EEVblog Electronics Community Forum
Electronics => Open Source Hardware => Topic started by: awallin on January 02, 2020, 07:28:43 pm
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Are there any good ~1 GHz active probe open designs out there?
would simply a high-speed FET opamp work? comments, ideas?
The +/-2.5V supplies would be nice to generate from 5V USB - any ideas for a small low-noise dc-2-dc converter for that? (followed by low-noise LDOs)
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An FET operational amplifier will work however transient response will be compromised by feedback to the inputs even if configured as a voltage follower. I think you would actually get better performance using a medium input resistance and current feedback amplifier or even better, an operational transconductance amplifier like the OPA860 however package and circuit layout limitations will limit ultimate performance.
I think a better way is the classic JFET source follower input stage preceded by a x5 compensated divider and then bipolar emitter follower to drive a double terminated 50 ohm input yielding a total x10 attenuation. Without any gain stage, this makes best use of the parasitic elements in printed circuit board construction.
There are some more specialized amplifier ICs which someone else may suggest but I am not familiar with them.
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The main objective of an active scope probe is to reduce the 11 pF to 15 pF shunt capacitance of a normal divide by 10 passive scope probe. Secondary objective would be to avoid the 10x signal loss. In order to get the capacitance down you have to have the amplifier at the probe tip end of the scope cable.
Expensive active probes have only couple tenths of pF shunt capacitance. This is less than most common chip resistor parasitic shunt capacitance.
There are some low cost FET followers on ebay but they are capacitively coupled.
For response to D.C. you pretty much have to use op amp. For an op amp solution look for the highest slew rate op amps. Keep the feedback resistors values low to prevent effects of op amp neg input node stray capacitance and layout needs to avoid nearby ground stray capacitance at the op amp - input node. Too high feedback resistor with too much neg input node capacitance can make the op amp unstable.
The op amp output drive current must support the low value feedback resistor in parallel with 50 ohm load impedance output to scope. Scope input needs 50 ohm termination so cable is terminated properly. If scope does not have 50 ohm input option a BNC-T with 50 ohm terminator load will do.
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The parasitics will likely kill any hope of getting some kind of bandwidth. The schematic in the first post isn't going to work at all. It starts with an 100k / 15pf RC filter.
There is a long thread about making your own (differential) 1GHz probes: https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/ (https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/) which ended up with a neat product 8)
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For response to D.C. you pretty much have to use op amp.
JFETs work just fine down to DC and a JFET source follower has many advantages over a series feedback amplifier. Up to 500 MHz, the common active probe design in the past used two JFETs and two RF bipolar transistors.
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The schematic in the first post isn't going to work at all. It starts with an 100k / 15pf RC filter.
where do you get the 15pF figure? BAV99 is 2pF (maybe 4pF for both) OPA858 is negligible. but yeah, talking about copycatting from bad schematics (pin 2 BAV99 should go to +ve power rail)
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These old JFET probes (from Tektronix, HP, Philips...) not only have a low bias input but also a very low input C, which is what makes them really useful in HF measurements at mostly any point in the circuit.
However I'm amazed at the high input C of the hyperexpensive tektronix IsoVu (starting at 41dB$ according to the X-chapters), 20pf at 1MOhm with no input divider and 0.5V range, there goes all the bandwidth
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The schematic in the first post isn't going to work at all. It starts with an 100k / 15pf RC filter.
where do you get the 15pF figure? BAV99 is 2pF (maybe 4pF for both) OPA858 is negligible. but yeah, talking about copycatting from bad schematics (pin 2 BAV99 should go to +ve power rail)
The pads of the board and other capacitances add up quickly so 15pf is kind of a worst case number. And no, the BAV99 are drawn correctly if it was for a low impedance clamp circuit. Dumping excess current into supply rails isn't a good idea. But in this circuit the uneven leakage current through the diodes could introduce an assymetric loading of the input resistor. Dual diodes are often 2 dies in one package which in theory should come from the same wafer but there are no guarantees. With such a high input series resistor you don't really need an external clamping diode. The ESD diodes in the opamp are sufficient.
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What about this thing:https://github.com/dkramnik/Eagle-Public/tree/master/instrumentation-active-probe-elektor (https://github.com/dkramnik/Eagle-Public/tree/master/instrumentation-active-probe-elektor) ?
