[hpsdr] Hermes Pure Signal

[Warren C. Pratt]

Hi Jani,

No, I was not talking about Cartesian feedback.  I considered that 
technique but, for various reasons, decided to use adaptive 
predistortion instead.

73,
Warren  NR0V

On 12/18/2015 2:01 AM, HA5FT Jani wrote:
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>
>
> Hi Warren,
> I think, you are talking about the technique called cartesian feedback, which is widely used to linearize power amplifiers. It has some variant which could handle even time variable delays. There are a lot of paper floating on the internet about this technique. The performance is comparable to your ps algorithm, but some cases it is difficult to ensure the stability of the feedback loop.
> Jani, ha5ft
>
> 2015. dec. 17. dátummal, 17:13 időpontban Warren C. Pratt <warren at wpratt.com> írta:
>
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>>
>> Hi Helmut,
>>
>> I believe you are correct in all your statements.
>>
>> We had a team member who worked on applying the feedback technique that Alfred is using.  The bottom line is that I believe that team member then became very busy with work activities and so we abandoned the approach.  There are two issues with that approach ... one should be very solvable.  The solvable issue is getting the delay matched between the two paths ... this is really just a matter of determining the exact differential delay since I have already implemented software to provide arbitrary sub-sample delays.  The second issue which will have some effect on the quality of the result is that, in the dual-down-conversion approach, the CIC and FIR filters are matched in characteristics like overshoot and passband ripple.  I believe this matching allows us to achieve a better result.  I don't know how much better since we don't have a working example of the other approach.
>>
>> 73,
>> Warren  NR0V
>>
>>> On 12/17/2015 7:05 AM, Helmut wrote:
>>> ***** High Performance Software Defined Radio Discussion List *****
>>>
>>> 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
>>>
>>>
>>>
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