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> 1 GHz DIY differential probes
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Noy:
I'm interested in one but currently I'm unsure if it is the right probe for my needs.

I only have 350MHz scopes (Rigol MSO5000 (external termination) / Hameg HMO3524(internal 50 Ohm temination)).

I need a probe to measure / qualify µSD signals in SDR104 mode somtetimes eMMC in HS400 mode (200MHz clk)...

Sometimes "slow" LVDS / MIPI without eye measurments (cant produce eyes on my scopes)..

Currently i only have passive 350MHz probes (they have too high loading for the 200MHz clk) / Micsig 100MHz high voltage differential probe (divider too high for the small 1.8V signals)..

I think two single ended active probes with >=350MHz BW with High resistance and low capacitance input for low loading would be the best fit for my needs. I can use 2 probes + math to measure LVDS/MIPI.

And for the "fast" SDIO signals i use both probes to measure 1 Data + CLk at the time.

So your differential probe has 2k Ohm input impedance? I think that is to low for me? And the 1.4GHz is over the top for my scopes..?

So has anybody another good solution for my needs? A DIY active probe with qualified behaviour (i can build one myself but i don't have equipment to measure the bandwith / impedance) and not too expensive?

Or is the DIP1400 useable for my needs?


0xdeadbeef:
I'm using the differential probe from Elektor 4/2017 by Alfred Rosenkränzer discussed earlier in this thread. It has slightly different specs (~5kOhm differential resistance), but should be somewhat comparable.
At work, I'm using these probes mainly to measure a 40MHz LVDS signal  (MSC aka "microsecond channel") on a 600MHz LeCroy. Compared to the default 500MHz passive LeCroy probes, the difference is like night and day. I.e. with the passive probes, the signal looks like a distorted sine and with the active probes, it's a nearly perfectly rectangular signal.
Actually I also use the probe for single ended measurements (i.e. "-" connected to GND) when very steep edges are involved and when the capacitance of the passive probe would somewhat invalidate the measurement.
IMHO, an active probe with low capacitance is always worth it even on a 200MHz scope when looking at sharp edges. Of course if you don't need a differential probe, there might be cheaper options.
tggzzz:

--- Quote from: 0xdeadbeef on October 01, 2020, 02:03:02 pm ---I'm using the differential probe from Elektor 4/2017 by Alfred Rosenkränzer discussed earlier in this thread. It has slightly different specs (~5kOhm differential resistance), but should be somewhat comparable.
At work, I'm using these probes mainly to measure a 40MHz LVDS signal  (MSC aka "microsecond channel") on a 600MHz LeCroy. Compared to the default 500MHz passive LeCroy probes, the difference is like night and day. I.e. with the passive probes, the signal looks like a distorted sine and with the active probes, it's a nearly perfectly rectangular signal.

--- End quote ---

I expect you mean a 40Mb/s LVDS signal. The maximum frequency will be much higher, dependent solely on the transition time.

It is unsurprising you see a distorted waveform, given that a *10 "10Mohm" impedance probe has a (capacitive) input impedance of <100ohms at the frequencies you are likely to see on an LVDS signal.

A pair of resistive divider Z0 probes would be a better bet, since they have a much lower capacitance and a well defined input resistance of 500ohms (*10) or 1000ohms (*20). They are easy to make at home.
nctnico:

--- Quote from: Noy on October 01, 2020, 08:30:31 am ---I'm interested in one but currently I'm unsure if it is the right probe for my needs.

I only have 350MHz scopes (Rigol MSO5000 (external termination) / Hameg HMO3524(internal 50 Ohm temination)).

I need a probe to measure / qualify µSD signals in SDR104 mode somtetimes eMMC in HS400 mode (200MHz clk)...

Sometimes "slow" LVDS / MIPI without eye measurments (cant produce eyes on my scopes)..

Currently i only have passive 350MHz probes (they have too high loading for the 200MHz clk) / Micsig 100MHz high voltage differential probe (divider too high for the small 1.8V signals)..

I think two single ended active probes with >=350MHz BW with High resistance and low capacitance input for low loading would be the best fit for my needs. I can use 2 probes + math to measure LVDS/MIPI.

And for the "fast" SDIO signals i use both probes to measure 1 Data + CLk at the time.

So your differential probe has 2k Ohm input impedance? I think that is to low for me? And the 1.4GHz is over the top for my scopes..?

Or is the DIP1400 useable for my needs?

--- End quote ---
I think the DIP1400 is useful for your purposes. One of the advantages of a differential probe is that you get the signal between two point (close together) and not via a ground point connected to a long lead. For example: I have used the DIP1400 to troubleshoot a power supply problem on a SOC design because I wanted to make sure I wasn't looking at spikes induced into the ground connection. 2k Ohm should be OK for almost every digital signal; one of the design goals was to make the input impedance high enough so the amplitude error on signals using a 50 Ohm (single ended) connection is below 10%.
0xdeadbeef:

--- Quote from: tggzzz on October 01, 2020, 04:04:59 pm ---I expect you mean a 40Mb/s LVDS signal. The maximum frequency will be much higher, dependent solely on the transition time.

--- End quote ---
Well, one of the LVDFS channels is a 40MHz clock. So since when is frequency only a valid unit for sine waves? Of course the sharp edges have much higher frequency components. No need to mention that in this board, do we?


--- Quote from: tggzzz on October 01, 2020, 04:04:59 pm ---It is unsurprising you see a distorted waveform, given that a *10 "10Mohm" impedance probe has a (capacitive) input impedance of <100ohms at the frequencies you are likely to see on an LVDS signal.

--- End quote ---
For a 11pF probe, a 400MHz component will "see" a resistance of ~36Ohm, while the DC components will "see" 10MOhm. How is it surprising that this distorts the waveform?
Anyway, have you ever actually tried to measure a 40MHz square wave with a passive probe?


--- Quote from: tggzzz on October 01, 2020, 04:04:59 pm ---A pair of resistive divider Z0 probes would be a better bet, since they have a much lower capacitance and a well defined input resistance of 500ohms (*10) or 1000ohms (*20). They are easy to make at home.

--- End quote ---
A typical active probe has like 0.5pF capacitance, so anything you attempt to attach it somewhere will actually add a capacitance in the same magnitude or higher. I don't see how a purely resistive probe would help there. Plus, its low resistance makes it problematic for digital outputs which might as well create a burst clock only now and then.
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