3.2 Transmission Modes

The transmission modes are configurations applied to the PHY layer to improve the performance, efficiency and spectrum ripple of the output driver according to the impedance detected in the line.

Depending on the detected impedance, two transmission modes are defined in register REG_CFG_IMPEDANCE:
  • HIGH. Mode optimized for high impedance (Z > 20Ω)
  • VERY_LOW. Mode optimized for very low impedance (Z < 10Ω)
Additionally, the PHY layer can modify the signal gain in a way that offers optimum results when combined with the Transmission mode. This behavior is controlled by the REG_CFG_AUTODETECT_IMPEDANCE. There are three operation modes:
  • OFF: Transmission mode and gain are fixed
  • AGC: Transmission mode is fixed but the gain is managed by the Automatic Gain Control (AGC) block to achieve the signal level injection target
  • ON: Transmission mode and gain are managed dynamically to optimize the output according to the line impedance and signal level injection target

Each Transmission mode is optimized within an impedance range; therefore, the PHY layer does not work properly in terms of performance/efficiency when operating outside of such range. Suppose there is a set signal injection objective of 110 dBuV for both HI and VLO modes. The following figures show the response of forced modes in an impedance range from 0 to 50Ω.

Figure 3-1. Transmission Power on HI and VLO Modes Depending on the Line Impedance

The figures above show the signal level when both Transmission mode and gain are fixed (OFF mode). In each mode, the injected transmission signal is reaching the objective in its intended range, but out of such range, each mode is not achieving the injection objectives. Also, the response has a much more stable trend for impedance above 20Ω, but for lower values, the transmission driver is more dependent on the detected impedance.

It is also possible to enable a configuration feature where the Transmission mode is fixed but slight changes in gain are allowed (AGC mode). This allowed variation in gain is probably not enough to meet the injection objective.

To meet the objective in the entire impedance range (with some limitations in low impedance as explained above), rely on the operation mode recommended where an Impedance Detection and Adaptation algorithm is implemented in the Physical layer (ON mode).