Hi Warren,
Interesting re. changing gain of the op amps. If the Rx II MDS is
limited by the noise floor of the op amps, then it is not clear to
me that changing the gain will affect this in a beneficial way.
The spec sheet for the op amp suggests to me that the circuit is
well designed and the noise is limited by the op amp itself, not
by the gain resistors.
However, if you are interested in experimenting, I have a thought.
Those 10 ohm resistors before and after the switching mixer seem
to me to be non-essential and possibly decreasing the sensitivity.
I was thinking of bypassing them myself as an experiment. I was
thinking of this from the perspective of eliminating some thermal
noise and simple resistive loss. However, eliminating them would
also increase the gain of the op amps since the input impedance,
together with the feedback impedance, determines the gain. The
mixer itself presents, I think, a source impedance of 12 ohms, so
eliminating the 20 ohms of resistors should increase the voltage
gain by X3 or so. I would be very interested to hear what, if any,
change in MDS measurement eliminating those resistors would make.
(A caveat, I am writing this after two glasses of wine.)
73,
Roger
Hi Warren and Roger,
I believe that the noise figure of the SoftRock is determined by the
conversion loss in the "mixer" and any front end filtering.? This
will determine the MDS.? A low noise amplifier ahead of the Softrock
mixer should reduce the noise figure and improve the MDS, but could
reduce the dynamic range unless the op amp gain is reduced.? The
UHFSDR by WB6DHW has a lower MDS, but utilizes a low noise amplifier
at the front-end.
I'm also of the opinion that the 10 ohms resistors serve to swamp
out variations in the ON resistance of the mixer switches.? With out
the resistors, there could be considerable variation in the I and Q?
overall gains.? This could present some problems with balancing the
gains (and phase) of the two channels.
Just my 2 cents.
Milt
W8NUE
