[time-nuts] DMTD Mixer Terminations
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
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 15504 bytes
Desc: not available
More information about the time-nuts