EVAL-ADF7021-NDBEZ Analog Devices Inc, EVAL-ADF7021-NDBEZ Datasheet - Page 24

• Manufacturer Part Number: EVAL-ADF7021-NDBEZ
• Description: 426MHz To 429 MHz, External L
• Manufacturer: Analog Devices Inc
• Type: Transceiver, FSKr

Specifications of EVAL-ADF7021-NDBEZ

• Frequency: 420MHz ~ 440MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: ADF7021-N
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: Q5835434A

ADF7021-N
VOLTAGE CONTROLLED OSCILLATOR (VCO)
The ADF7021-N contains two VCO cores. The first VCO, the
internal inductor VCO, uses an internal LC tank and supports
842 MHz to 916 MHz and 421 MHz to 458 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 421 MHz to 458 MHz band (internal inductor VCO) and
the 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 pin) to reduce internal noise.
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 recentered, 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.
LOOP FILTER
R1_DB(19:22)
VCO_BIAS
CVCO PIN
Figure 37. Voltage Controlled Oscillator (VCO)
220µF
VCO
N DIVIDER
÷2
TO
÷2
RF_DIVIDE_BY_2
R1_DB18
MUX
TO PA
Rev. 0 | Page 24 of 64
A plot of the VCO operating frequency vs. total external
inductance (chip inductor + PCB track) is shown in Figure 38.
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-N 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 should be set depending on the frequency of
operation (as indicated in Table 9).
750
700
650
600
550
500
450
400
350
300
250
200
Figure 38. Direct RF Output vs. Total External Inductance
0
f
MIN
5
TOTAL EXTERNAL INDUCTANCE (nH)
(MHz)
f
MAX
(MHz)
10
15
20
25
30