Andreas Tschammer wrote: > Bruce, > > thanks for the valuable information. > > I am aware of the importance of the zero crossing slope to noise ratio in > this kind of setup. > I will only use this design as a first step, because it is easy to replicate > and seems to have > a sufficiently low noise floor for my applications. I also have to build the > isolation amps with > transformer coupling. But the most difficult part seems to find a good > reference oscillator ... > > > Andreas, DL2KCL > _______________________________________________ > Andeas
The LT1028 filter has some essential details missing like the source impedance (ie mixer IF port termination circuit). JPL and NIST favour an LC PI filter low pass filter to attenuate Rf above 100kHz or so followed by an RC low pass filter. The LM161has a maximum differential input of 5V before destruction. At least add a pair of inverse parallel diodes across its inputs to avoid potential destruction. Comparators with totem pole output stages (like the LM161) tend to oscillate when the input signal has a relatively low slew rate (this may be why such a complex output circuit is used). Driving the comparator from a relatively high impedance source (Thevenin equivalent = 5K in parallel with 10K) doesnt help, Even the early NIST designs: http://tf.nist.gov/timefreq/general/pdf/84.pdf are better, although far from optimum. Note that all amplifier/filter stages are diode clamped. JPL favour optically isolated outputs: US patents 44011953, 4425543 http://tmo.jpl.nasa.gov/progress_report/42-169/169B.pdf http://tycho.usno.navy.mil/ptti/1994/Vol%2026_25.pdf Don't copy their isolation amplifier its not that good (besides the LH0033 is out of production). For isolation amplifier designs with high performance see: http://tf.nist.gov/timefreq/general/pdf/1200.pdf http://tf.nist.gov/timefreq/general/pdf/498.pdf 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.
