Hello Klaus,
On 07/30/2013 02:20 AM, Klaus Dittrich wrote:
...
I wonder why this isolation amplifier is off by default,
but assume the reason is to save battery power which
does not matter in case of use as home station.
So I would like to know if there are any degradations
in IP3 or more probable in opposite sideband suppression
if this amplifier is permanent in the signal path because
of additional phase shifts.
As far as I remember this case is not included
in the OSB adjustment procedure.
It consumes about 10 to 15 mA, and when used at the 0 dB gain level
(strictly for isolation) it degrades MDS slightly. If used at the 10 dB
gain setting (10 dB preamp), it will improve MDS slightly. This circuit
has a very high IP3 so it's affect on IMD3 dynamic range is mostly due
to the change in MDS and signal gain to downstream circuits. This stage
is before the signal is split into quadrature components, so it doesn't
directly affect opposite sideband rejection.
I was astonished to see during OSB adjustment how
sensitve the procedure is to frequeny dependend phase
shifts which I never thought they could be of importance.
Every 10th mm on the circuit board and every ns in software
seems to matter.
Wayne, could you please drop a few words about this?
Getting high levels of OSB rejection requires very precise phase and
gain matching between the quadrature signal paths. 40 dB requires gain
matching to be about 0.1 dB and phase matching to be about 1 degree.
With precision components, 40 to 50 dB is about the practical limit of
what can be done in hardware. The rest is done in DSP. Every 20 dB
improvement requires about a 10x decrease in both phase and gain
mismatch, so 80 dB of OSB rejection requires the phase balance to be
about 0.01 degrees and the gain balance to be about 0.001 dB. It's
possible to get a deep null during OSB calibration, but it takes very
little change in the balance to move away from that point.
73,
Rich AC7MA
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