examples: Add --power command line option to tx_waveforms

If you run

    tx_waveforms --power -20 [other args]

it will try to set the out power to -20 dBm. The signal amplitude is
factored in, so changing --ampl will not change the actual TX power
unless it causes clipping, or becomes too low.

If the USRP does not support setting a power, the program will terminate
early. If it does support setting a power, but can't reach the requested
power, it will coerce, and print the actual, available power.

1 files changed, 19 insertions, 0 deletions

diff --git a/host/examples/wavetable.hpp b/host/examples/wavetable.hpp
index dc2a93c36..d18bd1fa5 100644
--- a/host/examples/wavetable.hpp
+++ b/host/examples/wavetable.hpp
@@ -27,17 +27,28 @@ public:
             // Fill with I == ampl, Q == 0
             std::fill(
                 _wave_table.begin(), _wave_table.end(), std::complex<float>{ampl, 0.0});
+            _power_dbfs = static_cast<double>(20 * std::log10(ampl));
         } else if (wave_type == "SQUARE") {
             // Fill the second half of the table with ampl, first half with
             // zeros
             std::fill(_wave_table.begin() + wave_table_len / 2,
                 _wave_table.end(),
                 std::complex<float>{ampl, 0.0});
+            _power_dbfs = static_cast<double>(20 * std::log10(ampl))
+                          - static_cast<double>(10 * std::log10(2.0));
         } else if (wave_type == "RAMP") {
             // Fill I values with ramp from -1 to 1, Q with zero
+            float energy_acc = 0.0f;
             for (size_t i = 0; i < wave_table_len; i++) {
                 _wave_table[i] = {(2.0f * i / (wave_table_len - 1) - 1.0f) * ampl, 0.0};
+                energy_acc += std::norm(_wave_table[i]);
             }
+            _power_dbfs = static_cast<double>(energy_acc / wave_table_len);
+            // Note: The closed-form solution to the average sum of squares of
+            // the ramp is:
+            // 1.0 / 3 + 2.0 / (3 * N) + 1.0 / (3 * N) + 4.0 / (6 * N^2))
+            // where N == wave_table_len, but it turns out be be less code if we
+            // just calculate the power on the fly.
         } else if (wave_type == "SINE") {
             static const double tau = 2 * std::acos(-1.0);
             static const std::complex<float> J(0, 1);
@@ -49,6 +60,7 @@ public:
                 _wave_table[i] =
                     ampl * std::exp(J * static_cast<float>(tau * i / wave_table_len));
             }
+            _power_dbfs = static_cast<double>(20 * std::log10(ampl));
         } else {
             throw std::runtime_error("unknown waveform type: " + wave_type);
         }
@@ -59,6 +71,13 @@ public:
         return _wave_table[index % wave_table_len];
     }
 
+    //! Return the signal power in dBFS
+    inline double get_power() const
+    {
+        return _power_dbfs;
+    }
+
 private:
     std::vector<std::complex<float>> _wave_table;
+    double _power_dbfs;
 };