Active Probe - Page 1
Forum section: electronics / general

author: olsenn
author_rank: Frequent Contributor
timestamp: April 03, 2012, 05:38:01 pm
content: 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.

author: quantumfall
author_rank: Regular Contributor
timestamp: April 03, 2012, 07:29:32 pm
content: 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.lynkxI know nothing about High Frequency like these.

author: Unknown
author_rank: Unknown
timestamp: April 03, 2012, 08:11:36 pm
content: 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.

author: free_electron
author_rank: Super Contributor
timestamp: April 04, 2012, 12:21:15 am
content: 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.

author: dfnr2
author_rank: Regular Contributor
timestamp: April 04, 2012, 04:28:56 am
content: Is this for a scope, or for a spectrum analyzer?Dave

author: tnt
author_rank: Regular Contributor
timestamp: April 04, 2012, 12:39:15 pm
content: 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)

author: olsenn
author_rank: Frequent Contributor
timestamp: April 04, 2012, 12:50:49 pm
content: QuoteThis is for a spectrum analyzer. I imagine the difference is AC coupled vs DC coupled???QuoteI 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 commentsNick

author: free_electron
author_rank: Super Contributor
timestamp: April 04, 2012, 07:00:49 pm
content: 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.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 12, 2012, 02:31:46 am
content: QuoteI 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

author: free_electron
author_rank: Super Contributor
timestamp: April 12, 2012, 03:20:17 am
content: I use them with the matching oscilloscopeso 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 volts2 : -3 volts3 : 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 pin4 : 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 volts9 : + 12 voltsground is the BNC chassis part.Here is the manual :http://cp.literature.agilent.com/litweb/pdf/54855-97008.pdfpage 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 300mAprobe manual :http://cp.literature.agilent.com/litweb/pdf/01152-97002.pdfCode:2k2+3v--------/\/\/\/\/------- -3v/|\|---/\/\/\-----> pin 3 (offset)1ksome 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:0|tracking gen ------------0-+-0----------- analyserrun the thru-calibration on the analyserCode:-------<probe---------analyser|0|tracking gen ------------0-+-0---- 50 ohmthe 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.

author: tnt
author_rank: Regular Contributor
timestamp: April 12, 2012, 08:14:00 am
content: 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.

author: free_electron
author_rank: Super Contributor
timestamp: April 12, 2012, 12:01:44 pm
content: 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.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 12, 2012, 12:23:44 pm
content: 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

author: free_electron
author_rank: Super Contributor
timestamp: April 12, 2012, 12:38:27 pm
content: 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 ...

author: dfnr2
author_rank: Regular Contributor
timestamp: April 12, 2012, 03:18:59 pm
content: Quote from: olsenn on April 04, 2012, 12:50:49 pmThat'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

author: dfnr2
author_rank: Regular Contributor
timestamp: April 12, 2012, 04:06:11 pm
content: Quote from: olsenn on April 12, 2012, 02:31:46 amWhoops, 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

author: tnt
author_rank: Regular Contributor
timestamp: April 12, 2012, 04:15:02 pm
content: 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.

author: Rufus
author_rank: Super Contributor
timestamp: April 12, 2012, 04:24:30 pm
content: Quote from: dfnr2 on April 12, 2012, 03:18:59 pm0.6pF 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?

author: tnt
author_rank: Regular Contributor
timestamp: April 12, 2012, 04:28:55 pm
content: nope.

author: free_electron
author_rank: Super Contributor
timestamp: April 12, 2012, 07:02:03 pm
content: Quotethe 1152 is the same probe as the HP54701a. just a different mating connector.QuoteWhy 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.QuoteNo 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.

author: dfnr2
author_rank: Regular Contributor
timestamp: April 12, 2012, 07:25:30 pm
content: Quote from: Rufus on April 12, 2012, 04:24:30 pmIf 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 outDoug Smith's 1 GHz probefor 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

author: Unknown
author_rank: Unknown
timestamp: April 12, 2012, 08:37:21 pm
content: Quote from: dfnr2 on April 12, 2012, 07:25:30 pmPlenty 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.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 12, 2012, 08:40:39 pm
content: 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.

author: Unknown
author_rank: Unknown
timestamp: April 12, 2012, 09:03:22 pm
content: 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 Z0probes designed to operate with 50 ohm inputs.

author: dfnr2
author_rank: Regular Contributor
timestamp: April 12, 2012, 10:04:23 pm
content: Quote from: free_electron on April 12, 2012, 07:02:03 pmI 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. . .

author: olsenn
author_rank: Frequent Contributor
timestamp: April 12, 2012, 10:16:29 pm
content: QuoteConsidering as I've already ordered the 1152A (and paid $150) I was a little scared to read this comment, haha, but after looking up the 54701a, I see that it costs 3x or more than the 1152A, so I guess everything is okay afterall. I will aggree though that it does suck that my soon to arive active probe isn't ready out of the box (doesn't have an N type connector and I need to hack together a power source for it).To be honest, I don't really know the pros and cons of an active probe vs a Z0 passive probe, all I know is that I recently purchased a $1500 spectrum analyzer with the primary intention of using it to test the frequency responce of filters etc, and it doesn't seem to work well when I just plug it in directly; after looking at the most recent video on "theSignalPath", it shows an active probe being used to do precisely what I want to do.

