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DC coupled 2.7 GHz Active Probe Project - Now Available!
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lasmux:
UPDATE, 17/04/2024: These are now available at http://www.lasmux.com/product/single-ended-active-probes/. 2.7GHz probe.

I've been working on an active probe design for around a year. The goal started off as creating a DC-coupled active probe to support a photon counting sensor I am also working on, but it was a very fun project and I spent so much time on it that now the plan is to sell it. Could I have some feedback on the probe/performance, and on the contents of the datasheet, before I start buying the first batch of parts... which will be quite expensive.

I have another post that I'm putting together where I'll go into the development process a bit more.

I'm making two versions, a 1GHz version, and a 2GHz version.

The datasheet is here: https://www.lasmux.com/wp-content/uploads/2023/07/LD-ASP-1G_2G.pdf



Quick specs:
Bandwidth: DC-1GHz, DC-2GHz
Input capacitance (measured at 1GHz): 0.7pF
Attenuation: 20x
DC input resistance: 1Mohm

1GHz version frequency response (linear and log axis):


2GHz version frequency response (linear and log axis):


Tip input impedance of both probes, depending on which ground lead is fitted:


The resistive ground lead can be used to stop a resonance developing on the ground connection, which reduced the input impedance above 1.5GHz. I talk about this a bit more in the datasheet.



In terms of step response for the system, I've 'only' got a 500MHz oscilloscope, so can't properly test the rising edge speed unfortunately. This is the probe measuring a 50 ohm terminated 100MHz signal, with a <100ps rise time. This greatly exceeds the bandwidth of the scope so there's some ringing. The trace looks basically identical if I measure the signal directly by the oscilloscope.

Currently I'm aiming for around £150 for the 1GHz version, and £185 for the 2GHz version.
lasmux:
There have been... quite a lot of iterations of the hardware design...



I started off with a BNC output on the probe itself, and the batteries mounted to the probe also, but then went for an SMA connector and an external 9V battery box. It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

I went through a lot of iterations on the amplifier and signal conditioning circuitry, and got very used to soldering 0402's, which made my life miserable. Also given the extremely small capacitances on the board, every single time I made a change to the passive networks, I'd have to clean the board in IPA to remove flux residues.
djsb:
Hi,
When will this be back in stock? Or is it initially for evaluation purposes only (as you mention hardware changes)? Thanks.
lasmux:
I've not made the first batch yet. I'd expect maybe six to eight weeks, assuming my PCB assembler doesn't mess me about.

If the response from this thread is positive, I'll make an initial batch of 25 and start the process immediately.
lasmux:
Here is a simplified schematic of the probe input to give you a gist of how it works.



C1 balances the parasitic capacitance across R2, to get a resistor/capacitor divider between R2/R3. As you could imagine, the parasitic capacitance across R2 is extremely low, given it's an 0402 passive. By varying C1 and observing the probe frequency response, I measured it at less than 0.1pF! This is not a lot of capacitance.

The capacitance of the probe tip is a lot more than 0.1pF (0.7pF) because there is some coupling between the probe tip and ground itself. This is not so difficult to avoid by just isolating that resistor from ground completely. Unfortunately, the more you pull ground away from R2, the worse the noise pick-up on the probe is. The net which connects R2 to the non-inverting amp input is high impedance, and if you pull the ground away from it, it picks up a lot of noise. One of my design iterations removed a lot of the ground planes around this net to really try to keep the tip capacitance down as low as possible, and the noise pickup was really bad. So you kinda have to choose your demon. The noise level is quite a lot better than other active probes (only 650uV), but because it's a x20 probe, I needed to keep the noise level to a minimum.

One thing which I could do in future is reduce the R2 to 200k or something, so I would have to correspondingly reduce R3, which would make that node much lower impedance and less succeptible to noise. I could then pull the ground a bit further back to try to further reduce the tip capacitance. Not sure.

The 1Mohm resistor R2 also protects the non-inverting input of the high speed FET input op amp from ESD damage. It does have ESD protection diodes built in, but the resistor helps keep them safe. A lot safer than just having discrete FETs on the inputs.
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