diff --git a/src/ria_toolkit_oss/sdr/thinkrf.py b/src/ria_toolkit_oss/sdr/thinkrf.py
index abdb5a5..ccbe9e9 100644
--- a/src/ria_toolkit_oss/sdr/thinkrf.py
+++ b/src/ria_toolkit_oss/sdr/thinkrf.py
@@ -306,15 +306,31 @@ class ThinkRF(SDR):
         raise NotImplementedError("ThinkRF radios do not expose a controllable bias-tee")
 
     def _derive_decimation(self, target_sample_rate: int | float) -> int:
-        """Round sample rate to nearest supported decimation."""
+        """
+        Derive decimation from target sample rate.
+        Always rounds DOWN decimation (UP sample rate) to meet or exceed user's requested rate.
+
+        Example: 30 MS/s requested → dec 4 (31.25 MS/s), NOT dec 8 (15.625 MS/s)
+        """
         if not target_sample_rate:
             return 1
         requested = float(target_sample_rate)
         if requested >= self.BASE_SAMPLE_RATE:
             return 1
-        desired = self.BASE_SAMPLE_RATE / requested
-        # Always round UP to next decimation (lower sample rate) to avoid exceeding user request
-        best = min((d for d in self.SUPPORTED_DECIMATIONS if d >= desired), default=self.SUPPORTED_DECIMATIONS[-1])
+
+        desired_decimation = self.BASE_SAMPLE_RATE / requested
+
+        # Round DOWN decimation (UP sample rate) to meet or exceed requested rate
+        # Find largest decimation that gives sample rate >= requested
+        valid_decimations = [d for d in self.SUPPORTED_DECIMATIONS if d <= desired_decimation]
+
+        if valid_decimations:
+            # Use largest valid decimation (gives sample rate >= requested)
+            best = max(valid_decimations)
+        else:
+            # Requested rate too low, use minimum decimation (max sample rate)
+            best = self.SUPPORTED_DECIMATIONS[0]
+
         return int(best)
 
     def enforce_sample_rate(self, requested_sample_rate: int | float, decimation: Optional[int] = None) -> tuple[int, float]: