diff --git a/artiq/gateware/drtio/wrpll/filters.py b/artiq/gateware/drtio/wrpll/filters.py
index 58b664754..41fb06291 100644
--- a/artiq/gateware/drtio/wrpll/filters.py
+++ b/artiq/gateware/drtio/wrpll/filters.py
@@ -3,8 +3,7 @@ helper_xn2 = 0
 helper_yn0 = 0
 helper_yn1 = 0
 helper_yn2 = 0
-
-previous_helper_tag = 0
+helper_out = 0
 
 main_xn1 = 0
 main_xn2 = 0
@@ -13,31 +12,31 @@ main_yn1 = 0
 main_yn2 = 0
 
 
-def helper(helper_tag):
+def helper(tag_diff):
     global helper_xn1, helper_xn2, helper_yn0, \
-        helper_yn1, helper_yn2, previous_helper_tag
+        helper_yn1, helper_yn2, helper_out
 
-    helper_xn0 = helper_tag - previous_helper_tag - 32768
+    helper_xn0 = 0 - tag_diff # *(2**22)
 
     helper_yr = 4294967296
 
     helper_yn2 = helper_yn1
     helper_yn1 = helper_yn0
-    helper_yn0 = (
-                 ((284885689*((217319150*helper_xn0 >> 44) +
-                              (-17591968725107*helper_xn1 >> 44))) >> 44) +
-                 (-35184372088832*helper_yn1 >> 44) -
-                 (17592186044416*helper_yn2 >> 44))
+
+    helper_yn0 = (284885690 * (helper_xn0
+                                + (217319150 * helper_xn1 >> 44)
+                                - (17591968725108 * helper_xn2 >> 44)
+                                ) >> 44
+                  ) + (35184372088832*helper_yn1 >> 44) - helper_yn2
 
     helper_xn2 = helper_xn1
     helper_xn1 = helper_xn0
 
-    previous_helper_tag = helper_tag
-
     helper_yn0 = min(helper_yn0, helper_yr)
     helper_yn0 = max(helper_yn0, 0 - helper_yr)
+    helper_out = 268435456*helper_yn0 >> 44
 
-    return helper_yn0
+    return helper_out
 
 
 def main(main_xn0):