Active Probe

[olsenn]

Does anyone know where I can buy a cheap active probe; preferably one that covers the 9KHz to 1.5GHz frequency range? I don't require extream linearity, if that helps bring the price down to << $500.

[quantumfall]

I have no idea if this is any use but you might be able to reconfigure it, http://www.elektor.com/magazines/2004/october/poor-man-s-1-ghz-active-probe.57219.lynkx

 I know nothing about High Frequency like these.

[alm]

Try eBay, though << $500 might be hard, < $500 should be possible. There are several Tek P6245 probes for $300-$400 on eBay, for example. Many of the active probes use proprietary connectors for the power supply, but some of the earlier ones (eg. Tek P6202, but this one is only 500 MHz) use external power bricks. External power supplies for the proprietary power connectors tend to be rare and expensive on the used market, but you may be able to hack something up. Finding pinout and voltages should be easy. With the separate three or four pin Fischer power connectors, you should be able to just cut one of and solder on a new one, since it's only carrying DC.

Resistive divider probes are much cheaper. DC loading is much higher (500-5000 kohm), but loading at 1.5 GHz is similar. The Tek P6156 and P6158 both have about 3 GHz of bandwidth. They are sometimes available for well under $100, though there are none on eBay at this time.

[free_electron]

there are frequently Agilent active probes on ebay. 1652A i believe. can be had for under 150$. These do 1.5 Ghz without problems and are excellent. But... they only mate with Agilent infiniium scopes... if you want to use them standalone you will have to make a power supply that gives +15 -15 and +5 volts into the probe. It's not hard to do. simply take the scope end apart ( the amplifier sits in the tip of the probe. ). the scope end has pogo pins to tap into the scope's power bus. Take the pogo's off and jerry rig your own supply.

[dfnr2]

Is this for a scope, or for a spectrum analyzer?

Dave

[tnt]

For what scope do you want it ?

I have a Agilent 1152A I can sell you.
By default the probe is not compatible with newer X series scope, but I have a mod I can do to use it with the newer scopes (basically just need one more LDO).
And if you don't have a agilent scope, I also have a document explaining how to provide power and offset control to the probe externally.

(not that all those mods are purely in the DC stuff, the high speed / high precision stuff is entirely in the probe head and not touched)

[olsenn]

Is this for a scope, or for a spectrum analyzer?

This is for a spectrum analyzer. I imagine the difference is AC coupled vs DC coupled???

I have a Agilent 1152A I can sell you.

I feel a little uneducated in this stuff, so I'll clarify what I want this for. I recently purchased a Rigol DSA815-TG spectrum analyzer which is rated for operation between 9KHz and 1.5GHz. Right now, since the input impendance of the SA is 50 ohms, I can't test the frequency responce of passive filters etc which is why I want an active probe.

I recently ordered some LMH6702MA ultra wideband op-amps through NI's sample order page, and I will attempt to rig something up with them once they arrive, but if you think this either won't work, won't work well, or you're willing to sell me a suitable professional probe for cheap, then please let me know.

I appreciate all comments

Nick

[free_electron]

Drat ! 1152A is what i meant.. where did i get that 1652 number ... brainfart i guess . Those things go sometimes for 100$ on ebay. you cant even begin to make something yourself at that price ( not with that build quality )

Those will do perfectly fine. I use them with my scopes. I have the connector pinout of the scope interface. yank out the pogo pins, solder 4 wires and make a simple power supply to feed the amplifier. the clamshell on the scope end is easy to remove. of you look next to he bnc you will see two little square hole. poke in two paperclips end the clam will pop open.

Note that this probe is about 150 Kiloohm input impedance and not 1 Mega or 10 Megaohm as you'd expect from a passive probe.

