Datasheet

ADF4360-8
Rev. A | Page 23 of 24
OUTPUT MATCHING
There are a number of ways to match the output of the
ADF4360-8 for optimum operation; the most basic is to use a
50 Ω resistor to V
VCO
. A dc bypass capacitor of 100 pF is
connected in series, as shown in Figure 27. Because the resistor
is not frequency dependent, this provides a good broadband
match. The output power in the circuit below typically gives
−9 dBm output power into a 50 Ω load.
100pF
04763-030
RF
OUT
V
VCO
50Ω
51Ω
Figure 27. Simple ADF4360-8 Output Stage
A better solution is to use a shunt inductor (acting as an RF
choke) to V
VCO.
This gives a better match and, therefore, more
output power.
Experiments have shown that the circuit shown in Figure 28
provides an excellent match to 50 Ω over the operating range of
the ADF4360-8. This gives approximately 0 dBm output power
across the specific frequency range of the ADF4360-8 using the
recommended shunt inductor, followed by a 100 pF dc blocking
capacitor.
L
100pF
04763-031
RF
OUT
V
VCO
50Ω
Figure 28. Optimum ADF4360-8 Output Stage
The recommended value of this inductor changes with the VCO
center frequency. A graph of the optimum inductor value vs.
frequency is shown in Figure 29.
CENTRE FREQUENCY (MHz)
INDUCTANCE (nH)
300
250
150
200
100
0
50
0 100 200 300 5000400
04763-032
Figure 29. Optimum ADF4360-8 Shunt Inductor
Both complementary architectures can be examined using the
EVAL-ADF4360-8EB1 evaluation board. If the user does not
need the differential outputs available on the ADF4360-8, the
user should either terminate the unused output or combine
both outputs using a balun. Alternatively, instead of the LC
balun, both outputs may be combined using a 180° rat-race
coupler.