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

[Eric Schneider]

On Thu, 2010-01-21 at 13:50 -0800, Matt Ettus wrote:
> On 01/21/2010 12:40 PM, Eric Schneider wrote:

> > Linearity does seem to be an issue above 0.1 or so, based on the
> > intermod products I'm seeing.
> 
> 
> Well, it depends on what you mean by "issue".  The RFX2400 typical 
> midband OIP3 is about 37 dBm, which is about all you can expect for a 
> small, low voltage, wideband design like this.

I never claimed that my expectations were reasonable!  ;-)

> I am not sure if you are asking about DC offset correction (which shows 
> up as carrier feedthrough on the output) or if you are asking about the 
> DC bias points of the differential baseband signals.  I'll try to answer 
> both questions...

DC offset.

> So DC offset and IQ balance are independent of intermod, and reducing
> your amplitude won't help those.  The original intention was to store 

Agreed, but in my single tone tests, I see what I interpret as intermod
products between the test tone and the LO.  I guess intermod may not be
the proper term, as I assume it is an artifact of the mixer rather than
the power amps.  It looks like sum and difference terms of the test tone
and the LO (and their harmonics).

I was hoping that I could make a DC Offset adjustment to reduce the LO
component (and thus all the related products), but adjusting the DC
offsets of my test signal (in the flow graph) did not lead to any
meaningful improvement.  So either compensating for DC offset in the
test signal does not work, or the the offset is pretty much as good as
it gets out of the box.

> If you actually mean the DC bias point, then there is a way to handle 
> this, see the next answer.

I think I'm getting the point that it probably isn't worth the effort...

> By default we use 20mA as the DAC output current.  You can lower this 
> all the way down to 2mA which would give you 20dB lower power.  The 
> problem is that reducing the differential amplitude (which is what you 
> want) also happens to reduce the DC bias point (which you don't want), 
> and the inputs of the IQ modulator can be somewhat sensitive to this. 
> You can actually reduce the DAC gain by about 3-6dB without too much 
> problem.  You should try it in your case to see if it gives you the 
> effect you want.

Just to make sure that I am understanding clearly, are you saying that I
can go down 3-6dB without adjusting the bias point settings, or that
going down more than that cannot be compensated via the register
settings and must be addressed via hardware mods?


Thank you very much for your patience with me!

--ETS