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Old 19th Jun 2020, 2:27 pm   #41
G0HZU_JMR
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Default Re: Hi-Z input probe by Bob Pease.

Could be. The 4.7pF cap also really helps to manage the negative resistance at the probe input as demonstrated in the video.

I've just built a 'real' version of the (drain) boostrapped JFET buffer amplifier using a 2N5485 and a MMBTH10 BJT. I've added the necessary bias components and run it from an external PSU. It is nearly all in SMD and it is very small! I have built this in a way that is quite different from the Bob Pease probe. I have used a layout that minimises stray capacitance to free space. So there are no 'wires' or component legs anywhere. It is point to point SMD construction under a microscope and the whole thing is about 8mm x 5mm in size. I think this is why I get the predicted input capacitance of about 0.2pF.

I have cheated slightly because I have fed the gate bias in via the VNA bias tee so I have lost the (0.05pF) capacitance of the SMD input shunt resistor. But see the result below. It's a bit boring because it agrees very well with the previous (simulated) version of the circuit where I use s parameter models for the transistors.

This result makes sense to me because I have been able to (almost) cancel the input Cdg and Cgs capacitance of the 2N5485 JFET with the boostrap and a 10k source resistor that mimics a current source. I've tried very hard to minimise any free space capacitance.

Note that if I measure a precision 0.3pF 0603 packaged SMD microwave capacitor in the place of this circuit my VNA/jig measures a flat 0.35pF out to 200MHz. The jig has some fringing capacitance so this adds to the result.
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Old 19th Jun 2020, 5:58 pm   #42
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Default Re: Hi-Z input probe by Bob Pease.

To show that you don't need to spend megabucks on a lab VNA I tried testing the little bootstrap circuit with my 30 nanovna this afternoon.

I haven't used my nanovna for ages because the overall experience has always been spoiled by the lack of decent PC software to go with it. But I charged it up today and had a go at configuring it to calibrate up into my s parameter test fixture. This should exploit the full potential of the nanovna.

I was hoping things would have improved but the latest 'saver' PC software seems as buggy and error prone as ever. It took me many attempts wading through various workarounds to get it to spit out a valid S1p data file that didn't contain obvious data errors across the sweep. The whole point of a modern VNA is to be able to extract valid network models to a computer.

Anyway, I finally managed to extract a bug free s1p data file from it and the result is as below. Despite the crappy PC software, the little nanovna is a brilliant device. Well worth buying even if it is used standalone without any PC software. This tool should do a much better job of measuring the Pease probe compared to any attempt to reverse engineer the input impedance using a series test capacitor. Note that I didn't use the bundled calibration kit, I used my own and this allows the calibration reference plane to be set right at the gate of the JFET and this hugely improves the accuracy of the measurement.

The plot below is quite good although it does get very noisy below 10MHz.
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Old 19th Jun 2020, 8:48 pm   #43
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Default Re: Hi-Z input probe by Bob Pease.

I managed to get all the parts to make the Pease probe today and I've now made an initial (ugly) version. See below for a plot of series capacitance Cs and resistance Rs.

The parallel capacitance looks very similar and I think the series plot is more intuitive in this case. Note that this version does not have the full length probe tips pins fitted. The typical capacitance is about 1pF. The result is maybe 0.2pF greater than I expected but this is probably accounted for by the free space capacitance of the short tip wires and a bit of fringe capacitance in the test setup. There is the expected amount of negative resistance and this probe can easily be made to go unstable and oscillate when probing inductive loads. I think that if I connect to it more directly (without the tip wires) the capacitance might dip just below 1pF.

I found that the best way to interface it to a scope was with a correctly compensated x10 scope probe connected via a tip to BNC adaptor. This does have the penalty of dividing the voltage by 10 but it does give the most faithful reproduction of waveforms presented at the input to the probe.

I think I'll have a go at making a nicer version that fits into a probe housing. I'm not sure what to do about the negative resistance issue. One workaround is to simply avoid probing circuits with inductors!
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Old 19th Jun 2020, 10:40 pm   #44
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Arrow Re: Hi-Z input probe by Bob Pease.

