diff --git a/instrument/Bilby_Definition.xml b/instrument/Bilby_Definition.xml
index 81029ac258ae427777382b1c5f79f82a6478b94b..baa7b5eabc3295fd9b5a62f63f452d9a2aff8810 100644
--- a/instrument/Bilby_Definition.xml
+++ b/instrument/Bilby_Definition.xml
@@ -26,7 +26,7 @@
   <component name="Source" type="source">
     <location x="0.0" y="0.0" />
     <parameter name="z"> 
-      <logfile id="L1_chopper_value" eq="1*value"/>
+      <logfile id="L1_chopper_value" eq="-1*value"/>
     </parameter>
   </component>
   <type name="source" is="Source" />
@@ -503,4 +503,4 @@
    
   
 </instrument>
-  
\ No newline at end of file
+