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PRP1 - Low cost 2GHz power rail probe
Calvin:
Hi,
triggered by Your thread I quickly built a probe based on the circuit suggested by Tommi Otsavaara in his thesis "1 GHz Power Rail Probe".
Seemingly he orientated on the Keysight N7020 probe that Sharia kindly dared to open up for us ;)
The major differences to the Levido DIY-probe seems the higher output resistor of the LF-path of 590R which requires the second inverting stage to have a gain of -11.8, and the increased Offset range of up to 50V
I had some OP270 lying around, which is a low en-noise dual OP27, which in simulation results in very nice low noise, but it´s decompensated and faster brother OP37 would be better suited to the second inverter gain stage.
In the Levido probe where the second inverter is just a buffer with a gain of -1 the OP27 may get problems due to the required higher load currents.
As alternatives TI lists some ´Soundplus audio-OPAs´ like the OPA1655/1656 with lownoise, sufficient GBW and high current outputs.
If higher supply lines are possible the JFET-Input OPA827 appears interesting.
Due to the overall high offset-gain of 118 the Otsavaara probe´s offset-setting becomes critically sensitive and rather requires multiturn-potis even though potis are used for coarse and fine tuning already.
Another point is the lowpass filtering of the LF-path.
The Otsavaara relies on simple RC-Filtering, which in this case leads to a shelving-filter character.
In simulation this adds ~0.3dB of output signal level to the HF-path.
The Levido probe on the other hand utilizes a RLC-filter that follows a LP-filter curve much closer, hence attenuates the LF-path alot better.
@tszaboo
increasing the Offset range from +-24V to +-50V or even more (R&S claims up to 60V) seems easy ... just a bit of shuffling values of the Potis and the offset-input resistor.
Do You plan on offering a probe with higher offset range also?
regards
Calvin
tszaboo:
--- Quote from: Calvin on September 01, 2024, 08:51:18 am ---Hi,
triggered by Your thread I quickly built a probe based on the circuit suggested by Tommi Otsavaara in his thesis "1 GHz Power Rail Probe".
Seemingly he orientated on the Keysight N7020 probe that Sharia kindly dared to open up for us ;)
The major differences to the Levido DIY-probe seems the higher output resistor of the LF-path of 590R which requires the second inverting stage to have a gain of -11.8, and the increased Offset range of up to 50V
I had some OP270 lying around, which is a low en-noise dual OP27, which in simulation results in very nice low noise, but it´s decompensated and faster brother OP37 would be better suited to the second inverter gain stage.
In the Levido probe where the second inverter is just a buffer with a gain of -1 the OP27 may get problems due to the required higher load currents.
As alternatives TI lists some ´Soundplus audio-OPAs´ like the OPA1655/1656 with lownoise, sufficient GBW and high current outputs.
If higher supply lines are possible the JFET-Input OPA827 appears interesting.
Due to the overall high offset-gain of 118 the Otsavaara probe´s offset-setting becomes critically sensitive and rather requires multiturn-potis even though potis are used for coarse and fine tuning already.
Another point is the lowpass filtering of the LF-path.
The Otsavaara relies on simple RC-Filtering, which in this case leads to a shelving-filter character.
In simulation this adds ~0.3dB of output signal level to the HF-path.
The Levido probe on the other hand utilizes a RLC-filter that follows a LP-filter curve much closer, hence attenuates the LF-path alot better.
@tszaboo
increasing the Offset range from +-24V to +-50V or even more (R&S claims up to 60V) seems easy ... just a bit of shuffling values of the Potis and the offset-input resistor.
Do You plan on offering a probe with higher offset range also?
regards
Calvin
--- End quote ---
Hi Calvin. I didn't see this thesis before, it's quite interesting.
The circuits looking like the N7020 probe: There is an expectation on how the probe is supposed to work, if you deviate from it it would work differently. When taking measurements, the probe should be "transparent" meaning that it shouldn't show any behavior which makes you doubt :whether it's the DUT or your test setup is making certain outcomes. Wat I'm trying to say there is only so many ways this circuit can be built, if you deviate from it, it's a different probe.
The thesis is quite ambitious for a BSC. I run into the same issue, a semester is not enough to design a board build it test it, but more importantly to document it. Because you get your grade based on the documentation.
His circuit is a good effort on the analog path. His power supply is very noisy, these DC-DC modules can easily have 100mV ripple on the output. When you measure some high voltage, the potmeter is set to high, you would be measuring all the noise and ripple coming from that DC-DC, amplified. Even though he has a RC filter with an electrolytic cap. I don't think he built the power supply, probably used a bench power supply.
Not to diminish his efforts for a BSC thesis this is very good.
I don't think I would want to offer it with a higher voltage for now. Going for 48V would mean that I need to comply with the Low Voltage Directive, that I'm not willing to do now. If you are interested, I could accidentally ;) send you a probe that has it's resistor replaced, and it could measure higher voltages. It would also have higher noise levels.
temperance:
I saw this thesis. What I don't understand is all the fuzz about the coupling capacitor on page 23. The thesis also blames R&S for not plotting such data on page 11.
On the plot shown on page 23 is a reference to a GRM188R72A104KA35 capacitor which is a X7R capacitor. This cap must be an NP0 or C0G type. His supervisors should have caught that.
Neganur:
--- Quote from: temperance on September 01, 2024, 02:02:48 pm ---On the plot shown on page 23 is a reference to a GRM188R72A104KA35 capacitor which is a X7R capacitor. This cap must be an NP0 or C0G type. His supervisors should have caught that.
--- End quote ---
It's "just" a B.Eng thesis. The supervisors do not typically pay attention so detailed on that level (sorry, Heikki ;D). I would have slapped his wrist for not labeling the y-axis though :scared:
PS: 1206, 100nF, 100V C0G/NP0 from Kemet is pretty darn expensive on Digikey. ~10 EUR for one. Maybe that was a money choice rather than a design choice.
temperance:
--- Quote ---PS: 1206, 100nF, 100V C0G/NP0 from Kemet is pretty darn expensive on Digikey. ~10 EUR for one. Maybe that was a money choice rather than a design choice.
--- End quote ---
I've seen different prices some time ago on the mouser website. Let's see.
Some 100 V C0G capacitors in 1206 can be had for less than €1. 200...250 V versions in 1206 are available for less than €2. Both in single quantity.
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