MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
13
calibration plane are in series with it. The actual value (in pF) for most capacitors does increase with
frequency
and may reach infinity when the capacitive element and its stray inductance become series-
resonant
. This frequency is called the device's
Series Resonant Frequency
(where X=0). Bypass
capacitors are sometimes intentionally operated at or near this frequency, but for most applications, the
frequencies will be far below it. In addition to the display, the analyzer's
Impedance
meter displays the
reactance (X in ohms) of the capacitor.
4.5 Inductance (Function-4)
Access the
Inductance
mode by stepping the
Mode
switch to the
Inductance
identification screen. The
top line of the working display shows the
Frequency
in MHz and the
Inductive Reactance
(X
L
)
of the
DUT at that particular frequency. The lower line shows the
Inductance
in
uH
. Inductance is calculated
using the measured Reactance (X
L
)
and the VFO frequency.
Important Note:
Measurements become inaccurate below 7 ohms or above 650 ohms. If component
reactance falls into an inaccurate range, the error messages
L(X<7)
,
L(X=0)
or
L(Z>650
) will be
displayed.
Reactance Sign:
The MFJ-269D measures reactance (
X
) and mathematically converts it to an inductance
value
X
L
, but the processor can't actually determine if the reactance it measures is inductive or
capacitive. You can usually confirm the sign by adjusting the VFO. If tuning
down in frequency
decreases reactance
, the reactance is likely inductive (+jX) because inductors normally exhibit decreased
reactance with a decrease in frequency.
To measure an Inductor:
1.
Turn the analyzer on and step the
Mode
switch three times to bring up the
Inductance
identification
screen.
2.
Connect the inductor (DUT) across the
Antenna
connector using the shortest leads possible, or with
the lead length normally used in your working circuit to include stray inductance in the measurement.
3.
Adjust the
VFO
(Tune)
to your frequency of interest. If an error sign comes up, choose the closest
frequency where no warning appears. The
L(X=0)
warning indicates the inductor looks like a near
perfect short to the analyzer's bridge and the frequency is too low (or the inductor too small) to measure.
The digital display and the analog
Impedance
meter both present the DUT's reactance
(X)
in ohms.
When measuring an inductor, its displayed value will sometimes change with the test frequency. This
happens because of stray capacitance between coil windings and in the leads going to the
Antenna
connector. At RF, the value of an inductor (in uH) may appear substantially different from its "rated"
value that was determined at a lower frequency.
With increasing frequency, measured inductance usually
increases and, at some high frequency, the coil may become self-resonant and appear as an open circuit
(or a trap) with infinite reactance.
At some very low frequency, it may look like a short.
Inductance
in uH