Multi-band operation is where two or more baseband signals are up-converted (mixed) to individual carriers and then combined to generate a single composite analog output. In the RF-DAC tiles, this involves combining multiple DUC blocks together to drive the analog outputs.
The RF-DAC multi-band feature supports the following configurations:
- Real Dual-Band
2x multi-band real data per pair. One RF-DAC analog output of the pair is enabled; the other is off. Both RF-DAC digital DUCs are enabled.
- I/Q Dual-Band
2x multi-band I/Q data per pair. Both RF-DACs in the pair are enabled, one for I and one for Q. Both RF-DAC digital DUCs are enabled.
- Real Quad-Band
4x multi-band real data per tile. Four input streams through four DUCs are combined to drive the DAC0 output in real mode. All other RF-DACs are off.
- I/Q Quad-Band
4x multi-band I/Q data per tile. Four input streams through four DUCs are combined to drive the DAC0 output as I and DAC1 output as Q. All other RF-DACs are off.
Latency between outputs in a multi-band pair is matched and latency between pairs is matched, irrespective of the mode.
When multi-band is off, the I and Q datapaths pass straight through the multi-band logic block (as shown below). When multi-band is on, I and Q of each datapath are combined and passed to the next DSP block in the chain in front of the RF-DAC. Multi-band can be turned on per RF-DAC, for only the I datapath or the Q datapath, or for both I and Q.
To avoid potential overflow when two signals are added up, a -6 dBV scaling is introduced after the adder of each I and Q. Hence, -6 dBV scaling for dual bands and -12 dBV scaling for quad bands are applied. The default -6 dB scaling can be disabled via RFdc API.
RF-DAC multi-band is implemented by connecting multiple RF-DAC DUC blocks to an RF-DAC analog block. Each DUC block handles one band of data, and can mix this up to any carrier frequency. The output is then summed before being sent to the analog datapath and RF-DAC output. This is shown in the following figure.
Two example multi-band configurations are shown. The RF-DAC tile consists of four DUC blocks (digital datapaths) and four RF-DAC-analog blocks (analog datapaths). In the left hand example, the lower RF-DAC pair in the tile is configured as 2x real multi-band, while the top pair are independent RF-DACs. Because the bottom pair uses two DUC blocks, DAC1 is off, and DAC0 outputs the dual-band signal. In the right hand example, the lower pair is configured as 2x I/Q multi-band, while the top pair is off.