[time-nuts] DMTD Mixer Terminations

Brian Kirby kilodelta4foxmike at gmail.com
Sat Feb 27 17:41:25 UTC 2010

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
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