ADRF6750 Analog Devices, Inc., ADRF6750 Datasheet - Page 19

• Manufacturer Part Number: ADRF6750
• Description: 950 Mhz To 1575 Mhz Quadrature Modulator With Integrated Fractional-n Pll And Vco
• Manufacturer: Analog Devices, Inc.

Lock Detect (LDET)
LDET (Pin 44) signals when the PLL has achieved lock to an
error frequency of less than 100 Hz. On a write to Register CR0,
a new PLL acquisition cycle starts, and the LDET signal goes
low. When lock has been achieved, this signal returns high.
Voltage-Controlled Oscillator (VCO)
The VCO core in the ADRF6750 consists of two separate VCOs,
each with 16 overlapping bands. Figure 56 shows an acquisition
plot demonstrating both the VCO overlap at roughly 1260 MHz
and the multiple overlapping bands within each VCO. The
choice of two 16-band VCOs allows a wide frequency range to
be covered without a large VCO sensitivity (K
poor phase noise and spurious performance. Note that the VCO
range is larger than the 2× LO frequency range of the part to
ensure that the device has enough margin to cover the full
frequency range over all conditions.
The correct VCO and band are chosen automatically by the
VCO and band select circuitry when Register CR0 is updated.
This is referred to as autocalibration.
+IN
–IN
HI
2.5
2.3
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5
HI
800
D2
D1
900
CLR2
CLR1
U1
U2
Figure 55. PFD Simplified Schematic
Q1
Q2
Figure 56. V
1000
DOWN
UP
DELAY
1100
LO FREQUENCY (MHz)
TUNE
1200
vs. LO Frequency
U3
1300
1400
CHARGE
PUMP
1500
VCO
) and resultant
1600
1700
CP
Rev. 0 | Page 19 of 40
The autocalibration time is set to 50 μs. During this time, the
VCO V
and is connected to an internal reference voltage. A typical
frequency acquisition is shown in Figure 57.
After autocalibration, normal PLL action resumes and the
correct frequency is acquired to within a frequency error of
100 Hz in 170 μs typically.
For a maximum cumulative step of 100 kHz, autocalibration
can be turned off by Register CR24, Bit 0. This enables cumu-
lative PLL acquisitions of 100 kHz or less to occur without the
autocalibration procedure, which improves acquisition times
significantly (see Figure 58).
The VCO displays a variation of K
the band and from band to band. Figure 59 shows how the
K
is the average value for each of the frequency bands. Figure 59
is useful when calculating the loop filter bandwidth and
individual loop filter components.
VCO
Figure 58. PLL Acquisition Without Autocalibration for 100 kHz Step
varies across the full LO frequency range. Also shown
100M
100k
100k
10M
10k
100
10k
100
1M
1G
TUNE
1k
10
1k
10
0
0
AUTOCAL
TIME (µs)
is disconnected from the output of the loop filter
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Figure 57. PLL Acquisition
50
ACQUISITION TO 100Hz
TIME (µs)
TIME (µs)
100
VCO
ACQUISITION TO 100Hz
as V
TUNE
150
varies within
ADRF6750
200