EVAL-ADF7021DBZ5 Datasheet, PDF (24/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Data Sheet
VOLTAGE CONTROLLED OSCILLATOR (VCO)
The ADF7021 contains two VCO cores. The first VCO, the
internal inductor VCO, uses an internal LC tank and supports
862 MHz to 950 MHz and 431 MHz to 475 MHz operating
bands. The second VCO, the external inductor VCO, uses an
external inductor as part of its LC tank and supports the RF
operating band of 80 MHz to 650 MHz.
To minimize spurious emissions, both VCOs operate at twice
the RF frequency. The VCO signal is then divided by 2 inside
the synthesizer loop, giving the required frequency for the
transmitter and the required local oscillator (LO) frequency for
the receiver. A further divide-by-2 (RF_DIVIDE_BY_2) is
performed outside the synthesizer loop to allow operation in
the 431 MHz to 475 MHz band (internal inductor VCO) and
80 MHz to 325 MHz band (external inductor VCO).
The VCO needs an external 22 nF capacitor between the CVCO
pin and the regulator (CREG1) to reduce internal noise.
VCO BIAS
R1_DB(19:22)
LOOP FILTER VCO
÷2
220µF
MUX
÷2
CVCO PIN
TO
N DIVIDER
DIVIDE-BY-2
R1_DB18
Figure 36. Voltage Controlled Oscillator (VCO)
TO PA
Internal Inductor VCO
To select the internal inductor VCO, set R1_DB25 to Logic 0,
which is the default setting.
VCO bias current can be adjusted using R1_DB[19:22]. To
ensure VCO oscillation, the minimum bias current setting under
all conditions when using the internal inductor VCO is 0x8.
The VCO should be re-centered, depending on the required
frequency of operation, by programming the VCO_ADJUST
bits (R1_DB[23:24]). This is detailed in Table 9.
External Inductor VCO
When using the external inductor VCO, the center frequency of the
VCO is set by the internal varactor capacitance and the combined
inductance of the external chip inductor, bond wire, and PCB track.
The external inductor is connected between the L2 and L1 pins.
A plot of the VCO operating frequency vs. total external
inductance (chip inductor + PCB track) is shown in Figure 37.
750
700
650
600
f_MAX (MHz)
550
500
450
400
350
300
f_MIN (MHz)
250
200
0
5
10
15
20
25
30
TOTAL EXTERNAL INDUCTANCE (nH)
Figure 37. Direct RF Output vs. Total External Inductance
The inductance for a PCB track using FR4 material is approxi-
mately 0.57 nH/mm. This should be subtracted from the total
value to determine the correct chip inductor value.
Typically, a particular inductor value allows the ADF7021 to
function over a range of ±6% of the RF operating frequency.
When the RF_DIVIDE_BY_2 bit (R1_DB18) is selected, this
range becomes ±3%. At 400 MHz, for example, an operating
range of ±24 MHz (that is, 376 MHz to 424 MHz) with a single
inductor (VCO range centered at 400 MHz) can be expected.
The VCO tuning voltage can be checked for a particular RF
output frequency by measuring the voltage on the VCOIN pin
when the part is fully powered up in transmit or receive mode.
The VCO tuning range is 0.2 V to 2 V. The external inductor
value should be chosen to ensure that the VCO is operating
as close as possible to the center of this tuning range. This is
particularly important for RF frequencies <200 MHz, where
the VCO gain is reduced and a tuning range of <±6 MHz exists.
The VCO operating frequency range can be adjusted by
programming the VCO_ADJUST bits (R1_DB[23:24]). This
typically allows the VCO operating range to be shifted up or
down by a maximum of 1% of the RF frequency.
To select the external inductor VCO, set R1_DB25 to Logic 1.
The VCO_BIAS_CURRENT should be set depending on the
frequency of operation (as indicated in Table 9).
Table 9. RF Output Frequency Ranges for Internal and External Inductor VCOs and Required Register Settings
Register Settings
RF Frequency
Output (MHz)
VCO to
Be Used
RF Divide (VCO_INDUCTOR) (RF_DIVIDE_BY_2)
by 2
R1_DB25
R1_DB18
(VCO_ADJUST)
R1_DB[23:24]
(VCO_BIAS)
R1_DB[19:22]
900 to 950
Internal L No
0
0
11
8
862 to 900
Internal L No
0
0
00
8
450 to 470
Internal L Yes
0
1
11
8
431 to 450
Internal L Yes
0
1
00
8
450 to 650
External L No
1
0
XX
4
200 to 450
External L No
1
0
XX
3
80 to 200
External L Yes
1
1
XX
2
Rev. B | Page 24 of 64