ADF4110/ADF4111/ADF4112/ADF4113
RF
OUT
100pF
18Ω
18Ω
VCO
100pF
18Ω
LOOP
FILTER
2
CP
FREF
IN
8
REF
IN
INPUT OUTPUT
ADF4111
ADF4112
ADF4113
GND
CE
CLK
DATA
LE
LOCK
DETECT
14
MUXOUT
1
R
SET
100pF
6
5
RF
RF
A
B
IN
2.7kΩ
51Ω
IN
100pF
AD5320
12-BIT
V-OUT DAC
POWER SUPPLY CONNECTIONS AND DECOUPLING
CAPACITORS ARE OMITTED FOR CLARITY.
SPI COMPATIBLE SERIAL BUS
Figure 34. Driving the RSET Pin with a D/A Converter
a tuning range as wide as an octave. For example, cable TV
tuners have a total range of about 400 MHz. Figure 36 shows an
application where the ADF4113 is used to control and program
the Micronetics M3500-2235. The loop filter was designed for
an RF output of 2900 MHz, a loop bandwidth of 40 kHz, a PFD
frequency of 1 MHz, ICP of 10 mA (2.5 mA synthesizer ICP
multiplied by the gain factor of 4), VCO KD of 90 MHz/V
(sensitivity of the M3500-2235 at an output of 2900 MHz), and
a phase margin of 45°C.
USING A D/A CONVERTER TO DRIVE THE RSET PIN
A D/A converter can be used to drive the RSET pin of the
ADF4110 family, thus increasing the level of control over the
charge pump current, ICP. This can be advantageous in wide-
band applications where the sensitivity of the VCO varies over
the tuning range. To compensate for this, the ICP may be varied
to maintain good phase margin and ensure loop stability. See
Figure 34.
SHUTDOWN CIRCUIT
In narrow-band applications, there is generally a small variation
in output frequency (generally less than 10ꢀ) and a small
variation in VCO sensitivity over the range (typically 10ꢀ to
15ꢀ). However, in wideband applications, both of these
parameters have a much greater variation. In Figure 36, for
example, there is a −25ꢀ and +17ꢀ variation in the RF output
from the nominal 2.9 GHz. The sensitivity of the VCO can vary
from 120 MHz/V at 2750 MHz to 75 MHz/V at 3400 MHz
(+33ꢀ, −17ꢀ). Variations in these parameters change the loop
bandwidth. This in turn can affect stability and lock time. By
changing the programmable ICP, it is possible to get compensa-
tion for these varying loop conditions and ensure that the loop
is always operating close to optimal conditions.
The attached circuit in Figure 35 shows how to shut down both
the ADF4110 family and the accompanying VCO. The ADG701
switch goes closed circuit when a Logic 1 is applied to the IN
input. The low cost switch is available in both SOT-23 and
MSOP packages.
WIDEBAND PLL
Many of the wireless applications for synthesizers and VCOs in
PLLs are narrow band in nature. These applications include the
various wireless standards like GSM, DSC1800, CDMA, and
WCDMA. In each of these cases, the total tuning range for the
local oscillator is less than 100 MHz. However, there are also
wideband applications for which the local oscillator could have
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