[olsenn]

Drat ! 1152A is what i meant.. where did i get that 1652 number ... brainfart i guess . Those things go sometimes for 100$ on ebay. you cant even begin to make something yourself at that price ( not with that build quality )

Those will do perfectly fine. I use them with my scopes. I have the connector pinout of the scope interface. yank out the pogo pins, solder 4 wires and make a simple power supply to feed the amplifier. the clamshell on the scope end is easy to remove. of you look next to he bnc you will see two little square hole. poke in two paperclips end the clam will pop open.

I just ordered a 1152A on ebay. Can you please tell me which pins are ground and V+, and what voltage V+ should be? It looks like there are 7 gold plated pins sticking out of the unit... do you have a pinout for these? Actually, if you have a photo of your setup that you can add to this, I would greatly appreciate it

[free_electron]

I use them with the matching oscilloscope so no need for me to kludge around.

The probe has 9 pins : The mating surface ( the part that would be on the scope ) has the following signals( from left to right )


1 : +3 volts
2 : -3 volts
3 : offset   ( this is driven by a dac in the scope ) connect a 2k2 trimmer between +3 and -3. the wiper goes via a 1k resistor to this pin
4 : SDA ( this is used by the scope to read the calibration data )
5 : Ring ( this is the probe ID that tells the scope what is attached. you do not connect this. leave floating )
6 : SCL ( this is used by the scope to read calibration data )
7 : Rp  : this tells the scope what the termination should be for this probe : you leave this floating. The probe needs 50 ohm termination and your analyzer has this.
8 : -12 volts
9 : + 12 volts

ground is the BNC chassis part.

Here is the manual :  http://cp.literature.agilent.com/litweb/pdf/54855-97008.pdf page 132 has a nice drawing.

simply take a supply with a 7812 and 7912 and a lm317 and lm337.
You will need to put heatsinks on the regulators. The probe draws about 300mA

probe manual : http://cp.literature.agilent.com/litweb/pdf/01152-97002.pdf

Code: [Select]

                  2k2
+3v--------/\/\/\/\/------- -3v
              /|\
               |
                ---/\/\/\-----> pin 3 (offset)
                         1k



some background info ( not relevant to you, but as general knowledge)

the agilent scopes detect a probe being plugged because the voltage on the RING pin changes. internally in the scope is a current source. every type of probe has a different resistor between ring and ground. so if you plug the probe the voltag on the RING pin goes from 5 volts to whatever the resistor sets. the scope reads this and knows if it has a 1:1 1:5 1:10 1:20 or some other probe.

the scope also sends a current into the Rp pin. a resistor there also tells the scope if it needs to change to 50 ohm or stay in 1m mode. if no resistor is fitted there the pin stays at 5 volt and the user can select using the buttons on the scope. if a resistor is fitted the scope goes to that particular mode , changes its input attenuator to the correct scaling gain and the user is locked out of changing the input scaler. this guarantees the display on the scope is automatically set correctly ( plug in a current probe and the scope will switch to 50 ohms termination and change scale to ampere / division. )
So the RINg and Rp sgnals tell the input system what we are dealing with. This is all happening in the analog domain on the infiniium scopes.
The scope then tries to read the eeprom using SDA and SCl ( this is a standard I2C bus. most probes have a 24c08 or 24c16 eeprom fitted )
This eeprom can hold calibration constants like bandwidth curves , compensation curves etc... the scope uses this info to fine-tune its acquisition system.

In your case you don't care. you will use the calibration loop on the spectrum / network analyser to calibrate away both probe and your tracking generator.

to do absolute measurements you may want to verify where the probe rolls off and how flat it really is.

to do this. take a bnc cable , a T piece and a 50 ohm terminator.

Code: [Select]

                           0
                           |
tracking gen ------------0-+-0----------- analyser
run the thru-calibration on the analyser

Code: [Select]

                            -------<probe---------analyser
                           |
                           0
                           |
tracking gen ------------0-+-0---- 50 ohm


the curve you now see is the gain bandwidth curve for the probe. store that in memory of the analyser. you can then use that to compensate your measurements.

if i got a few fre eminutes i'll draw a nice schematic and pcb ( sigle sided so you can etch at home ) something with 2 BNC connectors so you can make this a 'universal probe' pwer supply.

