ADRF6750 Analog Devices, Inc., ADRF6750 Datasheet - Page 20

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

ADRF6750
QUADRATURE MODULATOR
Overview
A basic block diagram of the ADRF6750 quadrature modulator
circuit is shown in Figure 60. The VCO generates a signal at the
2× LO frequency, which is then divided down to give a signal at the
LO frequency. This signal is then split into in-phase and quadrature
components to provide the LO signals that drive the mixers.
The I and Q baseband input signals are converted to currents by
the V-to-I stages, which then drive the two mixers. The outputs
of these mixers combine to feed the output balun, which provides a
single-ended output. This single-ended output is then fed to the
attenuator and, finally, to the external RFOUT signal pin.
Baseband Inputs
The baseband inputs, QBBP, QBBN, IBBP, and IBBN, must be
driven from a differential source. The nominal drive level of
0.9 V p-p differential (450 mV p-p on each pin) should be
biased to a common-mode level of 500 mV dc.
To set the dc bias level at the baseband inputs, refer to Figure 61.
The average output current on each of the
10 mA. A current of 10 mA flowing through each of the 50 Ω
resistors to ground produces the desired dc bias of 500 mV at
each of the baseband inputs.
ATTENUATOR
40
35
30
25
20
15
10
5
0
RFOUT TO
Figure 60. Block Diagram of the Quadrature Modulator
Figure 59. K
BALUN
LO FREQUENCY (MHz)
VCO
vs. LO Frequency
SPLITTER
PHASE
QUAD
AD9779
V-TO-I
V-TO-I
VCO
÷2
outputs is
IBBP
IBBN
QBBP
QBBN
Rev. 0 | Page 20 of 40
The differential baseband inputs (QBBP, QBBN, IBBN, and
IBBP) consist of the bases of PNP transistors, which present
a high impedance of about 30 kΩ in parallel with roughly 2 pF
of capacitance. The impedance looks like 30 kΩ below 1 MHz
and starts to roll off at higher frequency. A 100 Ω differential
termination is recommended at the baseband inputs, and this
dominates the input impedance as seen by the input baseband
signal. This ensures that the input impedance, as seen by the
input circuit, remains flat across the baseband bandwidth. See
Figure 62 for a typical configuration.
The swing of the AD9779 output currents ranges from 0 mA to
20 mA. The ac voltage swing is 1 V p-p single-ended or 2 V p-p
differential with the 50 Ω resistors in place. The 100 Ω differen-
tial termination resistors at the baseband inputs have the effect
of limiting this swing without changing the dc bias condition of
500 mV. The low-pass filter is used to filter the DAC outputs
and remove images when driving a modulator.
Another consideration is that the baseband inputs actually
source a current of 240 μA out of each of the four inputs. This
current must be taken into account when setting up the dc bias
of 500 mV. In the initial example based on Figure 61, an error
of 12 mV occurs due to the 240 μA current flowing through
the 50 Ω resistor. Analog Devices, Inc., recommends that the
accuracy of the dc bias should be 500 mV ±25 mV. It is also
important that this 240 μA current have a dc path to ground.
CURRENT OUTPUT DAC
CURRENT OUTPUT DAC
(EXAMPLE: AD9779)
(EXAMPLE: AD9779)
Figure 61. Establishing DC Bias Level on Baseband Inputs
Figure 62. Typical Baseband Input Configuration
OUT1_N
OUT2_N
OUT1_P
OUT2_P
OUT1_N
OUT2_N
OUT1_P
OUT2_P
50Ω
50Ω
50Ω
50Ω
50Ω
50Ω
50Ω
50Ω
FILTER
FILTER
LOW-
PASS
LOW-
PASS
100Ω
100Ω
IBBP
IBBN
QBBN
QBBP
ADRF6750
ADRF6750
IBBP
IBBN
QBBN
QBBP