One more comment the phase shifter is not included in that calculation. Bert Kehren In a message dated 7/24/2009 11:30:22 P.M. Eastern Daylight Time, [email protected] writes:
[email protected] wrote: > On the chance that I may be totally wrong let me suggest the following. In > my opinion you are looking at the wrong information. When you mix a 10 MHz > signal with a signal offset by 1 Hz you are subtracting from a 10 E 7 > signal 10 E 7 and as a result get 1 Hz representing 10 E 7. Counting it with a > counter with 1 ns resolution, you in effect get at the last digit 10 E 16. > Obviously some of the last digits are irrelevant due to noise and trigger > error. Using two mixers with the common offset oscillator you want the two > phases to be as close as possible to reduce the noise contribution of the > offset oscillator. That is why you want the phase shift. Measuring the time > difference between the zero crossings in my opinion is not the way to go. > I am inspired by the simplicity of the NBS unit (thank you Corby for the > info) to lay out a PC Board; reading some of the comments I would appreciate > any links to more info on that subject that may make the design more > effective. Any help would be greatly appreciate. > Bert Kehren WB5MZJ > **************A Good Credit Score is 700 or Above. See yours in just 2 easy > steps! > (http://pr.atwola.com/promoclk/100126575x1222377105x1201454426/aol?redir=http://www.freecreditreport.com/pm/default.aspx?sc=668072&hmpgID=115&bcd > =JulystepsfooterNO115) > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > Bert With a classical dual mixer system, the only useful measurement is the time difference between the zero crossings of the 2 beat frequencies. The limiters used distort the signal and amplify the zero crossing slew rate, thereby precluding meaningful measures of anything else but the zero crossing times. Time stamping the zero crossing times for each channel may be more useful rather than just measuring the time differences. The zero crossing circuit employed by NIST is far from optimum (Oliver Collins paper on the design of low jitter hard limiters wasn't published until May 1996), some of the JPL designs are better but are still some way from optimum. The timing jitter and phase drift of the ZCD output limits performance. JPL achieved a zero crossing jitter of around 40ns in their 1Hz beat frequency ZCDs, it should be possible to reduce this by a factor of 3 or so. There later DMTD systems use a beat frequency of around 100Hz or so with a 100MHz mixer input frequency. The zero crossing detector filter component tempcos will limit the phase shift stability for larger tau as will the mixer phase shift tempco (~ ps/C). It is also essential to avoid low frequency ground loops that affect the 2 zero crossing detectors. The mixer IF port grounds need to be low frequency isolated from the RF grounds of the other 2 ports. One needs to know the phase detector output noise in the flicker region in order to optimise the zero crossing detector design. Unfortunately phase detector flicker noise specifications don't appear on the datasheet so one has to measure it. One can do this using a low noise preamp for a sound card or a spectrum analyser. Using an apprpriate mixer IF port termination (a 50 ohm or even 500 ohm resistive termination may be far from optimum) will affect the beat frequency signal jitter. If one eliminates the limiters (however the distortion probably needs to be kept down) then, in principle a COSTAS receiver could be used to measure the phase shift between the beat frequencies independently of the individual beat frequencies and proximity to a beat frequency zero crossing. However ADC noise will limit the performance unless one uses 2 or more ADCs per beat frequency and uses correlation techniques to eliminate the ADC noise. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. **************An Excellent Credit Score is 750. See Yours in Just 2 Easy Steps! (http://pr.atwola.com/promoclk/100126575x1222585106x1201462830/aol?redir=ht tp://www.freecreditreport.com/pm/default.aspx?sc=668072&hmpgID=62&bcd=JulyExcfooterNO62) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
