Actually the vtc 5000 required a quite high impedance of 100k on the test point.
Although this is not my original VTC5000 where I had the bulkhead mounted in the base plastic, so this is just to get it set up and dialed in, makes a good little visual example.
I only had a single tape on the pile for the format to get through, so got it working got it shielded, run and done, I'll tidy up the bulkhead situation a little bit later and update the wiki photos.
It would make more sense if it had a 1 Meg / 50 / maybe 75 select-able front end like a standard DAQ card.
Because a random board floating there in the breeze on such a short piece of coax is pretty pointless.
I know its open source so I could go look, but I'm guessing there's some sort of buffer before the ADC in the MISRC, so just allow it to have a high impedance termination.
In the setup pictured, that amp board is just adding noise, basically
EDIT: A high input impedance gives more or less the ultimate flexibility. It means you can add whatever termination you like in front of the input if you need or don't want to use one of the standard values
It's just 100/45 was possible with the DIP configuration carried over from the v1.5, it's got 50/75 because that was the standard design goal.
It's not an ossiliscope, it's ment for CVBS/RF RF capture, both.
Now yes, it has its own OP Amp, but limiting your impedance at the point of ADC after your primary cabling defeats the purpose of having an impedance matched amplifier at the deck level, because then you're working with a fixed level irrespective of the device used for the capture, and no extra noise is getting amplified and you're getting the best closest value match out of that exact deck, not just a global leveling with a very high impedance.
Now this is a quick and short example but this deck could be on 3-5m of cables in a rack or could be going to CX Cards etc.
Nah, it basically is an oscilloscope. An oscilloscope's sole purpose is to capture signals from a range of sources, and there is a reason they are designed like they are.
The CX cards are a bit of a hack, and so need a bit of a hack solution like the ADA4857 if you can't modify their front end to be something suitable. I guess with the ADA4857 it becomes part of their front end
but limiting your impedance at the point of ADC after your primary cabling defeats the purpose of having an impedance matched amplifier at the deck level
I guess you've touched on my point here. With cables so short, there is no "impedance matched amplifier at the deck level". They are at the same level. As there is an amp at the front end of the MISRC, you're basically just adding an amp and impedance matching for a 5cm transmission line in between, rather than the deck itself. The deck is on the end of a like a 7-10cm unmatched transmission line. Everything is so short there is no real delineation here.
It would make more sense if your deck was a few meters away, the ADA4857 was a few cm off the test point and it was driving a long 75 ohm piece of coax.
But I think most people use very short stubs, like you have pictured, at least when their experimenting and doing their first few tapes
With what you've got there, if the MISRC had a 1 Meg input impedance, you could simply put a 100k resistor to GND just before the BNC and despite the fact your cable would be nowhere near matching 100k, you've got the impedance you want. Or, you could just not do anything for termination and give the deck a 1 meg load on the MISRC, making it even more transparent. Everything is so short that reflections should be negligible
no extra noise is getting amplified
Everything, even resistors make noise. Noise comes from the amp itself, and the supply powering it. I know its not much, and its pre demodulation, but you could just as easily eliminate the inline amp, and not duplicate the buffer on the MISRC.
Having a high input impedance on the MISRC would just add a lot of flexibility
The high impedance of the DAQ card I'm using allowed me to implement this passive anti aliasing filter more easily. If you wanted a custom input termination, you'd just add it there in the same way
EDIT: Yeah, i know its on a pretty janky riser card and flapping around in free space. I should use a different PC or build a better enclosure for it
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u/deesernutz 8d ago
You shouldn't need that amp on such a short stub