help with 120MHz comparator

Nov 04, 2003 3 Replies
T

I'm trying to convert a 120MHZ 1V p-p ac coupled sinewave to a LVTTL (3.3V) format for use as a 120MHz clock signal to an FPGA.



  1. PECL Comparator - I'm trying to avoid using PECL Comparators, because I would have to convert the PECL output to a LVTTL swing, which just brings me back around to the original problem.
  2. "normal" comparators - I've looked through Linear Tech and other places, and the max spec is 100MHz. Do you know of any LVTTL comparators with specs closer to 120MHZ??????????
  3. Transistor. There are many high frequency RF transistor with excellent small signal gains around their bias points but lousy large signal - DO you know of any discrete transistors that will switch fully at 120MHz rates?? Thanks
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D

I would recommend a LVDS receiver to convert your sinewave to a LVTTL signal. Use 2 resistors and a capacitor to bias one input at 1.2 volts and run a terminating resistor to the other input.

You could also feed the signal directly to the FPGA if it has a LVDS clock input although this will give more jitter.

Daniel Lang

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H

I got burned (years ago) using a PECL-TTL level shifter. It had a normal TTL output when I wanted a CMOS signal. Worked much better when we added a CMOS gate/buffer to clean it up.

I'd probably try a fast CMOS inverter. AC couple your signal in and feed a 1 megohm resistor from the output back to bias the input. Got a couple spare pins on your FPGA?

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A

Daniel,

Of, if it is has an SSTL or HSTL or GTL input (which has a reference voltage) you have the high speed comparator already built in to your FPGA.

Aust> I would recommend a LVDS receiver to convert your sinewave to

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