[tnt]

You description isn't entirely accurate (at least for some probes).

pin 7 is used to control what voltage is present on pin 1 & 2. Depending on the value it can be between +/- 3V  +/- 6V. (Infinite resistance = 3V, 40k = 4V, 20k = 5V, short = 6V).
The 1152A has a 40k resistor meaning it wants 4V.
Note that the 3000-X series has a cost optimized version that only provides constant +/- 5V which is why it doesn't work with the 1152A by default.

See this thread for more infos :
https://www.eevblog.com/forum/projects-designs-and-technical-stuff/agilent-probe-interface-%28-autoprobe-and-probe-id%29/

Also FYI the 1152A doesn't use the -4V rail.

[free_electron]

I can't speak for the 2000 or 3000 series. I don't have those scopes. The 1152 was designed for the 54845a / 54855a / 54831 /54832. ( i have the last two machines. It works fine there. . It also works on my MSO7104 without problems ).

Also the link i posted to the service manual talks about 3.3 volts.

 But you are right. There may be more in that interface than we know. I,ll hook it up tonight to the scope and measure the voltages on the autoprobe interface while the probe is connected. Then we'll know for sure.

[olsenn]

So just to make sure I understand what is being said, if I apply +3V to Pin1, -3V to Pin2, -12V to pin8, and +12V to Pin9... and leave all other pins floating, the device should work?

There seems to be some confusion as to whether Pins 1+2 should be +-3V, +-3.3V, +-4V, +-5V, or +-6V. Would applying +-6V to these pins cause the probe to be destoyed?

Thanks

[free_electron]

Dont play around with it. I will hook it up to my scope tonight and measure the voltage on pins 1 and 2 . Then well know for sure.

@tnt : you have that pdf file ? Alciom removed it from their site ...

[dfnr2]

Is this for a scope, or for a spectrum analyzer?

This is for a spectrum analyzer. I imagine the difference is AC coupled vs DC coupled???

That's what I thought.  I recommend the HP54701a.  It's designed for a spectrum analzyer, with the proper N-connector.  If you have an HP analyzer, the probe will plug right in to the probe power connector--otherwise you will have to provide +15V (and ground of course).  It's a superb probe with a 2.5 GHz bandwidth, and you can find several on ebay right now, some fully kitted out with all the tips & accessories and case for $300-$400.

Good luck,

Dave

[dfnr2]

Drat ! 1152A is what i meant.. where did i get that 1652 number ... brainfart i guess . Those things go sometimes for 100$ on ebay. you cant even begin to make something yourself at that price ( not with that build quality )

Those will do perfectly fine. I use them with my scopes. I have the connector pinout of the scope interface. yank out the pogo pins, solder 4 wires and make a simple power supply to feed the amplifier. the clamshell on the scope end is easy to remove. of you look next to he bnc you will see two little square hole. poke in two paperclips end the clam will pop open.

I just ordered a 1152A on ebay. Can you please tell me which pins are ground and V+, and what voltage V+ should be? It looks like there are 7 gold plated pins sticking out of the unit... do you have a pinout for these? Actually, if you have a photo of your setup that you can add to this, I would greatly appreciate it

Whoops, I should have read ahead before replying.  Although you probably don't want to hear this, I would recommend you re-ebay the 1152A, which will be a kludge at best, and will introduce artifacts at worst, and go for a proper probe.  You will be out the cost of shipping, and some ebay fees, but even so, you should still come in under $500. 

Also, depending on your application, you may want to look into the HP 84705a preamplifier.  It's not an actual probe, but if you are looking at making, for example, E&M field measurements, or antenna characterization, it might be the best choice.  Also can be had in working condition for <$400 if you look carefully.

If you keep the 1152A, you may want to think about cracking the case, removing the BNC, and soldering a 50 ohm N right on to the board, and soldering on proper power connections wrapped around a ferrite, and putting it in an RF box.


