diff --git a/pslab/instrument/waveform_generator.py b/pslab/instrument/waveform_generator.py
index 07c3523..d618a3f 100644
--- a/pslab/instrument/waveform_generator.py
+++ b/pslab/instrument/waveform_generator.py
@@ -140,7 +140,8 @@ def generate(
             Frequency in Hz. Can be a list containing two different values when
             'channel' is ['SI1', 'SI2']. Must be greater than 0.1 Hz. For
             frequencies below 1 Hz the signal is noticably attenuated by AC
-            coupling.
+            coupling. Input frequency lists are not modified; the actual
+            generated frequencies are returned separately.
         phase : float, optional
             Phase between waveforms when generating waveforms on both SI1 and
             SI2 in degrees. The default is 0.
@@ -157,6 +158,7 @@ def generate(
             channels = ["SI1", "SI2"][:channels]
 
         channels, frequency = _listify(channels, 2, frequency)
+        frequency = frequency.copy()
         table_size = len(channels) * [None]
         timegap = len(channels) * [None]
         prescaler = len(channels) * [None]
diff --git a/tests/test_waveform_frequency_input.py b/tests/test_waveform_frequency_input.py
new file mode 100644
index 0000000..0c2779c
--- /dev/null
+++ b/tests/test_waveform_frequency_input.py
@@ -0,0 +1,40 @@
+"""Requested analog frequencies remain distinct from generated frequencies."""
+
+from unittest.mock import Mock
+
+import pytest
+
+from pslab.connection import ConnectionHandler
+from pslab.instrument.waveform_generator import WaveformGenerator
+
+
+@pytest.mark.parametrize(
+    "channels,requested",
+    [("SI1", [1001]), ("SI2", [2003]), (["SI1", "SI2"], [1001, 2003])],
+)
+def test_generate_does_not_replace_requested_frequency_list(channels, requested):
+    generator = WaveformGenerator(Mock(spec=ConnectionHandler))
+    original = requested.copy()
+    actual = generator.generate(channels, requested)
+    assert requested == original
+    assert actual != original
+    names = channels if isinstance(channels, list) else [channels]
+    assert actual == [generator._channels[name].frequency for name in names]
+
+
+def test_failed_generation_does_not_partially_replace_requested_frequencies():
+    generator = WaveformGenerator(Mock(spec=ConnectionHandler))
+    requested = [1001, 0.05]
+    with pytest.raises(ValueError, match="Frequency must be greater"):
+        generator.generate(["SI1", "SI2"], requested)
+    assert requested == [1001, 0.05]
+    assert generator._device.mock_calls == []
+
+
+def test_list_and_scalar_requests_program_identical_commands():
+    list_generator = WaveformGenerator(Mock(spec=ConnectionHandler))
+    scalar_generator = WaveformGenerator(Mock(spec=ConnectionHandler))
+    actual = list_generator.generate(["SI1", "SI2"], [1001, 1001], phase=90)
+    scalar_actual = scalar_generator.generate(["SI1", "SI2"], 1001, phase=90)
+    assert actual == scalar_actual
+    assert list_generator._device.mock_calls == scalar_generator._device.mock_calls