Quote:
Originally Posted by G0HZU_JMR View Post
You might get better waveform edges with the faster transistors but I think the original circuit will struggle here.
When you refer to 'faster transistors', I assume you're referring to the BJTs. The original design uses 2N3904 (fT 300 MHz) and 2N3906 (fT 250 MHz). My version uses BFY90 (fT 750 MHz) and 2N4241 (fT 2000 MHz). I could replace the BFY90 with a BFR91A (fT 6000MHz) but I would have thought that the 2N4241 should be fast enough for this cct.
I think it's time for a version 3 build * - when time permits!

*
ver. 0: the B.P original design.
ver. 1: my version with BF256, BFY90, 2N4241
ver. 2: as ver.1 but with 2N4416 (x2)

Al.
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Old 19th Jun 2020, 10:48 pm   #45
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Default Re: Hi-Z input probe by Bob Pease.

Quote:
When you refer to 'faster transistors', I assume you're referring to the BJTs.
Yes, but I think I worded it poorly. What I meant was that you are using faster transistors than the original 2N3904 and 2N3906 BJTs and that might explain why you are seeing a better waveform with minimal ringing.

I did try my copy of the original with a square wave drive and fed the output of the probe to an unterminated scope via a short run of RG58 coax. I did see some ringing but I'm using the original 2N3906 PNP BJT at the output. Since that test I've fitted a x10 scope probe to the scope and then connected the tip of the x10 probe to the Pease probe output using a scope tip to BNC coaxial adaptor. This seems to work a bit better although it still struggles a bit once the waveform gets above about 2Vpkpk.
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Old 9th Jul 2020, 3:59 am   #46
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Default Re: Hi-Z input probe by Bob Pease.

I wonder if you guys have seen this thread on another forum re RF Probes
https://www.eevblog.com/forum/rf-mic...18-transistor/.

In terms of any direct relevance to this thread, I guess the key term is "Hi-Z", or possible lack of it. I cannot spot any specific number being quoted for the input Z of the probe on eevblog. Is it of any interest or not quite in the park?

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Old 10th Jul 2020, 12:48 pm   #47
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Default Re: Hi-Z input probe by Bob Pease.

Hi!
I had a quick look at that and the associated threads - yes, it does look interesting. On account of the pressure I'm currently under - lots of urgent things demanding my immediate attention - I'll have a further in-depth look when I can.
However, having said that, the essential problem with any probe is its input capacitance and the ability to make a very low inductance connection to the 0v. / gnd. reference point when it is in use. An input capacitance at the probe tip needs to be not more that 1 pF and an inductance not more that a few nH for use with high-Z sources. It's more than component selection that is important: physical layout and mechanical construction is also very critical. The only ones I've seen that come anywhere near the 'ideal probe' are made by Tektronix - and they are exceptionally expensive!

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Old 11th Jul 2020, 8:45 pm   #48
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Default Re: Hi-Z input probe by Bob Pease.

I had a quick look at the BJT probe on Eevblog and it may prove useful for certain tasks. However, the input Rp will only be about 20k ohm across LF through into VHF. The input capacitance Cp is probably below 1pF.

I think it will generate negative resistance up in the VHF region but probably not enough to cause problems with stability unless you deliberately went looking for instability. The probe will probably be at its best when probing RF paths with lowish impedance and small signal levels (below 1mW?).

So if you like to build stuff up at VHF using 50R in/out stages then this probe might be useful because it looks to have plenty of bandwidth. It won't be much use if you want to probe high impedance circuits across LF through to VHF as the 20k ohm Rp will load the circuit a lot. I'd imagine that some care would be needed with the PCB layout for the probe if it was built with NEC 2SC3356 transistors. It could go unstable up in the GHz region with a poor layout.

I haven't tried to build it or do much analysis on it because it doesn't have much appeal to me.

I also managed to find the classic JFET app note with the bootstrapped 2N4416 + BJT. This circuit can deliver an input capacitance below 0.25pF in theory. This is very similar to the bootstrapped circuit I built and tested in post #41. You can see in post #41 I managed to get about 0.25pF when measured on a VNA although my version didn't have the bootstrapped 10Meg gate resistor. I hid this resistor in the external VNA bias tee.

It is possible to get very low input capacitance with circuits like this but the 2N4416 + BJT circuit below will generate an awful lot of negative resistance up at HF and into VHF. This means it would have to be used with some care or it could go unstable.
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