Is it my probes, my scope or something else?

[Mario87]

Hi all,

I have a Siglent SDS1104X-E which has been hacked with all licences and up to 200MHz (model no. within the FW reads as 1204X-E). Normally I do not probe anywhere near these frequencies, however I am currently doing some FPGA HDMI work and to generate 1080p images I am operating at an internal clock frequency of 148.5 MHz on the FPGA.

This 148.5 MHz is pulled out via the FMC connector on the dev board, to an ADV7513 HDMI encoder and it is working fine (now), but before I got it working and even now after it is working I was doing some probing and the signals should all be 1.8V from the FPGA to the HDMI encoder.

When I check HSYNC, VSYNC, RGB signals, etc they all show at 1.8V peak which is correct. However, when I probe the clock signal it only reads 1.25V to 1.35V peak and I am wondering, is this because of one of the following, or is it something else??

- Am I loading the signal with the scope probe (set to 10x, and if I change to 1x the reading drops down to around 950mV as it is loaded up more)?

- The hack didn't truely make it a 200MHz scope (however the thread for the hack has people actually testing this and proving it works, but I don't have a sig gen that goes that high to test myself)

- I am still using the stock PP510 (100MHz) probes that came with the scope, would a set of 200MHz probes resolve this?

Is there something else I am overlooking? Would just be nice to know what is going on.

[bdunham7]

- Am I loading the signal with the scope probe (set to 10x, and if I change to 1x the reading drops down to around 950mV as it is loaded up more)?

- The hack didn't truely make it a 200MHz scope (however the thread for the hack has people actually testing this and proving it works, but I don't have a sig gen that goes that high to test myself)

- I am still using the stock PP510 (100MHz) probes that came with the scope, would a set of 200MHz probes resolve this?

It most likely is loading the signal with the probes.  It is unlikely that the hack 'didn't work', I don't think anyone has ever reported that as a problem.  You don't need "200MHz" probes (the PP510 has a good 200MHz+ response anyway with a low impedance source), what you need is a lower input capacitance probe.  Of course it still needs to have sufficient BW (200MHz+) but what it is labeled and marketed as doesn't necessarily reflect that reality.  With the PP510, in 10X you are putting ~18pF across the load, and ~85pF in 1X.  This results in source loading that depends on a whole slew of complex factors, but just to give you an indication, at 150MHz 18pF has a Z of 58 ohms, 85pF is 13 ohms. 

What  you need is a probe with less capacitance and the "200MHz" PP215 isn't it.  The solution is likely to be a 100X probe, which is only feasible due to the low-noise front-end of your scope.  One example that might be readily available to you is the Testec TT-HV-250 (or 150) which are 100X, 4pF input capacitance.  It is hard to find high-impedance probes that are better than that--my old 100X 10M Probemaster 4910-2 probes are ~2.5pF, but the new ones seem to be more like 5-6pF.  In 10X, the Tektronix P6139A is 10M 8pF and is considered a "500MHz" probe.

[tggzzz]

Ordinary "high" impedance *10 probes are low impedance at 100MHz - calculate 1/2*pi*f*C, where f=100MHz and C=15pF.

You will need to avoid using 15cm ground leads on the probe: their inductance will cause resonance at ~100MHz. FFI: https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

Best affordable solution is a Z0 resistive divider probe. Fortunately you can easily make those yourself with a 470ohm resistor, 50ohm coax, and a 50ohm terminator. Solder the resistor and ground lead to the relevant points on your circuit. FFI: https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[Mario87]

It most likely is loading the signal with the probes.  It is unlikely that the hack 'didn't work', I don't think anyone has ever reported that as a problem.  You don't need "200MHz" probes (the PP510 has a good 200MHz+ response anyway with a low impedance source), what you need is a lower input capacitance probe.  Of course it still needs to have sufficient BW (200MHz+) but what it is labeled and marketed as doesn't necessarily reflect that reality.  With the PP510, in 10X you are putting ~18pF across the load, and ~85pF in 1X.  This results in source loading that depends on a whole slew of complex factors, but just to give you an indication, at 150MHz 18pF has a Z of 58 ohms, 85pF is 13 ohms. 

What  you need is a probe with less capacitance and the "200MHz" PP215 isn't it.  The solution is likely to be a 100X probe, which is only feasible due to the low-noise front-end of your scope.  One example that might be readily available to you is the Testec TT-HV-250 (or 150) which are 100X, 4pF input capacitance.  It is hard to find high-impedance probes that are better than that--my old 100X 10M Probemaster 4910-2 probes are ~2.5pF, but the new ones seem to be more like 5-6pF.  In 10X, the Tektronix P6139A is 10M 8pF and is considered a "500MHz" probe.

Thanks, makes perfect sense and was what I was more leaning towards (loading the signal). I hadn’t gone to the effort of calculating the capacitive reactance, but I had seen others say the stock PP510 probes had pretty good response at higher than rated frequencies and as you mention, no-one has ever complained of the hack not working previously, so was unlikely to be that.

I may look at some of the probes you mentioned but as I originally mentioned, I don’t usually probe at frequencies this high and just being able to see there was a clock signal at the right freq was good, but when it wasn’t working as expected I did wonder if the FPGA wasn’t properly generating the clock as it was lower than expected when probed.

So a better probe with lower capacitance would have helped there by showing me a value closer to what I actually expected.

Mainly was just wanting to figure out what was happening, so thank again for the info