diff --git a/artiq/firmware/runtime/rtio_clocking.rs b/artiq/firmware/runtime/rtio_clocking.rs
index 3277a2794..99a47fe3f 100644
--- a/artiq/firmware/runtime/rtio_clocking.rs
+++ b/artiq/firmware/runtime/rtio_clocking.rs
@@ -67,6 +67,19 @@ pub mod crg {
 
 #[cfg(si5324_as_synthesizer)]
 fn setup_si5324_as_synthesizer() {
+    // 125 MHz output from 10 MHz CLKIN2, 504 Hz BW
+    #[cfg(all(rtio_frequency = "125.0", si5324_ext_ref, ext_ref_frequency = "10.0"))]
+    const SI5324_SETTINGS: si5324::FrequencySettings
+        = si5324::FrequencySettings {
+        n1_hs  : 10,
+        nc1_ls : 4,
+        n2_hs  : 10,
+        n2_ls  : 300,
+        n31    : 75,
+        n32    : 6,
+        bwsel  : 4,
+        crystal_ref: false
+    };
     // 125MHz output, from 100MHz CLKIN2 reference, 586 Hz loop bandwidth
     #[cfg(all(rtio_frequency = "125.0", si5324_ext_ref, ext_ref_frequency = "100.0"))]
     const SI5324_SETTINGS: si5324::FrequencySettings