ADRF6655
REGISTER 7—EXTERNAL VCO CONTROL
With R6[2:0] set to 111, the external VCO control register is
programmed as shown in Figure 77.
The external VCO enable bit allows the use of an external VCO in
the PLL instead of the internal VCO. This can be advantageous in
cases where the internal VCO is not capable of providing the desired
frequency, or where the internal phase noise of the VCO is higher
than desired. By setting the external VCO enable bit (DB22) to 1,
and setting Bit DB15 to Bit DB10 of Register 6 to 0, the internal
VCO is disabled and the output of an external VCO can be fed into
the part differentially on Pin 38 and Pin 37 (LOP and LON).
Because the loop filter is already external, the output of the loop
filter simply needs to be connected to the external, tuning voltage
pin of the VCO. See the Using an External VCO section for more
information.
EXTERNAL
RES
VCO
ENABLE
RESERVED
CONTROL BITS
DB23 DB22 DB21 DB20 DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
0
XVCO
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 C3(1) C2(1) C1(1)
XVCO EXTERNAL VCO ENABLE
0
INTERNAL VCO
1
EXTERNAL VCO
Figure 77. External VCO Control Register (R7)
Rev. 0 | Page 33 of 44