DISCUSSION OF PERFORMANCE
THEORY OF OPERATION
The following refers to the schematic in Figure 1.
A1 and A2 are connected as a phase-shift oscillator
circuit with the frequency set by the external
capacitors C1 and C2. Q4 is included in the
feedback loop of A1 as a gain control element.
The oscillator output is fed to the chopper amplifier
which develops an absolute value representation of
the
oscillator
output.
The
chopper
output
is
compared to a precision DC reference in integrator
amplifier A3. This DC error signal is used ot control
the gain setting FET Q4.
As in all precision zener based DC references, the
drift of the zener becomes nonlinear at temperature
extremes. The chopper amplifier drift characteristic
is
complementary
to
this
nonlinearity
and
compensates for the reference drift.
APPLICATION INFORMATION
Figure 1 shows the connections for the SWR200
including the two frequency setting capacitors. The
frequency is:
The frequency stability is directly related to the
stability of the capacitors, therefore stable capacitors
like NPO ceramic, or polycarbonate or polystyrene
film should be used.
Two
separate
ground
pins
are
provided
for
accurate ground sensing. This minimizes errors due
to drops in the ground pin which can become a
significant source of error in sockets.
The offset of the SWR200 is fully specified for
initial offset and drift and is low enough that it can
normally be neglected. In applications which are
especially sensitive to offset the output can be AC
coupled. Proper capacitor sizing and high impedance
sensing will minimize errors due to capacitive
coupling.
f =
10
C
C
-5
SCHEMATIC
FIGURE 1
1
2
SWR200DS REV. D JUNE 1995
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