Hi The gotcha here is that saturation / slew limiting is one of the few things that will give you *better* data than the oscillators are really doing. Most error sources have the "nice" property of making things worse.
Bob On Feb 27, 2010, at 6:40 PM, Bruce Griffiths wrote: > The LT1037 is shown with a gain of ~1690x, if this amplifier is used to > amplify the beat frequency signal, it will saturate. > Opamp recovery from saturation is poorly documented and may be very slow. > It would be better to use some diodes in the amplifier feedback network to > limit the large signal gain to 5x (so that the LT1037 remains stable as it > isn't unity gain stable). > This will ensure a somewhat faster recovery from overload as the LT1037 then > avoids saturation and the opamp input stage remains in the linear region. > > Bruce > > Bob Camp wrote: >> Hi >> >> Assuming that the junction of the back to back diodes goes trough a chunk of >> coax to get to the counter: >> >> You are forming a low pass filter with the 10K resistor and the coax >> capacitance. The LT1037 is quite happy driving a 600 ohm load. You could >> easily drop the impedance at that point below 300 ohms. That should give you >> a faster edge into the counter. >> >> You also should check the slew rate performance of the 1037. You don't want >> the op amp to be slew rate limited. >> >> Bob >> >> >> On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote: >> >> >>> I am in the process of designing a DMTD system. As an experiment to do >>> basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in >>> series to ground with a 47 ohm metal film resistor. Where the capacitor >>> and resistor meets, another resistor is attached (390 ohms) that goes to >>> ground. The idea is to provide a 50 ohm termination at 20 Mhz and a >>> lighter termination at audio frequencies. I seen this is a NBS note and I >>> can say, its a starting point for my experiments. >>> >>> This (my) system is designed for 10 Mhz, using a 10 hertz beat. A >>> schematic is attached of what I am experimenting with at the moment. A >>> HP5370B is the recording instrument. The noise floor from 1 days >>> observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 >>> sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. >>> It will be interesting when the project is completed to see how much >>> improvement there will be. >>> >>> As I understand (or learning..) mixer performance is the key to the DMTD >>> system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 >>> mhz may be a better termination (for the IF port) for this mixer. A 16 pF >>> capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be >>> 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I >>> understand, a capacitor terminated mixer will give a triangle wave output, >>> which is very beneficial to the design - as the end result is to get >>> maximum slope out of the mixer. I would say, unqualified as I am, the >>> capacitor termination matches the 20 mhz signal, and helps attenuates the >>> harmonics of the mixer, and has no , or very little effect on the audio >>> frequencies that we are interested in. >>> >>> And saying/rambling on... that if maximum slope is needed, its needed on >>> the 10 hertz beat signal - so maybe a capacitive termination on the 10 >>> hertz signal only and something resistive on the 20 mhz >>> signal........another idea use the 16 pF direct off the mixer, then a >>> series resistor for isolation and then a large capacitor on the 10 hertz >>> beat for maximum slope. >>> >>> At the present, I am awaiting parts to build a low noise preamp base on the >>> THAT1512 so I can make better measurements on the mixer. Bruce has >>> provided a lot of good suggestions and helpful comments on my project and >>> Ulrich has provided me quite a bit of user support on his program, Plotter. >>> Thanks to all. >>> >>> Comments ? Brian KD4FM >>> <DMTD_Plans.pdf>_______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
