Stanford Research Systems SIM954 Operation And Service Manual page 37

3.7 Typical Application: a High Voltage Isolated, Low Noise, DC-DC Converter
SIM954 300 MHz Dual Inverting Driver Amplifier
of the driver signal (200 m for 1 MHz on 50
perform best without impedance matching LC-circuits at one or both
ends.
The transformer's secondary winding is connected to a simple half-
wave rectifier made from fast switching diodes such as the 1N4148.
In order to achieve minimum switching noise, the SIM954 is used
to drive the circuit with a sinusoidal voltage rather than a square
wave (as in ordinary switching power supply circuits). This ensures
that there are no spectral components beside the main operating
frequency present at the output of the driver. After the transformer,
the switching of the rectifier diodes produces significant switching
transients which have to be filtered. 1nF capacitors in parallel with
the two diodes slow diode turn on and turn o times down. Slower
transients significantly reduce noise in comparison to conventional
converter circuits where ultrafast diodes are used to achive highest
possible converter e ciency.
The rectified current is filtered by a pair of 10 F ceramic capacitors
followed by two sets of beads and ceramic and Tantalum capacitors.
In this circuit, six-aperture through hole beads (Fair-Rite 6611 type
43) were used, but high impedance multi-layer surface mount beads
are preferable in applications which are very noise sensitive and have
to improve the performance of this demonstartion circuit. In general,
the lowest ESR (Equivalent Series Resistance) capacitors have to be
used. Multiple ceramic capacitors in parallel are much better than
a single capacitor with the same equivalent capacitance because the
parallel circuit reduces lead inductance and ESR. More capacitance to
suppress the fundamental frequency can be added using high quality
tantalum or organic electrolyte capacitors.
Multiple consecutive LC filter stages should be used for optimum
results, with the first stages using RF beads to suppress the high-
est freqency components first before rejecting the fundamental fre-
quency and lower harmonics in the later stages. Proper RF design
techniques and a ground plane are absolutely necessary to achieve
the shown results.
The residual switching noise of this design were mainly dependent
on wiring geometry and the size of the current loop outside of the
core. If lowest possible switching noise is critical, the magnetic fields
from the core and the current loops have to be shielded with suitable
RF shields. Use of tightly twisted wires to reduce magnetic coupling
is vital. Traces carrying AC currents should be kept short and be
routed above a ground plane or sandwiched between two ground
planes on inner layers.
If voltage regulation is necessary, low-drop-out voltage regulators
3 – 13
coax) will typically