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#1 2007-Feb-27 16:24:54

zrcao
Member
From: Vienna, VA
Registered: 2007-Jan-24
Posts: 121

Tx Gain control

Code:

void WarpRadio_BaseBandTxGain(unsigned int* baseaddr, char value, unsigned char radios);
void WarpRadio_TxVGAGainControl(unsigned int* baseaddr, char value, unsigned char radios);

The first function looks like to control the DAC output power. So what is the DAC output voltage range? What is the DAC output impedence? In fact, I would like to understand how to calculate the DAC output power from the digital sample sequence we feed into the DAC.

The second function should be the radio IC VGA control. Interestingly, on page 17 of maxim 2829 data sheet, the figure of position (2, 2) shows the output TX power vs. Gain Settings. The output power should depend on the input power, right? So do you know what is the input power for that figure? It would be great if you can put up some Rice measurement online.

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#2 2007-Feb-27 17:20:40

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 5159

Re: Tx Gain control

The DAC output voltage range is fixed by resisotrs on the board. The DAC's gain settings are also fixed and are used only for small gain variations when calibrating the I/Q channels, which (thankfully) is not generally necessary.

Both of the functions you list affect only the radio. The MAX2829 has two stages of Tx gain control- 2 bits at baseband and 6 bits at RF. WarpRadio_BaseBandTxGain configures the baseband gain and accepts values in [0,3] where 3 is maximum gain. WarpRadio_TxVGAGainControl configures the RF gain and accepts values in [0,63] where 63 is maximum gain.

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#3 2007-Feb-28 17:30:27

zrcao
Member
From: Vienna, VA
Registered: 2007-Jan-24
Posts: 121

Re: Tx Gain control

The following is the response from our team's RF specialist, who briefly studied the data sheet of MAX2829 regarding my question posted on this topic.

The input power should be 100mVrms into 60kohm, see page 10 of the datasheet for TX. You really donít want to input anything else. The reason why TX vs gain is specified without concern for input power is that when a high impedance 60kohm (instead of 50 ohm) differential pair is used as the baseband input stage, it wants to see a specific voltage swing. In other words, the dynamic range of the differential pair is fairly small. It cares about voltage swings, rather than power (if you divide V^2/ 60 kohm, you get rather low power (-40dBm), but the voltage swing is good 100mVrms.

High impedance inputs are typical of all baseband stages to make chip to chip communications easier. (you donít need very strong drivers to drive 60kohm, unlike 50 ohm).  To summarize, the baseband input wants to see 100mVrms voltage swing.

So, new question is, if we translate this voltage swing requirement into digital domain, what is the RMS number of bits we should use for samples to make sure the DAC output swing is around 100mVrms?

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#4 2007-Mar-02 22:34:55

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 5159

Re: Tx Gain control

I haven't done this calculation myself, but it should be straightforward given the Radio Board Schematics (DAC on pg. 4) and the AD9777 DAC datasheet.

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