diff --git a/hostsoftware/src/outputsettings.rs b/hostsoftware/src/outputsettings.rs
index 13e5a69..f7cb548 100644
--- a/hostsoftware/src/outputsettings.rs
+++ b/hostsoftware/src/outputsettings.rs
@@ -1,9 +1,16 @@
+use eframe::egui::Color32;
 use eframe::egui::{self};
 
 use crate::serial;
 use crate::widgets;
 use crate::CheapsdoControl;
 
+const PLL_MIN_FREQ: u32 = 600_000_000;
+const PLL_MAX_FREQ: u32 = 900_000_000;
+
+const OUT_MIN_FREQ: u32 = 500_000;
+const OUT_MAX_FREQ: u32 = 225_000_000;
+
 impl CheapsdoControl {
     pub fn output_control(&mut self, ui: &mut egui::Ui, ctx: &egui::Context) {
         ui.vertical(|ui| {
@@ -16,6 +23,8 @@ impl CheapsdoControl {
                     widgets::frequency_input(
                         self.ms1_frequency_id,
                         &mut self.pll_settings.ms1_frequency,
+                        PLL_MIN_FREQ,
+                        PLL_MAX_FREQ,
                         &ctx,
                         ui,
                     );
@@ -25,6 +34,8 @@ impl CheapsdoControl {
                     widgets::frequency_input(
                         self.ms2_frequency_id,
                         &mut self.pll_settings.ms2_frequency,
+                        PLL_MIN_FREQ,
+                        PLL_MAX_FREQ,
                         &ctx,
                         ui,
                     );
@@ -45,6 +56,8 @@ impl CheapsdoControl {
                         widgets::frequency_input(
                             self.output_frequency_id[i],
                             &mut self.pll_settings.outputs[i].frequency,
+                            OUT_MIN_FREQ,
+                            OUT_MAX_FREQ,
                             &ctx,
                             ui,
                         );
diff --git a/hostsoftware/src/widgets.rs b/hostsoftware/src/widgets.rs
index 99352d5..8f8c0f9 100644
--- a/hostsoftware/src/widgets.rs
+++ b/hostsoftware/src/widgets.rs
@@ -1,11 +1,13 @@
 use cheapsdo_protocol::Multisynth;
-use eframe::egui::{self, RichText};
+use eframe::egui::{self, Color32, RichText};
 
 use crate::formatters;
 
 pub fn frequency_input(
     id: egui::Id,
     freq: &mut u32,
+    min_freq: u32,
+    max_freq: u32,
     ctx: &egui::Context,
     ui: &mut egui::Ui,
 ) -> egui::Response {
@@ -16,10 +18,18 @@ pub fn frequency_input(
     } else {
         formatters::format_frequency(*freq as f64)
     };
+
+    let color = if !has_focus && (*freq <= min_freq || *freq >= max_freq) {
+        Some(Color32::DARK_RED)
+    } else {
+        None
+    };
+
     let resp = ui.add_sized(
         [100.0, 20.0],
         egui::TextEdit::singleline(&mut tmp)
             .id(id)
+            .text_color_opt(color)
             .horizontal_align(egui::Align::Max),
     );
     if has_focus {