ADRF6750ACPZ-R7 datasheet(20/40 Pages) AD

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ADRF6750

Rev. A | Page 20 of

LO FREQUENCY (MHz)

Figure 59. KVCO vs. LO Frequency

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.

VCO

V-TO-I

IBBP

IBBN

QBBP

QBBN

BALUN

RFOUT TO

ATTENUATOR

QUAD

PHASE

SPLITTER

÷2

Figure 60. Block Diagram of the Quadrature Modulator

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 AD9779 outputs is

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.

Ω

IBBP

IBBN

QBBN

QBBP

OUT1_P

OUT1_N

OUT2_N

OUT2_P

ADRF6750

CURRENT OUTPUT DAC

(EXAMPLE: AD9779)

Figure 61. Establishing DC Bias Level on Baseband Inputs

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.

Ω

100

Ω

100

Ω

IBBP

IBBN

QBBN

QBBP

OUT1_P

OUT1_N

OUT2_N

OUT2_P

ADRF6750

CURRENT OUTPUT DAC

(EXAMPLE: AD9779)

LOW-

PASS

FILTER

LOW-

PASS

FILTER

Figure 62. Typical Baseband Input 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.

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