Output Side Transformer - Stanford Research Systems SIM954 Operation And Service Manual

3.4 Transformers

3.4.3 Output Side Transformer

SIM954 300 MHz Dual Inverting Driver Amplifier
R4
50
V1
SINE(0 1 1e6)
Figure 3.7: 180 power splitter circuit
circuit topology which adds isolation between the two output ports
without sacrificing any signal power.
In the properly terminated balanced circuit the center tap node is
a virtual ground, and no current will flow through resistor R1. If
power is reflected into the transformer by the load on either of the
splitter outputs (i.e., in this case the SIM954 inputs), some of it will
be transfered to this load resistor and will not be visible at the other
output port (although some power will still make it to the input port
because the circuit does not have perfect isolation between all ports).
It should be noted that the SIM954 requires an isolated power splitter
in di erential driver applications. High frequency oscillations have been
observed with some non-isolated splitters.
It is convenient to approximate the necessary
multiples of 7 : 5 or 10 : 7 turn ratios. The resulting matching
errors are small and can usually be neglected. And while they do
result in a non-ideal isolation characteristics of the circuit, this can
(at least theoretically) be reduced by lowering the resistance of the
internal isolation resistor from 25
implementation the di erence is likely going to be lost in errors
caused by component tolerances and stray impedances.
These phase splitters are commercially available from many sources
(e.g., Mini-Circuits), but suitable transformers can also be easily
made from toroidal RF cores.
The more interesting and challenging case is operating the SIM954
with an output transformer. Care must be taken that no DC compo-
nents are present on the circuit's output when driving a transfomer
directly. This can either be achieved with a DC block like a series
capacitor, or by means of an input transformer.
SIM954
180 Degree Power Splitter
100µH
L2
49µH
L1
R1
49µH
L3
SIM954
24.5
.tran 1e-5
K123 L1 L2 L3 1
--- C:\Program Files\LTC\SwCADIII\SIM954PowerSplitter.asc ---
to 24.5 . However, in a typical
3 – 9
R2
50
R3
50
2 turn ratios with