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
Description: Adobe PDF document
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