diff --git a/gr-analog/python/analog/wfm_tx.py b/gr-analog/python/analog/wfm_tx.py
index eab16fe9b9..599a9bb6cd 100644
--- a/gr-analog/python/analog/wfm_tx.py
+++ b/gr-analog/python/analog/wfm_tx.py
@@ -51,10 +51,12 @@ def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=75e3, fh=-1.0):
 
         if do_interp:
             interp_factor = quad_rate // audio_rate
-            interp_taps = filter.firdes.low_pass(interp_factor, # gain
-                                                 quad_rate,     # Fs
-                                                 19000,         # cutoff_freq
-                                                 4000)          # transition_width
+            interp_taps = filter.optfir.low_pass(interp_factor,  # gain
+                                                 quad_rate,  # Fs
+                                                 16000,  # passband cutoff
+                                                 18000,  # stopband cutoff
+                                                 0.1,  # passband ripple dB
+                                                 40)  # stopband atten dB
             print("len(interp_taps) =", len(interp_taps))
             self.interpolator = filter.interp_fir_filter_fff(
                 interp_factor, interp_taps)