[time-nuts] DMTD Mixer Terminations
lists at rtty.us
Sat Feb 27 23:24:07 UTC 2010
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.
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
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