diff --git a/data_logging/tests/test_core.py b/data_logging/tests/test_core.py
new file mode 100644
index 0000000..0cc945e
--- /dev/null
+++ b/data_logging/tests/test_core.py
@@ -0,0 +1,60 @@
+import json
+import os
+import tempfile
+from pathlib import Path
+import pytest
+
+import sys
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / 'src'))
+
+from data_logging import core
+
+
+def test_log_data_creates_and_appends_valid_json(tmp_path, monkeypatch):
+    log_file = tmp_path / 'log_data.json'
+
+    # Patch the target output path in the function if it uses fixed path
+    monkeypatch.setattr(core, 'Path', lambda p=None: tmp_path if p is None else Path(p))
+
+    data_points = [
+        {"timestamp": "2024-06-01T12:00:00Z", "temperature": 22.5, "signal_noise_ratio": 35.1},
+        {"timestamp": "2024-06-01T12:05:00Z", "temperature": 22.7, "signal_noise_ratio": 34.8}
+    ]
+
+    for dp in data_points:
+        core.log_data(dp['timestamp'], dp['temperature'], dp['signal_noise_ratio'])
+
+    assert log_file.exists(), 'Log file should be created by log_data()'
+
+    with open(log_file, 'r', encoding='utf-8') as f:
+        content = f.read().strip()
+
+    assert content, 'Log file must not be empty.'
+
+    # Interpret as JSON lines
+    lines = content.splitlines()
+    parsed = [json.loads(l) for l in lines]
+
+    assert all(isinstance(p, dict) for p in parsed)
+
+    expected_keys = {'timestamp', 'temperature', 'signal_noise_ratio'}
+    for p in parsed:
+        assert expected_keys.issubset(p.keys())
+        assert isinstance(p['timestamp'], str)
+        assert isinstance(p['temperature'], (float, int))
+        assert isinstance(p['signal_noise_ratio'], (float, int))
+
+    # Check append behavior
+    assert len(parsed) == len(data_points)
+
+
+def test_log_data_handles_invalid_input(tmp_path):
+    log_file = tmp_path / 'log_data.json'
+
+    sys.path.insert(0, str(Path(__file__).resolve().parents[1] / 'src'))
+
+    # Expect that invalid temperature type raises an error
+    with pytest.raises((TypeError, ValueError)):
+        core.log_data('2024-06-01T12:00:00Z', 'invalid', 34.2)
+
+    assert not log_file.exists(), 'Invalid input should not create a file.'
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