Datasheet
ADF4350
Rev. A | Page 13 of 32
OUTPUT STAGE
The VCO shows variation of K
V
as the V
TUNE
varies within the
band and from band-to-band. It has been shown for wideband
applications covering a wide frequency range (and changing
output dividers) that a value of 33 MHz/V provides the most
accurate K
V
as this is closest to an average value. Figure 21
shows how K
V
varies with fundamental VCO frequency along
with an average value for the frequency band. Users may prefer
this figure when using narrowband designs.
The RF
OUT
A+ and RF
OUT
A− pins of the ADF4350 are connected
to the collectors of an NPN differential pair driven by buffered
outputs of the VCO, as shown in Figure 22. To allow the user to
optimize the power dissipation vs. the output power requirements,
the tail current of the differential pair is programmable by
Bits [D2:D1] in Register 4 (R4). Four current levels may be set.
These levels give output power levels of −4 dBm, −1 dBm, +2
dBm, and +5 dBm, respectively, using a 50 Ω resistor to AV
DD
and ac coupling into a 50 Ω load. Alternatively, both outputs
can be combined in a 1 + 1:1 transformer or a 180° microstrip
coupler (see the Output Matching section). If the outputs are
used individually, the optimum output stage consists of a shunt
inductor to V
VCO
. The unused complementary output must
be terminated with a similar circuit to the used output.
80
70
60
50
40
30
20
10
0
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6
07325-133
VCO SENSITIVITY (MHz/V)
FREQUENCY (GHz)
An auxiliary output stage exists on Pins RF
OUT
B+ and RF
OUT
B−
providing a second set of differential outputs which can be
used to drive another circuit, or which can be powered down
if unused. The auxiliary output must be used in conjunction
with the main RF output. It cannot be used with the main
output powered down.
Another feature of the ADF4350 is that the supply current to
the RF output stage can be shut down until the part achieves
lock as measured by the digital lock detect circuitry. This is
enabled by the mute till lock detect (MTLD) bit in Register 4 (R4).
Figure 21. K
V
vs. Frequency
In fixed frequency applications, the ADF4350 V
TUNE
may
vary with ambient temperature switching from hot to cold.
In extreme cases, the drift causes V
TUNE
to drop to a very low
level (<0.25 V) and can cause loss of lock. This becomes an
issue only at fundamental VCO frequencies less than 2.95 GHz
and at ambient temperatures below 0°C.
VCO
RF
OUT
A+ RF
OUT
A
–
BUFFER/
DIVIDE-BY-
1/2/4/8/16
07325-010
In cases such as these, if the ambient temperature decreases
below 0°C, the frequency needs to be reprogrammed (R0 updated)
to avoid V
TUNE
dropping to a level close to 0 V. Reprogramming
the part chooses a more suitable VCO band, and thus avoids
the low V
TUNE
issue. Any further temperature drops of more
than 20°C (below 0°C) also require further reprogramming.
Any increases in the ambient temperature do not require repro-
gramming.
Figure 22. Output Stage