[time-nuts] FE-5680A clock shaping (sine -> square wave)

Bob Camp lists at rtty.us
Mon Jan 2 15:41:01 UTC 2012


Hi

In most cases it's a T section (two coils, one cap) low pass filter tacked on to the output of a logic gate (or FPGA output). Net result is a reasonable sine wave *if* you terminate it correctly. Based on the observations posted on the list, the filter in the FE-5680 seems to be set up for a 50 ohm resistive load and is mis-terminated when run into a scope probe. Not to surprisingly, you get a lot less output voltage into the correct load than into an open circuit. With a filter, the difference can be significantly more than 2:1. The logic gate / FPGA is putting out about 3 volts p-p, so that will be the "filter went away" output level. The matched value will be quite a bit lower to keep the gate / FPGA from going a bit nuts power wise. 

Bob


On Jan 2, 2012, at 9:52 AM, ehydra wrote:

> Is it possible to sketch the circuit? I can SPICE it.
> 
> Symmetry limiting is the holy grail and it is questionable if a discrete design is way better than one of the chips.
> 
> 
> Here is another limiter circuit (by Chris Trask):
> http://ehydra.dyndns.info/NG/LTspice/Negative%20Impedance%20LO%20Driver.pdf
> 
> 
> - Henry
> 
> 
> David schrieb:
>> What kind of performance would you expect in this application?  Low
>> jitter?  50 ohm output?  TTL or better signal levels?  Fast rise and
>> fall times?  Duty cycle correction?
>> After reading your post I was thinking about how to go about it and
>> ended up with an 8 transistor discrete design using a differential
>> amplifier input and pair of current mirror transconductance amplifiers
>> for the output.  I have been looking into designing a pulse generator
>> for oscilloscope calibration and have an interest in GPSDOs so maybe I
>> will prototype this as well just to see what kind of performance a
>> bunch of 2N3904 and 2N4401 jelly bean transistors can provide.
>> On Thu, 29 Dec 2011 21:14:30 -0800, John Beale <beale at bealecorner.com>
>> wrote:
>>> In case it's useful... there are many ways to get a square wave out from a sine wave in, but one straightforward way is with a comparator. Some work better than others. The slow ones won't work at all at 10 MHz, and the very fast comparators (MAX999, ADCMP600, LT1116 etc.) are more expensive, and perhaps harder to work with. I tried a MAX9013 in SO-8 package and it works well for the job. You can see my schematic, circuit and scope plots at the bottom of this page:
> 
> 
> -- 
> ehydra.dyndns.info
> 
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