I think it's been around for ages and the ones sold on ebay for 20€ (based on this circuit) provide decent results.
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What you need is this http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf (http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf)
The author is active here on the forum too.
I built three pieces and now I am doing actially a power supply with this IC http://www.ti.com/product/LM27762 (http://www.ti.com/product/LM27762)
I'll piggy back it to the board and supply it from USB.
Close enough.
Best regards, Jiri
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What you need is this http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf (http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf)
The author is active here on the forum too.
I built three pieces and now I am doing actially a power supply with this IC http://www.ti.com/product/LM27762 (http://www.ti.com/product/LM27762)
I'll piggy back it to the board and supply it from USB.
Close enough.
Best regards, Jiri
I would suggest using one or two LiPo batteries and recharge them via USB.
Much one less cable that is being dragged along the bench when trying to measure something :)
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I'm also interested in one or two. I know the branadic design. Maybe we should respin it.
Maybe with USB supply and a newer /better Opamp? I'm not sure...
I need only 500MHz for my Rigol MSO5000 i want to compensate the lower impedance of the passive probes at the higher frequencies.
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Note that the dual JFET in the Tektronix design achieves nominal zero offset voltage due to the matched FETs and the equal source resistors on the input follower.
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Note that the dual JFET in the Tektronix design achieves nominal zero offset voltage due to the matched FETs and the equal source resistors on the input follower.
It does but grading single JFETs into pairs is almost as good and Tektronix often did exactly this. Graded pairs are less desirable in a production environment however.
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Dear ntcnico
Is DIP1400 1.4GHz differential probe currently avaliable?
I have an Operating manual dated 2019 any new version avaliable?
What about the price of DIP1400 1.4GHz differential probe and how I can buy it.
Thanks for your answer
Roberto
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Pretty awesome new device for a DIY active probe:
Ti BUF802
https://www.ti.com/lit/ds/symlink/buf802.pdf?ts=1643347428244&ref_url=https%253A%252F%252Fwww.google.com%252F (https://www.ti.com/lit/ds/symlink/buf802.pdf?ts=1643347428244&ref_url=https%253A%252F%252Fwww.google.com%252F)
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Good luck getting any TI parts these days. They made some great parts, but it doesn't matter if they are unobtainium.
John
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The Texas Instruments BUF802 looks ideal for a simple as possible design. It can be improved by adding a divider to the input to lower its relatively high input capacitance which will otherwise limit performance. Typically a x5 input divider would be used so that with the x2 attenuation of a double terminated output, x10 attenuation would result. Many x10 active probes do exactly this.
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No shortage of BUF802s at DigiKey or TI's direct-sales outlet.
However, you really want a differential probe. :) Buy one from nctnico.
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No shortage of BUF802s at DigiKey or TI's direct-sales outlet.
You are correct, sir! To be fair, a number of TI parts are hard to get these days, but apparently not this one.
However, you really want a differential probe. :) Buy one from nctnico.
I am a happy customer.
John
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I got 5 samples from TI (TI direkt store) for the BUF802 within 2 days from Singapur so no problems for now..
Now i have to try to find "hobbytime" to do the schematic for a single ended active probe..
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I designed a simple probe using three 2SC3356 transistors
http://www.github.com/profdc9/ (http://www.github.com/profdc9/)
https://github.com/profdc9/RFUtilityKnife (https://github.com/profdc9/RFUtilityKnife)
You could also use a BFR93 rather than 2SC3356 (I used this because it was available at LCSC cheap, it is a 7 GHz transistor).
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Are there any good ~1 GHz active probe open designs out there?
would simply a high-speed FET opamp work? comments, ideas?
The +/-2.5V supplies would be nice to generate from 5V USB - any ideas for a small low-noise dc-2-dc converter for that? (followed by low-noise LDOs)
I have an FET RF probe, it works well. It uses what must be a few dollars in parts.. It does not load the circuit, so the scope can see RF better.
Basically thats what it does. I got mine on ebay...It works well..
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no reason for it
Use a Zo probe like Tektronix P6156
https://w140.com/tekwiki/wiki/P6156
See Tektronix Circuits Concept book Oscilloscope Probe Circuits
Jon
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A forum search for 'z0 probe' (note z zero) will yield a lot of threads.
Edit: Actually, so will 'Zo probe' (although the z0 ones are more relevant).