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AD6122ACP

CDMA 3 V Transmitter IF Subsystem with Integrated Voltage Regulator

Analog Devices 

AD6122

FROM EXTERNAL

VOLTAGE REGULATOR

AD6122

LDOE

LDOB

LDOC

REFOUT

1.23V

Figure 26. Configuration for Bypassing the Low Dropout

Regulator

ROOFING FILTER

Because the outputs of the AD6122 modulator are open collec-

tor, the parasitic capacitances seen at the output of the modula-

tor, and inputs of the IF amplifiers, are high enough to create a

low-pass filter, which may attenuate the IF signal. Consequently,

the parasitic capacitance must be cancelled by using external

inductors to form a parallel resonant circuit. The external in-

ductors and the internal parasitic capacitors form what is known

as the roofing filter, with the resonant frequency given by

Equation 2.

f0 =

1

2 π LCPAR

(2)

where f0 is the IF frequency, in Hertz, CPAR is the total parasitic

capacitance in Farads, and L is the value of external inductors,

in henrys.

The roofing filter may be composed of the pull-up inductors

required on the open collector outputs of the I and Q modula-

tor. This configuration is shown in Figure 27. The 10 nF ca-

pacitors are used for ac coupling.

AD6122

MODOPP

2CPAR

L/2 VCC

2CPAR

MODOPN

IFINN

L/2

10nF

PARALLEL

RESONANT

CIRCUIT

10nF

IFINP

10nF ATTENUATOR

Figure 27. Roofing Filter Configuration

The attenuator is discussed in the next section entitled Measur-

ing Adjacent Channel Protection Ratio (ACPR).

In order to confirm whether the roofing filter has been correctly

designed, sweep the LO frequency and view the output of the IF

amplifier on a spectrum analyzer. The signal should peak at the

IF frequency if the inductor value is correct. The Q of the filter

should be low enough so that variations in the parasitic capaci-

tances should be negligible.

The value of inductor required will be a function of the IF fre-

quency at which we are operating. The values of inductors used

during characterization at Analog Devices are shown in Table

II. Because the exact value will also be a function of printed

circuit board layout, we will have to vary the value from those in

Table II to those required for our board.

Table II. Roofing Filter Inductor Values

IF Frequency (MHz)

50–125

126–200

201–275

276–350

Value of Roofing Filter

Inductor (nH)

470

150

68

27

It should be noted that the roofing filter is only required when

cascading the output from the I/Q modulator to the input of the

IF amplifiers. If we are driving into the IF amplifiers directly, no

roofing filter is required, however, pull-up inductors are required

in order to set the dc voltage of the open collector modulator

outputs.

MEASURING ADJACENT CHANNEL POWER RATIO

(ACPR)

At maximum IF gain and specified input conditions (500 mV

p-p baseband inputs), the output of the I/Q modulator is 11 dB

greater than the P1 dB (one dB compression point) of the IF

amplifiers. This configuration maximizes the ratio of signal to

LO feedthrough and also maximizes the signal to noise ratio.

Once these ratios are maximized, we can attenuate the noise,

signal and LO feedthrough without affecting the ratios. There-

fore, attenuation is required between the I/Q modulator and the

IF amplifiers.

In order to determine exactly how much attenuation is required,

we must recognize that ACPR is a function of the attenuation

from the modulator outputs to the IF amplifier inputs. As a

result, in order to determine how much attenuation is required,

we must first know how good an ACPR performance is desired.

If too much attenuation is applied, the ACPR will be very good,

but, the IF amplifier’s output power level will be low, possibly

resulting in poor signal to noise ratio and possibly requiring

additional amplification external to the AD6122.

An appropriate method that can be used to provide the correct

amount of attenuation between the modulator outputs and the

IF amplifier inputs is a simple differential voltage divider. The

topology and its design equations are shown in Figure 28 and

Equations 3 and 4. The input impedance of the IF amplifiers is

typically 1 kΩ. As a result, if we design resistor R2 to be much

less than 1 kΩ, we can neglect the effects of the IF amplifier’s

input impedance on the attenuator.

REV. B

–13–

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