- VHDL source: psi_fix_dds_18b
- Testbench source: psi_fix_dds_18b_tb.vhd
This entity implements an 18-bit direct digital synthesizer (DDS). The sine-wave is generated using the entity psi_fix_lin_approx_sin_18b and it has an error of less than one LSB for all values. As a result, there are no significant spurs in the generated spectrum (significant in terms of above the quantization noise floor) as shown in the figure below.
The DDS supports single-channel or multi-channel TDM implementation.
| Name | type | Description |
|---|---|---|
| phase_fmt_g | psi_fix_fmt_t | phase format width => generally counter length |
| tdm_channels_g | positive | time division multiplexed number of channels |
| ram_behavior_g | string | ram behavior read before write |
| rst_pol_g | std_logic | reset polarity active high = '1' |
| rst_sync_g | boolean | reset sync or async |
| Name | In/Out | Length | Description |
|---|---|---|---|
| clk_i | i | 1 | clk system |
| rst_i | i | 1 | rst system |
| restart_i | i | 1 | This signal can be used to start the DDS again at the phase offset. This is useful if 100% reproducible outputs must be generated several times. |
| phi_step_i | i | rasterized | Phase step between two consecutive output samples. The phase step is given in 2π (0.5 corresponds to π). The phase step can be changed at runtime safely. |
| phi_offset_i | i | phase_fmt_g | Phase offset of the generated signal. The phase offset is given in 2π (0.5 corresponds to π). The phase offset can be changed at runtime safely. |
| vld_i | i | 1 | AXI-S handshaking signal that can be used to generate samples at any rate. For continuous operation (one sample per clock cycle) , the signal can be left unconnected. The inputs restart_i, phi_step_i and phi_offset_i are latched on vld_i=1. |
| dat_sin_o | o | 17 | sinus output |
| dat_cos_o | o | 17 | cosine output 90 degree phase shifted |
| vld_o | o | 1 | frequency sampling output valid |
The total pipeline delay of the DDS is 10 clock cycles.
When using the DDS in multi-channel TDM configuration, the inputs (phi_step_i, phi_offset_i, restart_i) of all channels must be applied in turns