Voltage Controlled Oscillator (Vco); Choosing A Vco For Best System Performance - Analog Devices ADF7021-V Manual

VOLTAGE CONTROLLED OSCILLATOR (VCO)

To minimize feedthrough and spurious emissions, the external
VCO must be chosen to operate at a minimum of twice the
required RF frequency. The VCO frequency is divided by 2 inside
the synthesizer loop, providing the required frequency for the
transmitter and for the local oscillator (LO) of the receiver. For
improved phase noise performance, an additional divide-by-2
can be enabled by setting the RF_DIVIDE_BY_2 bit (Bit DB18)
in Register 1.
As an example, for 80 MHz operation, a 160 MHz external VCO
could be used with the RF_DIVIDE_BY_2 bit disabled, or a
320 MHz VCO could be used with the RF_DIVIDE_BY_2 bit
enabled to support operation in the 80 MHz band. Assuming
that both VCOs have similar phase noise performance, the
320 MHz design using the additional divide-by-2 should result
in improved transmit ACP, as well as improved ACR, blocking,
and image rejection in the receiver.
The maximum VCO frequency of operation supported on the
ADF7021-V is 1920 MHz, which results in a maximum RF
channel frequency of 960 MHz using a 2× VCO or 480 MHz
using a 4× VCO.
REF
÷R PFD/CP
TCXO/XTAL
ADF7021-V
Figure 37. Voltage Controlled Oscillator (VCO)
EXTERNAL
COMPONENTS
LOOP
VCO
FILTER
÷2
MUX
÷N
÷2
SYNTH
FREQUENCY
The VCO tuning voltage can be checked for a particular RF
output frequency by measuring the voltage on the CPOUT pin
when the part is fully powered up in transmit or receive mode.
The VCO tuning range of the external VCO must be 0.2 V to 2 V.
The input impedance of the L2 pin is programmable and can
be selected to have a high impedance value or 50 Ω impedance,
depending on the VCO selected. The impedance of this pin can
be set using the BUFFER_IMPEDANCE bit (Bit DB17) in
Register 1.
CHOOSING A VCO FOR BEST SYSTEM
PERFORMANCE
The interaction between the RF VCO frequency and the refer-
ence frequency can lead to fractional spur creation. When the
synthesizer is in fractional mode (that is, the RF VCO and
reference frequencies are not integer related), spurs can appear
on the VCO output spectrum at an offset frequency that
corresponds to the difference frequency between an integer
multiple of the reference and the VCO frequency.
These spurs are attenuated by the loop filter. They are more
noticeable on channels close to integer multiples of the reference
where the difference frequency may be inside the loop bandwidth
(thus, the name integer boundary spurs). The occurrence of
these spurs is rare because the integer frequencies are around
multiples of the reference, which is typically >10 MHz. To avoid
having very small or very large values in the fractional register,
choose a suitable reference frequency.
In addition to spurious considerations, the selection of a high
TO
performance VCO with very low phase noise is essential to
PA
minimize the ACP performance of the transmitter and to
maximize the ACR and blocking resilience of the receiver.
Rev. 0 | Page 23 of 60
ADF7021-V