Products > Test Equipment
DC coupled 2.7 GHz Active Probe Project - Now Available!
lasmux:
--- Quote from: tszaboo on July 26, 2023, 09:26:00 am ---Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.
--- End quote ---
I'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.
I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faff
https://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134
The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.
tszaboo:
--- Quote from: lasmux on July 26, 2023, 10:38:38 am ---
--- Quote from: tszaboo on July 26, 2023, 09:26:00 am ---Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.
--- End quote ---
I'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.
I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faff
https://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134
The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.
--- End quote ---
Problem? No, I have those 1GHz active probes at work (N2795), and saw how versatile the accessory package is. According to my caliper, the pins are 0.7mm. I don't know if they sell the probe pins separate.
The equivalent input on these are 120Ohm series, 1pF 1Mohm parallel to ground.
JohnG:
--- Quote from: lasmux on July 25, 2023, 04:36:49 pm ---Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).
Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:
--- End quote ---
I measure such signals on a routine basis, so it doesn't seem so unusual to me (maybe I am an outlier, though).
Regarding the Agilent/Keysight probe example, usually such a graph shows probe derating due to some reliability or safety limitation. The same rolloff is typical for completely passive probes which will not exhibit slew rate limiting. The data sheet was not very clear on this, but from the impedance plot, an 8V AC amplitude signal at 1 GHz will give you about 50ish mA AC RMS of probe tip current, which is not insignificant. These probes have an adjustable offset of +/-12V, which is how they arrive at the 20V max input. This is not slew rate limiting, this is a way to get around the AC voltage limitations of the probe tip amplifier. It's useful for looking at noise on a voltage bus, for example, by adjusting the offset to the bus voltage so you have the full AC dynamic range centered around the bus voltage.
As I mentioned, I'd be happy to test the probe. I have a Bodnar pulser and an HP8131 pulse generator, and an old, but very functional, 6 GHz scope. I should warn you ahead of time that I'm not real quick about getting to these things, so if you find someone locally, you are likely to get a quicker turnaround. Also, if it is really slew rate limited to 1V/ns at the output, it's not something I can use.
John
lasmux:
--- Quote from: tszaboo on July 26, 2023, 12:33:13 pm ---
--- Quote from: lasmux on July 26, 2023, 10:38:38 am ---
--- Quote from: tszaboo on July 26, 2023, 09:26:00 am ---Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.
--- End quote ---
I'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.
I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faff
https://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134
The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.
--- End quote ---
Problem? No, I have those 1GHz active probes at work (N2795), and saw how versatile the accessory package is. According to my caliper, the pins are 0.7mm. I don't know if they sell the probe pins separate.
The equivalent input on these are 120Ohm series, 1pF 1Mohm parallel to ground.
--- End quote ---
My probe divides the signal prior to the amplifier, and then re-amplifies it. The N2795 doesn't attenuate at all to begin with, and then I guess has the amplifier gain configured to divide by 10 perhaps. I'd guess that that could also be why their ESD damage tolerance is so low also, the input is only protected by the 120R resistor.
lasmux:
--- Quote from: JohnG on July 26, 2023, 01:12:55 pm ---
--- Quote from: lasmux on July 25, 2023, 04:36:49 pm ---Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).
Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:
--- End quote ---
I measure such signals on a routine basis, so it doesn't seem so unusual to me (maybe I am an outlier, though).
Regarding the Agilent/Keysight probe example, usually such a graph shows probe derating due to some reliability or safety limitation. The same rolloff is typical for completely passive probes which will not exhibit slew rate limiting. The data sheet was not very clear on this, but from the impedance plot, an 8V AC amplitude signal at 1 GHz will give you about 50ish mA AC RMS of probe tip current, which is not insignificant. These probes have an adjustable offset of +/-12V, which is how they arrive at the 20V max input. This is not slew rate limiting, this is a way to get around the AC voltage limitations of the probe tip amplifier. It's useful for looking at noise on a voltage bus, for example, by adjusting the offset to the bus voltage so you have the full AC dynamic range centered around the bus voltage.
As I mentioned, I'd be happy to test the probe. I have a Bodnar pulser and an HP8131 pulse generator, and an old, but very functional, 6 GHz scope. I should warn you ahead of time that I'm not real quick about getting to these things, so if you find someone locally, you are likely to get a quicker turnaround. Also, if it is really slew rate limited to 1V/ns at the output, it's not something I can use.
John
--- End quote ---
I think because of how I divide the voltage prior to the amplifier, I am less succeptible to damage induced by high amplitude/high frequency signals. Last time I checked (edit. in spice) for a 28V p-p signal at 2GHz into the probe, there would be less than 1mA RMS going into the op amp, with most of the power dissipated within the input resistor network, which can handle it. I do need to run some more simulations on this though. But thanks for the clarification, I had misunderstood that the dynamic range line on that graph was not referring to the probe dynamic range in general, but more focusing on the voltage derating.
The slew rate is limited to 2V/ns at the output (remember the 10x output attenuation). For example, your HP8131 has a maximum p-p voltage of 5V, with a 200ps transition time. The slew rate limited rise time on that signal would be just slew rate limited to 250ps on my probe (theoretically). So only slightly longer, maybe here the bandwidth is reduced to around 1.3-1.4GHz for such signals. For the Bodnar pulser, the maximum p-p voltage is only 1.2V, so the slew rate limitation of the probe wouldn't affect the reading, it would be bandwidth limited here and wouldnt achieve the 40ps rise time.
If no-one else volunteers, I would be very grateful if you'd have a look at the probe. It'll be really interesting to see how it performs on a fast oscilloscope!
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