utils: Get signal above noise floor when finding optimal gain

Ensure that the signal is at least visible above the noise floor before
attempting to identify gain compression. Another option that was
considered involved starting from the top of the gain range and working
down to avoid this issue, however, that option could expose the device
to unsafe amounts of incoming signal power.

Signed-off-by: mattprost <matt.prost@ni.com>

1 files changed, 31 insertions, 9 deletions

diff --git a/host/utils/usrp_cal_utils.hpp b/host/utils/usrp_cal_utils.hpp
index 1b9df9d03..3750be9cb 100644
--- a/host/utils/usrp_cal_utils.hpp
+++ b/host/utils/usrp_cal_utils.hpp
@@ -263,15 +263,15 @@ static uhd::usrp::multi_usrp::sptr setup_usrp_for_cal(
  **********************************************************************/
 UHD_INLINE void set_optimal_rx_gain(uhd::usrp::multi_usrp::sptr usrp,
     uhd::rx_streamer::sptr rx_stream,
-    double wave_freq = 0.0)
+    double wave_freq = 0.0,
+    double gain_step = 3.0)
 {
-    const double gain_step                  = 3.0;
-    const double gain_compression_threshold = gain_step * 0.5;
-    const double actual_rx_rate             = usrp->get_rx_rate();
-    const double actual_tx_freq             = usrp->get_tx_freq();
-    const double actual_rx_freq             = usrp->get_rx_freq();
-    const double bb_tone_freq               = actual_tx_freq - actual_rx_freq + wave_freq;
-    const size_t nsamps = size_t(actual_rx_rate / default_fft_bin_size);
+    const double gain_step_threshold = gain_step * 0.5;
+    const double actual_rx_rate      = usrp->get_rx_rate();
+    const double actual_tx_freq      = usrp->get_tx_freq();
+    const double actual_rx_freq      = usrp->get_rx_freq();
+    const double bb_tone_freq        = actual_tx_freq - actual_rx_freq + wave_freq;
+    const size_t nsamps              = size_t(actual_rx_rate / default_fft_bin_size);
 
     std::vector<samp_type> buff(nsamps);
     uhd::gain_range_t rx_gain_range = usrp->get_rx_gain_range();
@@ -290,6 +290,10 @@ UHD_INLINE void set_optimal_rx_gain(uhd::usrp::multi_usrp::sptr usrp,
     // this by looking for the gain setting where the increase
     // in the tone is less than the gain step by more than the
     // gain compression threshold (curr - prev < gain - threshold).
+    // This routine starts searching at the bottom of the gain range
+    // rather than the top in order to minimize the chances of
+    // exposing frontend components to a dangerous amount of power
+    // from the incoming signal.
 
     // Initialize prev_dbrms value
     usrp->set_rx_gain(rx_gain);
@@ -297,6 +301,24 @@ UHD_INLINE void set_optimal_rx_gain(uhd::usrp::multi_usrp::sptr usrp,
     prev_dbrms = compute_tone_dbrms(buff, bb_tone_freq / actual_rx_rate);
     rx_gain += gain_step;
 
+    // First, get the signal above the noise floor
+    while (rx_gain <= rx_gain_range.stop()) {
+        usrp->set_rx_gain(rx_gain);
+        capture_samples(usrp, rx_stream, buff, nsamps);
+        curr_dbrms = compute_tone_dbrms(buff, bb_tone_freq / actual_rx_rate);
+        delta      = curr_dbrms - prev_dbrms;
+
+        // Check that the signal power is not already high
+        if (curr_dbrms >= 0)
+            break;
+        // Check that the signal power has increased as the gain increases
+        if (delta >= gain_step - gain_step_threshold)
+            break;
+
+        prev_dbrms = curr_dbrms;
+        rx_gain += gain_step;
+    }
+
     // Find RX gain where signal begins to clip
     while (rx_gain <= rx_gain_range.stop()) {
         usrp->set_rx_gain(rx_gain);
@@ -305,7 +327,7 @@ UHD_INLINE void set_optimal_rx_gain(uhd::usrp::multi_usrp::sptr usrp,
         delta      = curr_dbrms - prev_dbrms;
 
         // check if the gain is compressed beyone the threshold
-        if (delta < gain_step - gain_compression_threshold)
+        if (delta < gain_step - gain_step_threshold)
             break; // if so, we are done
 
         prev_dbrms = curr_dbrms;