Datasheet

DS_1x66B_001 73M1866B/73M1966B Data Sheet
Rev. 1.6 57
8.8.1 Transmit and Receive Level Control
Refer to Section 8.5 for information about 73M1x66B levels.
8.8.1.1 Transmit Gain Scaling
The first gain stage in the transmit signal path is the digital gain whose value is controlled by writing to
Register 0x08 (TXDG). The second gain stages are the analog gains that are controlled by Register
0x14[7] (TXBST) and Register 0x14[6:5] (DAA). As a general rule to prevent clipping of the analog gain
stages, it is important to choose a value of the digital gain such that the transmit data after multiplied by
TXDG does not exceed +1.25 dBm. So for correct use of the gain controls the appropriate mix of digital
and analog settings must be used. Generally speaking, for best S/N performance, it is advisable to make
the digital words, after digital gain scaling, as large as possible without going over the +1.25 dBm limit.
Example:
If +2 dBm transmit level is desired for the case where the maximum input is 0 dBm:
Set analog gain to +3 dB (i.e. TXBST and DAA = 0) add attenuation of –1 dB by setting TXDG to
01101100 (-6 + 3.5 + 1 + 0.5 = -1); therefore, +3 - 1 = +2. Note in this case any digital gain could
cause clipping at high input levels in the analog circuitry.
Table 34 lists transmit level analog gain adjustment settings based upon the values of TXBST and DAA.
Table 34: Transmit Gain Control
TXBST
0x14[7]
DAA1
0x14[6]
DAA0
0x14[5]
Gain,
Nom. (dB)
0 0 0 +3.0
0
0
1
0.0
0
1
0
-4.0
0 1 1 -8.0
1 0 0 +6.0
1 0 1 +6.0
1 1 0 +2.0
1 1 1 -2.0
Table 35 shows a recommended gain settings for various transmit levels. With 0 dBm of Tx Data and
default setting of DAA1:0 = 01, Txbst=0, TXDG=00h, the transmit level is slightly off at –0.25 dBm.
TXDG=00000010 is required to achieve 0 dBm transmit level. For Tx level > 6 dBm, Tx Data is assumed
less than 0 dBm such that the product of Tx Data and TXDG is less than 1.25 dBm.
Table 35: Recommended Gain Setting
TX Level
Analog Gain
Digital Gain
Ana+Dig
dBm TxBst DAA1 DAA0 dB TXDG dB dB
-26
0
1
1
-8
1100_0010
-17.75
-25.75
-25
0
1
1
-8
1100_1010
-16.75
-24.75
-24 0 1 1 -8 1101_0010 -15.75 -23.75
-23 0 1 1 -8 1101_1010 -14.75 -22.75
-22 0 1 1 -8 1110_0110 -13.75 -21.75
-21 0 1 1 -8 1110_1110 -12.75 -20.75
-20 0 1 1 -8 1000_0010 -11.75 -19.75
-19 0 1 1 -8 1000_1010 -10.75 -18.75
-18 0 1 1 -8 1001_0010 -9.75 -17.75