Am 11.01.2018 um 10:57 schrieb Stephan Sandenbergh:
I plotted the result for a few oscillator drift rate values. It seems that
the 'extra' error introduced by the imperfect ADC time base would be
negligible for many applications for OCXO drift rates or better. This is
likely the reason why it is often ignored.
There is no reason to assume that an ADC time base should be more
imperfect than a down converter time base.
On Mon, Dec 25, 2017 at 3:11 AM Attila Kinali <att...@kinali.ch> wrote:
As Tim Shoppa mentioned, you do not want to have a ratio with small
integers
between the LO frequency and the sampling frequency, as any feedthrough of
the LO and its harmonics will lead to a DC offset and spurs. The amplitude
of both will depend on the exact phase relation between the LO frequency
and the sampling frequency, which is usually stable, but not time-nuts
stable.
No, what I really want is having no LO frequency at all. I'd like to
start with a Pascall
class 100 MHz osc, maybe locked to the house reference. Multiply up to
800, 1200 or
2400 MHz using barndoor-wide filters that have a constant delay on the
center frequency.
The spurii are 100 MHz far away or multiples thereof at the later stages.
The ADC would be an Analog Devices AD9680, AD9208 or similar from TI.
These are
dual ADCs already, with 2 of them we could play most of the tricks of
the Timepod,
just sampling directly in L-band included.
Use the built-in DDS and down converters with small integers that
produce no birdies.
Filter and decimate like hell. Here we get the phase noise performance
back that we
have lost in multiplication. This down conversion/filtering/decimation
is available twice
in each ADC chip, no need for DIY.
When we have long words at a comfortably slow sample rate, we can
transfer them
via the JESD204B links to a mid size ZYNC system on chip, for further
processing in
its FPGA and/or CPUs, with Linux, network access and all the comfort we
are used to.
There are also interpolating/up sampling DACs for the transmitter if
needed. The
G5 phone system gives us nice building blocks to play with.
That all would fit on a 3*5 inch board, like some Red Pitaya on steroids.
< http://www.analog.com/en/search.html?q=ad9680 >
< https://www.redpitaya.com/c96/stemsuplabsup-125-14 >
cheers, Gerhard
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