EVAL-ADF7021DB2 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-ADF7021DB2' PDF : 44 Pages View PDF
ADF7021
VOLTAGE CONTROLLED OSCILLATOR (VCO)
To minimize spurious emissions, the on-chip VCO operates
from 1724 MHz to 1880 MHz. The VCO signal is then divided
by 2 to give the required frequency for the transmitter and the
required LO frequency for the receiver.
The VCO should be recentered, depending on the required
frequency of operation, by programming the VCO adjust bits
R1_DB[23:24]. The VCO is enabled by Bit R1_DB17.
A further frequency divide-by-2 is included after the PLL to
allow operation from 432 MHz to 470 MHz using the internal
VCO inductor and 80 MHz to 325 MHz using the external VCO
inductor. This divide-by-2 is enabled by setting R1_DB18 to 1.
The VCO needs an external 22 nF capacitor between the VCO
and the regulator 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 12. Voltage Controlled Oscillator (VCO)
TO PA
The ADF7021 on-chip VCO can use either an internal or
external tank inductor. The possible frequency ranges are given
in Table 5.
Table 5. RF Output Frequency Ranges for Internal/External
VCO Inductor and Divide-by-2 Enabled/Disabled
RF Frequency
Output (MHz)
Int/Ext VCO Inductor Divide-by-2
R1_DB25
R1_DB18
862 to 940
0
0
431 to 470
0
1
135 to 650
1
0
80 to 325
1
1
Internal VCO Inductor
To select the internal VCO inductor, set R1_DB25 to Logic 0,
which is the default setting.
VCO bias current can be adjusted using Bits R1_DB[19:22]. To
ensure VCO oscillation, the minimum bias current setting under
all conditions when using the internal VCO inductor is 0xA.
The VCO should be recentered, depending on the required
frequency of operation, by programming the VCO adjust bits
R1_DB[23:24].
Preliminary Technical Data
Using the External VCO Inductor
When using an external inductor, 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. A plot of the VCO operating frequency vs. total
external inductance (chip inductor + PCB track) is shown in
Figure 13. The inductance for a PCB track using FR4 material is
approximately 0.57 nH/mm. This should be subtracted from the
total value to determine the correct chip inductor value.
750
700
650
600
FMAX (MHz)
550
500
450
400
350
300
FMIN (MHz)
250
200
0
5
10
15
20
25
30
TOTAL EXTERNAL INDUCTANCE (nH)
Figure 13. VCO Operating Frequency vs. Total External Inductance
VCO bias current can be adjusted using Bits R1_DB[19:22]. To
minimize current consumption and to ensure reliable
operation, the bias current setting should be as indicated in
Table 6.
Table 6. Recommended VCO Bias Currents for External
VCO Inductor
RF Frequency Output (f)
VCO Bias R1_DB[19:22]
f < 200 MHz
0001
200 MHz < f < 450 MHz
0010
f > 450 MHz
0011
CHOOSING CHANNELS FOR BEST SYSTEM
PERFORMANCE
The architecture of fractional-N results in some level of the
nearest integer channel moving through the loop to the RF
output. These beat-note spurs are not attenuated by the loop, if
the desired RF channel and the nearest integer channel are
separated by a frequency of less than the LBW. The occurrence
of beat-note spurs is rare because the integer frequencies are at
multiples of the reference, which is typically >10 MHz. Beat-
note spurs can be significantly reduced in amplitude by
avoiding very small or very large values in the fractional register
using the frequency doubler.
Rev. PrI | Page 14 of 44
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