[time-nuts] Down-conversion to IF and sampling

Gerhard Hoffmann dk4xp at arcor.de
Thu Jan 11 08:19:26 EST 2018


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 <attila at 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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