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DC coupled 2.7 GHz Active Probe Project - Now Available!
lasmux:
I've done a bit of work on several points. Firstly, the effect of using the SMA-SMA cable. Secondly, looking at the noise pickup in the probe. Finally, looking at the issue where the probe responds differently depending on whether it's clamped or held.
Maybe unsurprisingly, using the SMA-SMA cable shows a fairly significant improvement in impedance matching to equipment with SMA connectors. It removes the need for an BNC-SMA adapter, which I've found in the past could be a fairly large source of error in the match. See for example this S21 measurement using the new cable:
Much cleaner with reduced ripple/reflections.
With regards to the noise pickup. After a bit of toying about, I have confirmed that the issue is relating to picking up noise on the signal input of the op amp. As the op amp isn't unity gain stable, its design gain unfortunately amplifies this noise. The obvious answer is shielding which can be achieved with copper tape around the front of the probe, near where the amplifier input is. There is much less benefit from wrapping any other part of the probe in copper tape, and it is not really recommended as it can damage the sticker on the probe. The copper tape can be grounded to the SMA connectior. Care must be taken to not have the tape any further forward than in the image, otherwise the ground starts to have an affect on the linearity of the probe, and even the probe loading on the measured signal if the tape is pushed much too far forward. Unfortunately, the noise pickup is dramatically improved the further forward the tape is pushed, so a balance must be achieved. With the tape as it is in the photograph, I see an improvement of the noise pickup of around -5dB, with little affect on the frequency response, although this is a little difficult to measure consistently given the nature of the radiators that I was subjecting the probe to. This is still going to be a fair bit of noise, but it is an improvement, with a fairly easy implementation if you do need to reduce the noise level.
As an aside, this is one of the reasons why adding a metal enclosure is not really viable, as the closer the metal is to the signal pin and associated circuitry, the higher the tip capacitance. I think it would be quite challenging to achieve the 0.7pF tip capacitance with a non-insulating enclosure.
Finally with regards to the issue where clamping the probe results in a poor looking frequency response. I've tested this and there is a difference in frequency response depending on whether the probe is clamped or held.
Clamped (not handheld)
Handheld (not clamped)
Basically, I think this is caused by a specific grounding configuration, and the addition of a hand modifying this somehow. The addition of the copper tape shielding doesn't help here as it's not actually modifying the ground connection of the probe. I think I see some improvement by using a more substantial ground connection off the ground pin of the probe, by basically soldering together some of the pins in the pin kit to make a DIY ground blade pin. I need to do some more testing to confirm this. Something I've not tried is to use a soldered ground connection with a copper braid, I think that might make quite a difference. I'll continue working on investigating this issue, however I'm not sure it'll ever be completely 'solved' as different grounding configurations will always have an effect on the probe response. Also on my equipment the effect is much more subtle, which makes looking for improvements more difficult. For now I'd say keep using it handheld, but hopefully if you don't the modification of the probe response would be relatively minor so long as your probe grounding is reasonable.
joeqsmith:
You make no mention of lowering input impedance to support low voltage signals where drive strength is less of a concern. Is this no longer being considered?
For the effects I show using the holder vs placing my hand on the probe being much more perturbed than what you show, hard to say. I could set it back up and try to see if I could add anything further to the discussion. The probe holder is all metal. It was laying on top of a ground plane. The DUT and cabling were all laid across the ground plane as well. There was no good way to mount your probe to my holder, so I used a paper clip which is also metal. The metal bar that the probes would normally attach to, was a fair distance away from the probe tip. Lots going on there. When placing my hand on the probe, the probe was still attached to the fixture in the same manor.
The point I was trying to make in the review is that how the probe was held made a difference above a GHz.
lasmux:
--- Quote from: joeqsmith on June 19, 2024, 01:15:11 pm ---You make no mention of lowering input impedance to support low voltage signals where drive strength is less of a concern. Is this no longer being considered?
--- End quote ---
Yeah, I tried this, it makes no difference at all to the noise pickup. Not surprising in retrospect as the existing op amp input node capacitive coupling to GND is going to provide a much smaller impedance to reduce noise pickup, than reducing the relatively large voltage divider resistor. ~5pF coupling to GND at the op amp input, at 500MHz is 63Ohms impedance. Reducing the resistor to ground on the input voltage divider from 20k to 1k (and the probe input impedance from 1MOhm to 50KOhm), isn't going to make much of a difference given the capacitance and the frequencies we're dealing with.
Yeah, it does sound like there's a lot going on there! In my case it was a plastic clip holding the probe. Maybe it will have some effect. I'll need to do some testing.
--- Quote from: joeqsmith on June 19, 2024, 01:15:11 pm ---The point I was trying to make in the review is that how the probe was held made a difference above a GHz.
--- End quote ---
Yeah for sure. Currently it looks like I will have to keep on marketing it as a probe which performs best/closest to the specification whilst handheld, but in general, I may need to make more clear that the response will also vary depending on the grounding setup. It is kinda implied in the test sheet etc, but I will make it more clear in the "Performance Insights" section of the datasheet, at least until I decide whether it is resolvable or not.
lasmux:
The issue with the probe response being different is definitely a grounding setup issue, where the ground between the local signal and the probe is insufficient. A hand touching the probe improves the probe grounding with some capacitive coupling between your body and the nearby DUT. This issue can be resolved with a bit of copper tape, connecting the SMA connector (which is grounded) to a DUT ground nearby the measured signal. I had the tape stuck to the 50 ohm terminator shell, but I think this could be any sensible ground connection on a PCB. This tape has a very similar effect as holding the probe and really smoothes out the response.
A couple of things to be careful of.
* The added tape shouldn't be directly connected to the ground directly next to the waveguide being measured, as it will affect the actual signal.
* Don't let the tape get too close to the front end of the probe, particularly the signal pin, as this will affect the response linearity of the probe.
So yeah, if you want to use the probe handheld, there should be no problems. If you want to use it in a clamp, you'll need to improve the probe grounding with some copper tape. Also, it does seem like all other active probes in general seem to suffer from variable probe response depending on the exact local conditions, so I think this is reasonable.
Clamped with copper tape disconnected vs connected
Without copper tape:
With copper tape:
joeqsmith:
--- Quote from: lasmux on June 19, 2024, 03:32:51 pm ---
--- Quote from: joeqsmith on June 19, 2024, 01:15:11 pm ---You make no mention of lowering input impedance to support low voltage signals where drive strength is less of a concern. Is this no longer being considered?
--- End quote ---
Yeah, I tried this, it makes no difference at all to the noise pickup. Not surprising in retrospect ...
--- End quote ---
As we have discussed, for digital you could potentially sacrifice dynamic range as well to improve the SNR. I have included the spreadsheet from my review which shows how the probes I have on-hand compare. Even my old Tektronix probe was only rated to +/-6V. Again, I'm not suggesting this would be a replacement for what you are offering but an extension.
--- Quote ---... it does seem like all other active probes in general seem to suffer from variable probe response depending on the exact local conditions, so I think this is reasonable. ...
--- End quote ---
Other snips from the review showing my setup as well as my with and without touching the probe. There's a fair bit of difference between what you show. I could try to compare how sensitive my other probes are to the proximity of my hand and holder to the body if you like. I don't recall the handing ever being a concern but that may be my ignorance showing. Most of the LeCroy probes were provided with their own holders. I got the idea for my Harbor Freight probe holder from the Tektonix PPM100 which I use for work.
https://www.ebay.com/itm/175205288879
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