diff --git a/NanoVNASaver/SITools.py b/NanoVNASaver/SITools.py
index aab140d..12e86d4 100644
--- a/NanoVNASaver/SITools.py
+++ b/NanoVNASaver/SITools.py
@@ -33,6 +33,8 @@ class Format(NamedTuple):
     allow_strip: bool = False
     parse_sloppy_unit: bool = False
     parse_sloppy_kilo: bool = False
+    parse_clamp_min: float = -math.inf
+    parse_clamp_max: float = math.inf
 
 
 class Value:
@@ -66,7 +68,7 @@ class Value:
         elif offset > fmt.max_offset:
             offset = fmt.max_offset
 
-        real = self._value / (10 ** (offset * 3))
+        real = float(self._value) / (10 ** (offset * 3))
 
         if fmt.max_nr_digits < 4:
             formstr = ".0f"
@@ -111,7 +113,17 @@ class Value:
         elif self.fmt.assume_infinity and value == "-\N{INFINITY}":
             self._value = -math.inf
         else:
-            self._value = decimal.Decimal(value, context=Value.CTX) * factor
+            try:
+                self._value = (decimal.Decimal(value, context=Value.CTX) *
+                               decimal.Decimal(factor, context=Value.CTX))
+            except decimal.InvalidOperation:
+                raise ValueError
+            # TODO: get formating out of RFTools to be able to import clamp
+            #       and reuse code
+            if self._value < self.fmt.parse_clamp_min:
+                self._value = self.fmt.parse_clamp_min
+            elif self._value > self.fmt.parse_clamp_max:
+                self._value = self.fmt.parse_clamp_max
         return float(self._value)
 
     @property
diff --git a/test/test_parseFrequency.py b/test/test_parseFrequency.py
index 8a2e631..860b20b 100644
--- a/test/test_parseFrequency.py
+++ b/test/test_parseFrequency.py
@@ -1,4 +1,5 @@
-#  NanoVNASaver - a python program to view and export Touchstone data from a NanoVNA
+#  NanoVNASaver
+#  A python program to view and export Touchstone data from a NanoVNA
 #  Copyright (C) 2019.  Rune B. Broberg
 #
 #  This program is free software: you can redistribute it and/or modify
@@ -14,13 +15,20 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-import sys
 import unittest
 
 # Import targets to be tested
-from NanoVNASaver import RFTools
-rft = RFTools.RFTools()
+from NanoVNASaver.RFTools import RFTools
+rft = RFTools()
 
+
+# TODO: should be tested against SITools.Value
+# RFTools.parseFrequency will hopefully go away in future
+# and be specialised by input field and device, like
+# parse_clamp_min=50000 for sweep input with
+# a nanovna version 1 attached ...
+# the hardware developer already announced a successor
+# which will have different limits
 class TestCases(unittest.TestCase):
 
     def test_basicSIUnits(self):
@@ -58,7 +66,7 @@ class TestCases(unittest.TestCase):
         # Current behavior: unusual SI values that are legal, but inappropriate
         # for this application provide unexpected outputs. This behavior is
         # based on the FULL set of SI prefixes defined in SITools (below).
-        #PREFIXES = ("y", "z", "a", "f", "p", "n", "µ", "m",
+        # PREFIXES = ("y", "z", "a", "f", "p", "n", "µ", "m",
         #            "", "k", "M", "G", "T", "P", "E", "Z", "Y")
         #######################################################################
         self.assertEqual(rft.parseFrequency('123EHz'), 123000000000000000000)