Good luck,

Dave

[tnt]

The HP54701a actually looks like the exact same hardware as the 1152A, only the connection/compbox is different but the active part seems the same.
BTW, the 1152A comp box has internal filtering for all the input rails.

[Rufus]

It's a superb probe with a 2.5 GHz bandwidth, and you can find several on ebay right now, some fully kitted out with all the tips & accessories and case for $300-$400.

0.6pF is 265 ohms at a gig and it is x10. Is it going to be better than a 450R 0603 stuck on the end of some 50 ohm coax?

[tnt]

nope.

[free_electron]

  HP54701a

the 1152 is the same probe as the HP54701a. just a different mating connector.

and soldering a 50 ohm N right on to the board

Why would you try to force-mate a connector with a board that is not designed for that. There is nothing wrong with the BNC on the probe.

soldering on proper power connections wrapped around a ferrite, and putting it in an RF box.

No need for that either. the tail of the probe has everything in it that is needed.
Apply power and that's it. Use a clean linear power regulator with good rejection like an LM317 and LM337 and you are golden.

I use the 1152 frequently ( with scopes , not with SA ) and they work perfectly fine.

[dfnr2]

0.6pF is 265 ohms at a gig and it is x10. Is it going to be better than a 450R 0603 stuck on the end of some 50 ohm coax?

If you're measuring a pure 1GHz sine wave, you can use almost anything you want as a probe, as long as you know the gain/attenuation, and it's not picking up or generating 1GHz noise.  Or even easier, you could use an RF detector hooked up to a DVM, and skip the SA altogether.  You can check out Doug Smith's 1 GHz probe for an idea of how to make a decent passive probe, BTW.

However, if you're measuring a broadband signal above 500-1000 MHz, then you'll have a hard time getting a flat response without an active probe.  Also, above 1GHz, you really have to know the what, why, and how of  your measurement, in order to factor in the probe loading effects, and other artifacts from your test equipment.

Dave

[alm]

However, if you're measuring a broadband signal above 500-1000 MHz, then you'll have a hard time getting a flat response without an active probe.

Plenty of passive probes go well beyond 1 GHz with excellent flatness (often better, since they contain less active and parasitic components to mess up the signal). Some example: 3 GHz, 1.5 pF, 6 GHz, 0.25 pF, 9 GHz, 0.15 pF. Impedance is much more constant than active probes. The 9 GHz probe has 500 ohm impedance at DC, and about 100 ohm at 9 GHz. HP 1152A is 100 kohm at DC and 100 ohm at 2.5 GHz. Main downside is the DC loading, AC loading (capacitance) can actually be lower than many active probes. Howard Johnson wrote some articles about their advantages for use in high-speed digital circuits.

Achieving > 1 GHz in a DIY design without a spectrum analyzer / tracking generator to tweak will be quite tricky, however.

[olsenn]

Remember, I need this for a spectrum analyzer, not an oscilloscope, and my SA has an input impedance of 50ohms; thus a passive probe won't work.

[alm]

What input impedance would you expect a > 1 GHz passive probe to require? Would you be able to design a passive probe with 3 GHz bandwidth in a 1 Mohm // 10 pF input? What would the loading of a capacitive voltage divider be at 3 GHz?

All the passive probes discussed in this thread, including the DIY designs, are resistive Z₀ probes designed to operate with 50 ohm inputs.

[dfnr2]

No need for that either. the tail of the probe has everything in it that is needed.
Apply power and that's it. Use a clean linear power regulator with good rejection like an LM317 and LM337 and you are golden.

I use the 1152 frequently ( with scopes , not with SA ) and they work perfectly fine.

I don't get it.  Why recommend that someone go out and buy a 1152A instead of a 54701a for a SA with an N connector? It's just not great advice.  Going with the 1152A really only makes sense if you have an infiniium.  I love my lecroy active probes, but I'm not going to recommend them here, either.  Now, if someone gets one free and wants to use it with another system, that's another story. . .