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DC coupled 2.7 GHz Active Probe Project - Now Available!
joeqsmith:
--- Quote from: lasmux on August 08, 2023, 10:52:05 am ---I think your second set of measurements will be better? With the splitter you will have two 50 ohm terminations (to ground, not VCC-2V) on the PECL driver, which it might not like? I've not used PECL before though to be honest, only a bit in simulation.
--- End quote ---
I had used a resistive splitter, so with the two ports loaded with 50 ohms, the PECL driver sees 50 ohms. The signal the scope sees is 6dB lower than the original. The reason I thought about running this test was to somewhat isolate the two ports.
https://www.microwaves101.com/encyclopedias/resistive-power-splitters
--- Quote ---Very interesting how the PP005 really does a lot of damage to the signal. The PP061 lowers the PECL voltage levels slightly with it's loading? But the shape of the loaded waveform is more similar. The P6202A waveform has almost no loading (at these frequencies), but the output waveform maybe has slightly more peaking on the rising edge? It's still qualitatively very similar.
--- End quote ---
From my previous posts where I provided some basic metrics for my probes, you can see the PP061 is a 10X probe, so the DCR was 500 ohms (which includes the scope's input). The waveguide was loaded with 50 ohms by the scopes other channel. Now we added another 500 ohms. Your comment about the P6202A "at these frequencies" is key. It's not a perfect 500 ohms and the response is not going to be flat as we start to look at faster signals.
Currently the scope is doing a good job of masking what the actual signal looks like.
***
Thinking about your previous comment, I am guessing you thought I was using a "T" which is not the same thing as a splitter.
joeqsmith:
They claim 1GHz, for $250 and it is not DC coupled. I would have no use for something like this.
https://www.ebay.com/itm/254162548019?hash=item3b2d44b533
$29 and again, not DC coupled.
https://www.ebay.com/itm/175751864133?hash=item28eba09f45
There doesn't appear to be a low cost 2GHz product that would compete with you. The only choice for the hobbyist is the used market...
joeqsmith:
--- Quote ---I had another look around, there's a little bit of testing of Zo probes done by... you!
....
Everyone seems to get very different results. Not quite sure what's going on.
--- End quote ---
Because much of what I had shown here by attempting to embed resistors in the cable and such, isn't really how I typically would make a probe, I wanted to show what something a bit more common. This is a 20X (953 ohm) probe. Zoomed in 1ns/div. Again, fairly low frequencies and the scope is hiding the details but hopefully it helps show that the results shouldn't vary by much. The cable (Teflon) and connector came from Pasternak. Surface mount resistor is stabilized with a bit of shrink tube. Not a whole lot invested. Maybe a half hour labor and $15.
Maybe we can run some of these on the VNA and my faster scope later on to help paint a clearer picture.
lasmux:
--- Quote from: joeqsmith on August 08, 2023, 12:22:20 pm ---Currently the scope is doing a good job of masking what the actual signal looks like.
[...]
Thinking about your previous comment, I am guessing you thought I was using a "T" which is not the same thing as a splitter.
--- End quote ---
Ah thanks about clarifying about the splitter. I had indeed misunderstood, and yes, I have the same problem with my oscilloscope. The true signal form is hidden behind the insufficient bandwidth.
With regards to those other probes, I have seen those. They're not great tbh. The first ones construction looks quite good, but the input capacitance is 3.5pF. This is really quite bad, not far off a good passive probe. As you said, not DC coupled either. The second one looks very similar to other active probes based on the BF-998 that are on ebay. I have a similar version from Ukraine. The frequency response linearity is awful (swings +/- 8db or so across the bandwidth), and the input capacitance is in reality much higher than the quoted 0.5pF. I measured the tip loading drop to 20 ohms at around 500MHz, which again, is really bad.
--- Quote from: joeqsmith on August 08, 2023, 05:48:50 pm ---
--- Quote ---I had another look around, there's a little bit of testing of Zo probes done by... you!
....
Everyone seems to get very different results. Not quite sure what's going on.
--- End quote ---
Because much of what I had shown here by attempting to embed resistors in the cable and such, isn't really how I typically would make a probe, I wanted to show what something a bit more common. This is a 20X (953 ohm) probe. Zoomed in 1ns/div. Again, fairly low frequencies and the scope is hiding the details but hopefully it helps show that the results shouldn't vary by much. The cable (Teflon) and connector came from Pasternak. Surface mount resistor is stabilized with a bit of shrink tube. Not a whole lot invested. Maybe a half hour labor and $15.
Maybe we can run some of these on the VNA and my faster scope later on to help paint a clearer picture.
--- End quote ---
Indeed, there is a very low loading here! Would definitely be interesting to see how it performs on the VNA/faster scope.
joeqsmith:
Before doing this, another option may be to make a new one that uses a high grade SMA rather than the BNC. I should have some cable and connectors on-hand. The tip loop is also fairly large on this one. I wouldn't want to give you the wrong impression about these homemade probes once we start speeding things up.
To look at the BW with the VNA, I am thinking we stay with the same waveguide. Terminate the backside to 50. Then attach the probe to the second port. Using my homemade probe, we should see 26dB. I would need to use my old Agilent for this. Maybe then check the tip capacitance as well. I could run your probe as well as my resistive probes but none of my active or standard 10X probes.
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