USB Counter with 50 Ohm output

[Vasilich2004]

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@dmysliwiec13 wrote:

Henrik is correct, if you could provide a little more information about your application we can both give you better suggestions on which hardware to use. The more information the better!

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there is not reason to discuss reflection due low frequency. device is IR Opolette 2940 but I can't find manual in Internet. I am pretty sure that the device has 50 Ohm resistor on input.

I ask very simple question. It is easy to test.

- connect any pulse to scope (10+ MOhm impedance)

- see TTL pulse (up to 5V)

- put scope in 50 Ohm impedance (or connect 50 Ohm resistor in parallel to input)

- if TTL pulse is up to 5 V - it is Ok. if TTL pulse drop to ~1V then output doesn't work.

Henrik's suggestion about 10 V is really wrong.

[Henrik_Volkers]

Your test assume a source impedance of (nealry) 0 (Zero) Ohm. 🙂 (and you are rigth, can be done with a driver and you will need 100mA drive current for that. )

And will give you reflections with typical TTL signals,  but you don't care  😄

I'm not wrong, I just used your information about TTL triggers .. and 50 Ohm is a typical impedance you use when it comes to high frequency and signal integrety....  and didn't knew you only need (lame) 20 Hz with a don't care rising edge....   (and in TTL a high signal doesn't need to be 5v 😉 )

And no, I'm not avare of a USB counter output that can drive 100mA ... but that's a question to the NI sales rep.

No  external trigger mentioned in the docs found, BUT you can control it via RS232 🙂  so why use a counter to set the rep. rate?

[Vasilich2004]

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@Henrik_Volkers wrote:

Your test assume a source impedance of (nealry) 0 (Zero) Ohm. 🙂 (and you are rigth, can be done with a driver and you will need 100mA drive current for that. )

And will give you reflections with typical TTL signals,  but you don't care  😄

I'm not wrong, I just used your information about TTL triggers .. and 50 Ohm is a typical impedance you use when it comes to high frequency and signal integrety....  and didn't knew you only need (lame) 20 Hz with a don't care rising edge....   (and in TTL a high signal doesn't need to be 5v 😉 )

And no, I'm not avare of a USB counter output that can drive 100mA ... but that's a question to the NI sales rep.

No  external trigger mentioned in the docs found, BUT you can control it via RS232 🙂  so why use a counter to set the rep. rate?

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Good! We are on the same technical page!

Low impedance input is used in noise enviroment to avoid wrong pulses.

RS232 (100+ KBod rate) could reach ~1 ms jutter and it will be very unstable.

[Henrik_Volkers]

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@Vasilich2004 wrote:

Low impedance input is used in noise enviroment to avoid wrong pulses.

RS232 (100+ KBod rate) could reach ~1 ms jutter and it will be very unstable.

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I don't know what you want to achieve and don't have the manual for the device, however the datasheet told something, that the pulse rate can be set by the serial, not only trigger a single measurement....

As a driver: A single transistor as a voltage follower (with a Vbe drop) should be fine to do the job 🙂

[Chishti]

Can you please tell me what's the impedance of Auxilary I/O of USRP2940. Is it 50Ohm like RX/ Tx channel. 

The issue is my signal got dumped at the input end of Auxilary I/O of USRP2940... It dumped from 3.3V to 1.5V.