[time-nuts] DMTD Mixer Terminations

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Feb 27 21:48:20 UTC 2010

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.
You will need a bit more filtering of the mixer IF output, or signal 
rectification effects in the bipolar opamp may be an issue.
The amplitude of the sum frequency component seen by the opamp input 
needs to be reduced to a value such that the effect of signal 
rectification by the opamp input stage is insignificant.
> 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.
Whilst in narrowband systems (a DMTD is a narrow band system) reactive 
termination of the mixer/phase detector RF port will reduce the noise, 
the idea is to reflect all of the sum frequency component back into the 
mixer. This can be done using a capacitive termination where the 
impedance of the capacitor is low at the (20MHz) sum frequency.
The capacitor impedance should be high at the (10Hz) difference 
frequency to avoid attenuating the difference frequency component.
Using a capacitor with a 50 ohm reactance at the sum frequency will not 
reflect all of the sum frequency back into the mixer.

Note with saturated mixer input ports capacitive termination as outlined 
above of the IF port will not produce a triangular beat frequency waveform.
The waveform should be quasi trapezoidal with rounded peaks.
The slew rate at the zero crossing will be increased and the noise to 
slope ration improved over that achieved with a more conventional 
> 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.
There are a series of NIST papers that show the effect of the IF port 
termination on the noise and beat frequency waveform.

Since its very easy to measure the beat frequency waveform slope and 
noise at the zero crossing its probably better to calibrate your 
speculations with actual measurements.

> 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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