[hpsdr] Pre-Distortion

[Mike Seguin]

For those interested in the effect of pre-distortion on FMHD (IBOC) 
signals, here are two spectrum analyzer shots.

http://users.burlingtontelecom.net/~n1jez@burlingtontelecom.net/images/WVPS3.jpg

This one is of an uncorrected IBOC signal showing 'spectral regrowth'. 
The main FM analog signal is centered at 107.9 MHz. Upper and lower HD 
carriers are clearly visible between 108/108.1 and 107.7/107.8

The nasty regrowth at 107.6 and 108.2

http://users.burlingtontelecom.net/~n1jez@burlingtontelecom.net/images/woxm.jpg

This is a shot of a corrected signal. Sorry it isn't as wide as the WVPS 
shot, but it has the IBOC "mask" drawn on it. You can the see the signal 
easily fits and the mask. There was a bit of regrowth further out, but 
easily 10 dB down from the mask.

It is pretty dramatic to see the pre-distortion correctors operate.

Mike, N1JEZ
Engineering
Vermont Public Radio

On 12/8/2013 4:07 AM, David R. Wilson wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello Helmut,
>
> I didn't remember the actual figures, however TV stations have a very
> difficult mask to fit into spectrum wise.  The same goes for HDRadio
> (IBOC) Cleaning that up was done with pre distortion routines.  I do
> remember seeing the scope display improve remarkably.  When you can
> compensate for the distortion (even in the final amp) it is amazing how
> clean things can get.
>
> 73,
> Dave
> KU4B
>
>
> On Sun, 2013-12-08 at 09:24 +0100, Helmut, DC6NY wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> Hi,
>>
>> just my two 2ct: The typical improvement is far more than 10 db. It can be
>> around 30 dB(!). I got - 55dBc on 2tone (equal -61 dB PEP) with my 800W
>> LDMOS/BJT amp. This means the same scale as the HPSDR exciter or no
>> degradation of spectral purity due to IM after 32 dB amplification.
>>
>> 73, Helmut, DC6NY
-- 

73,
Mike, N1JEZ
"A closed mouth gathers no feet"


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