[hpsdr] Hermes Pure Signal

[Helmut]

Hi,

I do understand the desire of Atlas-based-HPSDR owners for digital adaptive
pre-distortion and their question why nearly the same hardware doesn't work
with PS.
As far I know - hopefully Warren will correct me, when I'm wrong - two
separate RX-DDC-paths are employed in the current PS version: One path
converts the analogue 'distorted' feedback-signal from the amp into a 'fast'
data-stream, followed by the digital I-Q mixer and CIC filters for down
conversion into the baseband. The second equal RX-path is fed directly with
the 'fast undistorted' TX- data stream, collected at the DAC input and
up-converted from baseband to the operation frequency. As the nominal-actual
comparison has to be made in the baseband (or better in the PC) this
solution was chosen to provide the necessary correlation of the samples.
It's meanwhile an approved way to provide the essential information for
error correction of amplitude and phase, but  it's also evident that this
design requires a lot of FPGA resources and that the necessary 'high speed'
data transport between Mercury and Penny via the given Atlas structure is
not possible.
As mentioned in a previous post, Alfred, HB9EPU made another approach for
his Atlas-based HPSDR: He used the 'standard' RX-path in Mercury for the
feed-back signal in Dupe mode to feed one port of the baseband 'sample
comparator'. The other port is supplied with the corresponding 'clean' TX
samples. His kind of sample correlation makes things easy to apply on
Atlas-based-HPSDRs with good results. Two different solid state amps (300W
and 700W) show under his adaptive pre-distortion application acceptable
IM-performance of around 50 dBc.

Sorry, when using too much bandwidth.

73, Helmut, DC6NY