[cma-l] DAB trials

tlr at gairloch.co.uk tlr at gairloch.co.uk
Tue Mar 10 11:57:15 GMT 2015 

The amplifer could indeed be perfectly linear up to its rated CW output (they
aren't perfect of course), but the nature of the DAB modulated signal is such
that without pre-correction its power peaks are massively greater than the
average power, so the average power has to be kept low enough that it doesn't
drive the amplifier beyond its acceptably linear region on the peaks.
 
Remember we are talking about an SDR that is designed to be used for all manner
of modulation systems: AM, FM, DAB, DRM, SSB, GSM, wif-fi - any of these and
even some  not yet invented! It's output transceiver is capable of reasonably
linear operation up to 100mW continuous carrier power, and if, for example, it
were being used as an FM transmitter, then it could indeed be used close to
that, because FM is a continuous full power relatively slowly varying sine wave.
 
On the other hand, the COFDM modulation system used for DAB can include rapidly
varying peak modulation levels that are many times higher than the average level
(think of it like the '100-year wave' effect where a generally moderate sea can
suddenly throw up one huge wave when a whole pile of interfering waves happen to
constructively interfere). That isn't a 'fault' - it's an inherent aspect of the
design of the COFDM system.
 
The unfortunate side effect of this is that to avoid the amplifier being driven
outside acceptable linearity and producing spurious signals it must be run at a
relatively low average power level compared with its peak capability. For the
100mW transceiver on the SDR that Ofcom used they found it had to be kept down
to an average power of 5mW for transient peaks to be handled linearly enough (in
their case producing a 54dB carrier to noise ratio).
 
For a high power DAB system where the costs are shared by maybe 10 radio
stations, it turns out that it is more economic to use more advanced modulation
hardware t hat can 'pre-correct' the signal - ie slightly distort it to reduce
the ratio of peaks to average, then run the amplifiers more highly and to use
humungous six- or eight-cavity output filters to clean up the act. But for low
power transmitters serving just a couple of stations it turns out to be cheaper
to noiminally 'under-run' the amplifiers in terms of the average power versus
the amplifier's rated CW ability, to allow the use more basic pre- and post-
filtering.
 
Alex

> On 10 March 2015 at 10:25 Tony Bailey <ravensound at pilgrimsound.co.uk> wrote:
> 
>  I'm not so sure about that analysis, after all, the report implied that it
> was the SDR that was under-run at 5mW presumably because the output
> daughterboard was intended for AM/FM modulation?  Also, it was common IBA
> practice to run a several kilowatt bottle at a hundred watts or so, the excess
> input power being necessary to ensure class A linear operation.  The point
> being that the rf amp has to be linear, and it isn't, you can't just turn it
> down a bit.
> 
>  Tony Bailey
> 
> 
>  On 09/03/15 22:41, tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk> wrote:
> 
>      > >      Pete: We know what software and modulator they will probbaly be
>      > > using, but one of the key features of Ofcom's proposals for low-cost
>      > > DAB broadcasting is avoiding the use of the conventional large and
>      > > expensive DAB mask filters (typically 6-cavity high power devices) on
>      > > th efinal output.
> >       
> >      This can be achieved by a combination of tactics, including a very
> > under-run driver stage (eg 100mW stage running at only 5mW) and relatively
> > underrun RF amplifiers (eg maybe using an amplifier potentially capable of
> > say 250-500W continuous carrier power to produce a DAB signal of less than
> > 100W), so that there are fewer spectral regrowth components to be filtered.
> > Plus with the whole kit running at relatively low ERP anyway, the filtering
> > is not as critical as it would be for a high-power transmitter because the
> > absolute level of spurious products is low relative to neighbouring
> > high-power signals and aeronautical limits.
> >       
> >      Alex
> > 
> >      > On 09 March 2015 at 20:18 Pete <peter at engineeringradio.co.uk>
> >      > <mailto:peter at engineeringradio.co.uk> wrote:
> >      >
> >      >
> >      > It's obvious what type of exciter they will be using if you have been
> >      > following them. It's obvious it will be SDR based. The amps may be a
> >      > different matter. I don't think it is clear what they would be using.
> >      > They would probably use filtering to the spec set out for DAB.
> >       
> >       
> > 
> > 
> > 
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