10 • BX-245
© Box 73 Amateurfunkservice GmbH 2019
Power Switch-on
After switching-on power on the FA-VA5,
a start-up message appears on the display,
which includes the firmware version and
the current battery voltage. The factory
default setting of the menu language is
English
. The language can be changed to
German
by making following button
presses:
Long 2-second press on Left Button:
→
Operating Mode
→
Centre button: press 7
¥
→
Setup
→
1
¥
→
Language
→
2
¥
→
German
→
left button
→
.
n
Basic Calibration
(SOL Compensation)
Each additional connector and cable af-
fects the accuracy of the impedance mea-
surement on the device under test (DUT).
However, these unwanted errors can be
fully calibrated out by the
Short Open
Load
method (abbreviated to SOL method).
Three known calibration elements are
measured instead of the test object. The
Short element
applies a short circuit over
the BNC socket, the
Open
simulates an
open cable end and the
Load element
pro-
vides a resistance equal to the system
impedance (in our case 50 Ω). During cal-
ibration, the software of the FA-VA5
“measures” the reflections of a short cir-
cuit (0 Ohm = start of measurement scale)
and of an open circuit (Infinity Ohms =
end of measurement scale) as well as the
reference value Z = 50 Ω at the input (=
midpoint of scale). These values are the
three defined points of reflection. Due to
the physical properties (tolerances) of real
components, no absolutely exact values
will prevail at the input hence the devia-
tion must be detected and later eliminated
from the measurement result. The process
of acquisition is called “Calibration”.
Without calibration, the measurement re-
sult would be subject to a relatively fixed,
recurring and frequency-dependent error.
During the master calibration, the actual
input impedance is automatically mea-
sured at intervals of 100 kHz or 1 MHz for
all three reflection defined cases, and the
calculated correction data is stored perma-
nently in memory. The microcontroller
software retrieves and applies appropriate
correction factors later whenever calculat-
ing the measurement results. If the mea-
suring frequency is between two calibra-
tion points, then interpolation of the
correction is performed.
Three Calibration elements with accept-
able accuracy up to about 100 MHz can be
easily made by yourself using three BNC
coaxial cable plugs (50 Ω). The
Short
,
Open
and
Load elements
supplied with
the FA-VA5 will provide acceptable accu-
racy up to about 100 MHz. You can easily
make your own using three BNC coaxial
cable plugs (50 Ω). To make the
Short
cal-
ibration
element
, the inner conductor and
plug housing are short-circuited, whilst in
the
Open
calibration
element
the pin of
the inner remains unconnected (see attach-
ment) and the
Load element
has a low-
impedance 50 Ω SMD metal layer resistor
soldered between the inner conductor and
the housing of the plug.
High Quality BNC calibration option
An optional high-quality BNC calibration
set, which can be used up to 600 MHz is
available from [1]. The load element of
this calibration set is individually mea-
sured for highest accuracy and the mea-
sured data is provided in a printed instruc-
tion leaflet. This data is then entered in the
FA-VA5 Setup menu (see section Calibra-
tion Set Data).
The data supplied for the load element
consists of: picosecond delay, ohms resis-
tance, femtofarad parasitic capacitance,
and nanohenry parasitic inductance.
In addition the delay values for
Short
and
Open elements
should be entered: this is
always – 41.66 ps for the open element
and always –113.52 ps for the
Short ele-
ment
.