author: free_electron
author_rank: Super Contributor
timestamp: April 12, 2012, 10:34:12 pm
content: Quote from: dfnr2 on April 12, 2012, 10:04:23 pmbecause :1) the TS ( topicstarter ) has a RIGOL spectrum analyzer with tracking generator that does NOT have the HP probe power interface connector.2) the 1152a is 1/3 of the price than the 54701a for the same probe ...BNC to N adapters are 2$ in your local dustbin store.So , 1152 + homebrew three regulators ( stuff any hobbyist has laying around ) + adapter = still 1/2 of the 54701A.And if someone makes a nice PCB layout for this power supply it will allow a trove of people to use the 1152A..The reason the 1152 A is so cheap is because lots are avaialble and there is no demand. Very few people have infiniiums , and there is a newer generation of these probes for the new infinivision machines.

author: dfnr2
author_rank: Regular Contributor
timestamp: April 13, 2012, 02:48:48 am
content: Quote from: free_electron on April 12, 2012, 10:34:12 pmI see your point. But looking through Ebay, the cheapest working 1152A is $250, while a working 54701a is $350. Not that much of a savings. And a proper 50 ohm BNC to N adapter is going to eat up a chunk of that savings. The $2 BNC to N adapter belongs in the dustbin for a reason. To start with, it will--almost certainly--damage the N connector on the SA, sometimes after just one cycle.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 13, 2012, 12:51:21 pm
content: QuoteThe 1152A I purchased cost me $150USD + $20 shipping. I already own a BNC to N adapter, and I can't imagine something like that causing any harm to my device. The only real beef i have with it is that i am going to have to figure out how to power it and properly use it (bias its offset pins etc)

author: dfnr2
author_rank: Regular Contributor
timestamp: April 13, 2012, 02:01:37 pm
content: There's nothing about a BNC to N adapter that would intrinsically damage your SA. In fact, it's a very good idea to put some kind of adapter on the SA--even a N-N adapter (which some people call a "connector saver"), and leave it there for good, so any connections you make will be on the adapter. That way, if/when you manage to damage the threads, you just have to replace the adapter, and not repair the SA.That said, it's worth the investment to get decent quality adapters. The difference is that the cheap adapters have imperfect tolerances on the threads, and sometimes rough threads, and can scratch the high quality N adapter on the SA. If you buy even good adapters on Ebay, you should try to look carefully at the pics for nicks or damaged threads, and look for some kind of return policy.When going to be using the SA for an extended period, I leave a high quality 10 dB attenuator on the input. This not only protects the connector, but also protects the input against ESD damage.

author: dfnr2
author_rank: Regular Contributor
timestamp: April 13, 2012, 02:15:12 pm
content: Quote from: olsenn on April 04, 2012, 12:50:49 pmBy the way, I was just recently looking at the datasheet on that generator. It looks like a really nice piece of kit at a fantastic price. This particular device looks like a perfect machine for precompliance, plus a ton of RF stuff applications. I think Rigol is gunning to bring the middle market to the masses. What are your impressions of this device? It would great if you posted a blurb in the reviews forum.Dave

author: ejeffrey
author_rank: Super Contributor
timestamp: April 13, 2012, 04:24:59 pm
content: Quote from: olsenn on April 12, 2012, 08:40:39 pmThey make passive probes designed to work with 50 ohm inputs. They are often called 'low Z' probes or 'transmission line' probes. In the simplest form they are a 450 ohm SMD resistor soldered to the end of a coaxial cable which makes a 10:1 voltage divider with the source. You can easily make one yourself that is good to 1GHZ, if you are careful you can do a lot better.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 13, 2012, 04:34:26 pm
content: QuoteYou can follow the discussion on this item here:https://www.eevblog.com/forum/product-reviews-photos-and-discussion/spectrum-analyzer-rigol-dsa815/I promise to do a full review on it after a couple of months of getting used to how everything works. So far I am very pleased with it, and I don't imagine there are any alternatives that come close to the performance specs and functionality of the Rigol DSA815-TG.

author: free_electron
author_rank: Super Contributor
timestamp: April 13, 2012, 05:38:17 pm
content: i found the original paten tthat describes the autoprobe !Patent 5939875http://www.google.com/patents?id=X4cYAAAAEBAJ&pg=PA6&source=gbs_selected_pages&cad=4#v=onepage&q&f=falseIt has the entire lookup table and the full decoded matrix as well as schematics.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 16, 2012, 01:37:54 pm
content: QuoteDid you ever manage to figure out what the voltages need to be for the 1152A? I'm assuming it's +4V, 0V, -12V, and +12V??? Also, do I need to add any resistors between pins or do any weird things (or actually anything other than just applying power) to get the probe to work?

author: plesa
author_rank: Frequent Contributor
timestamp: June 05, 2013, 11:32:50 pm
content: The voltages and other useful information for Autoprobe interface are in the threadhttps://www.eevblog.com/forum/projects/agilent-probe-interface-(-autoprobe-and-probe-id)/msg43788/#msg43788

author: KJDS
author_rank: Super Contributor
timestamp: June 06, 2013, 11:59:48 am
content: out of interest, what impedances are the filters that you want to test designed to work with?