diff --git a/test/dma_tests.py b/test/dma_tests.py
index 54e42fe8e..719b8d699 100644
--- a/test/dma_tests.py
+++ b/test/dma_tests.py
@@ -595,10 +595,14 @@ def cw_loopback(uri, classname, channel, param_set, use_tx2=False, use_rx2=False
         RXFS = int(getattr(sdr, attr))
 
     A = 2 ** 15
-    fc = RXFS * 0.1
-    fc = int(fc / (RXFS / N)) * (RXFS / N)
+    if hasattr(sdr, "tx_sample_rate"):
+        FS = int(sdr.tx_sample_rate)
+    else:
+        FS = RXFS
+    fc = FS * 0.1
+    fc = int(fc / (FS / N)) * (FS / N)
 
-    ts = 1 / float(RXFS)
+    ts = 1 / float(FS)
     t = np.arange(0, N * ts, ts)
     if sdr._complex_data:
         i = np.cos(2 * np.pi * t * fc) * A * 0.5