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Re: [Discuss-gnuradio] Flex400 TX gain

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Flex400 TX gain
Date: Mon, 06 Mar 2006 17:34:06 -0800
User-agent: Thunderbird 1.5 (X11/20051201)

R and D wrote:
> The Flex400 driver (db_flex.py) has the following code:
>     def gain_range(self):
>         """
>         Return range of gain that can be set by this d'board.
>         @returns (min_gain, max_gain, step_size)
>         Where gains are expressed in decibels (your mileage may vary)
>         Flex Tx boards require that the PGA be maxed out to properly
> bias their circuitry.
>         """
>         g = self._u.pga_max()
>         return (g, g, 1.0)
> Am I understanding correctly that there is no way to adjust gain on
> the TX path? If I modified the code to allow programming of lower
> values in the PGA, what would the effect be? I mean, would that be a
> bad thing?

The TX PGA must be set at full scale on the Flex boards to ensure that
the mixer chip is properly biased.  You can adjust signal levels
digitally to transmit at lower power.  Since we have 14-bit DACs, this
should not be a problem unless you need very weak signals.

On the other transmit boards (BasicTX and LFTX) you can use the TX PGA
gains to adjust signal levels all you want.

None of this affects the RX PGA, which can be used across its full 20 dB
range with any daughterboard.


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