EVAL-ADF7021DBZ5 Datasheet, PDF (22/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Data Sheet
FREQUENCY SYNTHESIZER
REFERENCE INPUT
The on-board crystal oscillator circuitry (see Figure 31) can
use a quartz crystal as the PLL reference. Using a quartz crystal
with a frequency tolerance of ≤10 ppm for narrow-band appli-
cations is recommended. It is possible to use a quartz crystal
with >10 ppm tolerance, but to comply with the absolute
frequency error specifications of narrow-band regulations
(for example, ARIB STD-T67 and ETSI EN 300-220), compen-
sation for the frequency error of the crystal is necessary.
The oscillator circuit is enabled by setting R1_DB12 high. It is
enabled by default on power-up and is disabled by bringing CE
low. Errors in the crystal can be corrected by using the automatic
frequency control feature or by adjusting the fractional-N value
(see the N Counter section).
OSC1
OSC2
CP2
CP1
Figure 31. Oscillator Circuit on the ADF7021
Two parallel resonant capacitors are required for oscillation at
the correct frequency. Their values are dependent upon the
crystal specification. They should be chosen to make sure that
the series value of capacitance added to the PCB track capacitance
adds up to the specified load capacitance of the crystal, usually
12 pF to 20 pF. Track capacitance values vary from 2 pF to 5 pF,
depending on board layout. When possible, choose capacitors
that have a very low temperature coefficient to ensure stable
frequency operation over all conditions.
Using a TCXO Reference
A single-ended reference (TCXO, VCXO, or OCXO) can also be
used with the ADF7021. This is recommended for applications
having absolute frequency accuracy requirements of <10 ppm, such
as ARIB STD-T67 or ETSI EN 300-220. There are two options
for interfacing the ADF7021 to an external reference oscillator.
• An oscillator with CMOS output levels can be applied to
OSC2. The internal oscillator circuit should be disabled by
setting R1_DB12 low.
• An oscillator with 0.8 V p-p levels can be ac-coupled through
a 22 pF capacitor into OSC1. The internal oscillator circuit
should be enabled by setting R1_DB12 high.
Programmable Crystal Bias Current
Bias current in the oscillator circuit can be configured
between 20 µA and 35 µA by writing to the XTAL_BIAS bits
(R1_DB[13:14]). Increasing the bias current allows the crystal
oscillator to power up faster.
CLKOUT Divider and Buffer
The CLKOUT circuit takes the reference clock signal from the
oscillator section, shown in Figure 31, and supplies a divided-
down, 50:50 mark-space signal to the CLKOUT pin. The CLKOUT
signal is inverted with respect to the reference clock. An even
divide from 2 to 30 is available. This divide number is set in
R1_DB[7:10]. On power-up, the CLKOUT defaults to divide-by-8.
DV_DD
CLKOUT
ENABLE BIT
OSC1
DIVIDER
÷2
1 TO 15
CLKOUT
Figure 32. CLKOUT Stage
To disable CLKOUT, set the divide number to 0. The output
buffer can drive up to a 20 pF load with a 10% rise time at
4.8 MHz. Faster edges can result in some spurious feedthrough
to the output. A series resistor (1 kΩ) can be used to slow the
clock edges to reduce these spurs at the CLKOUT frequency.
R Counter
The 3-bit R counter divides the reference input frequency by an
integer of 1 to 7. The divided-down signal is presented as the
reference clock to the phase frequency detector (PFD). The divide
ratio is set in R1_DB[4:6]. Maximizing the PFD frequency reduces
the N value. This reduces the noise multiplied at a rate of 20 log(N)
to the output and reduces occurrences of spurious components.
Register 1 defaults to R = 1 on power-up.
PFD [Hz] = XTAL/R
Loop Filter
The loop filter integrates the current pulses from the charge
pump to form a voltage that tunes the output of the VCO to the
desired frequency. It also attenuates spurious levels generated by
the PLL. A typical loop filter design is shown in Figure 33.
CHARGE
PUMP OUT
VCO
Figure 33. Typical Loop Filter Configuration
The loop should be designed so that the loop bandwidth (LBW)
is approximately 100 kHz. This provides a good compromise
between in-band phase noise and out-of-band spurious rejection.
Widening the LBW excessively reduces the time spent jumping
between frequencies, but it can cause insufficient spurious attenua-
tion. Narrow-loop bandwidths can result in the loop taking long
periods to attain lock and can also result in a higher level of power
falling into the adjacent channel. The loop filter design on the
EVAL-ADF7021DBX should be used for optimum performance.
Rev. B | Page 22 of 64