Re: [Fwd: Re: [Discuss-gnuradio] Gain Range error / Tx LO Offset]

[Eric Schneider]

On Wed, 2010-01-20 at 18:11 -0800, Matt Ettus wrote:
> Because 0.2 amplitude doesn't clip.  On a typical 1800, it will start to 
> clip somewhere around 0.5 to 0.6, and a typical 2400 will be much closer 
> to 1.0.  But in any case, you want to back off on the power to get the 
> linearity you need.  How much linearity you need is very application 
> dependent.

Okay, my use of "clip" wasn't precise.  I was just parroting some recent
posts regarding 0.2 amplitudes and "clipping".  I haven't measured it
but eyeballing the compression point does seem to be around 0.5 as you
state.

Linearity does seem to be an issue above 0.1 or so, based on the
intermod products I'm seeing.

> There are users that are doing nonlinear modulations where clipping is 
> ok, like GMSK, and in that case they need to be able to get the full 
> power output (well over 20dBm for most boards).  If we scale the analog 
> electronics to give good linearity at digital amplitude of 1.0, then you 
> would not be able to run in saturated mode if you wanted to.  So we 
> scaled it so that it is a compromise between the two usage cases.

Okay, that makes perfect sense.  Just to be clear, I am in no way
challenging the design; just in case I may have come across that way.

For my application, I would like to have high linearity and keep most of
the DAC range. (I don't need high output power)  It would seem that
reducing the DAC/9862 output gain is a good way (the only way?) to do
that.

Thus my questions regarding testing offset/balance via a flowgraph, and
if the tx settings are stored on the DB EEPROM (as opposed to having to
reset them after every power cycle).

But the core question is: Am I correct to assume that I should be able
to lower the DAC gain as long as I make the required offset adjustments
as well?

> On the WBX we have closed-loop analog transmit power control, so you 
> could always run at .75 for great linearity and then can control your 
> power with the low-speed DAC over a 25-35 dB range.
> 

I'm really looking forward to playing with that board.  Quite a design
feat!

--ETS