes too now I have seen this Unfortunately not much (i.e. currently nothing) available on eBay in the UK....

[Converter]

Just in the treasury of knowledge. When connecting the HP54701 probe to the source, I used the shortest connection possible. Instead of a 50 ohm terminator for the generator load, I used several 3 dB attenuators connected in series. The generator allowed only up to 3 GHz, while the maximum attenuation did not exceed -1.5 dB. And from this value it is also necessary to subtract -0.8 dB - the difference in own losses in the measuring stand in the 3 GHz interval (normalization of the reference signal level in this test configuration is not possible). This means that the actual bandwidth of the probes is well above 3 GHz. In total, I tested HP54701 5 units. There are individual differences in frequency response, but on average it looks like this.