[Discuss-gnuradio] Questions about USRP TV RX agc setting

[HL_Wu]

Hi, all
     When I traced the tv_rx.py code, I found that in order to adjust the RF AGC/IF AGC
the AUX DAC inside the AD9862 must be used. But I had several questions about this.

1). the AUX DAC bit width: in the code segment below, there is a magic number 4096
(2e12), but the bit width of AUX DAC is only 8 bits width(2e8=256).What is the relationship
between the 8-bit and 4096?
2). the gain/voltage formula:
      Are there two formula approximation from the tuner datasheet ?
3). TV RX daughterboard
      In the schematics, it is necessary to have two signals: AUX_DAC_A, and io_rx_00
to control the IF AGC setting. What is the use of io_rx_00 signal?    

   # Gain setting
    def _set_rfagc(self,gain):
        assert gain <= 60 and gain >= 0
        if gain == 60:
            voltage = 4
        else:
            voltage = gain/60.0 * 2.25 + 1.25
        dacword = int(4096*voltage/1.22/3.3)    # 1.22 = opamp gain

        assert dacword>=0 and dacword<4096
        self.rfagc = dacword
        self._u.write_aux_dac(self._which, 1, dacword)

    def _set_ifagc(self,gain):
        assert gain <= 35 and gain >= 0
        if gain == 35:
            voltage = 3.5
        else:
            voltage = gain/35.0 * 2.1 + 1.4
        dacword = int(4096*voltage/1.22/3.3)    # 1.22 = opamp gain

        assert dacword>=0 and dacword<4096
        self.rfagc = dacword
        self._u.write_aux_dac(self._which, 0, dacword)

Owen Wu
ASMedia