The LT1037 is shown with a gain of ~1690x, if this amplifier is used to
amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network
to limit the large signal gain to 5x (so that the LT1037 remains stable
as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037
then avoids saturation and the opamp input stage remains in the linear
region.
Bruce
Bob Camp wrote:
Hi
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.
Bob
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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