AD8302 datasheet(19/24 Pages) AD

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REV. 0

AD8302

–19–

The comparator mode can be turned into a controller mode by

closing the loop around the VMAG and VPHS outputs. Figure 11

illustrates a closed loop controller that stabilizes the gain and phase

of a DUT with gain and phase adjustment elements. If VMAG and

VPHS are properly conditioned to drive gain and phase adjustment

blocks preceding the DUT, the actual gain and phase of the DUT

will be servoed toward the prescribed setpoint gain and phase given

in Equations 11 and 12. These are essentially AGC and APC

loops. Note that as with all control loops of this kind, loop dynam-

ics and appropriate interfaces all must be considered in more detail.

MAG

SETPOINT

PHASE

SETPOINT

VMAG

MSET

PSET

VPHS

INPA

INPB

MAG

AD8302

Figure 11. By applying overall feedback to a DUT via

external gain and phase adjusters, the AD8302 acts

as a controller.

APPLICATIONS

Measuring Amplifier Gain and Compression

The most fundamental application of AD8302 is the monitoring

of the gain and phase response of a functional circuit block such

as an amplifier or a mixer. As illustrated in Figure 12, direc-

tional couplers, DCB and DCA, sample the input and output

signals of the “Black Box” DUT. The attenuators ensure that

the signal levels presented to the AD8302 fall within its dynamic

range. From the discussion in the Dynamic Range section, the

optimal choice places both channels at POPT = –30 dBm refer-

enced to 50

Ω, which corresponds to –43 dBV. To achieve this,

the combination of coupling factor and attenuation are given by,

CB + LB = PIN – POPT

(15)

CA + LA = PIN + GAINNOM–POPT

(16)

where CB and CA are the coupling coefficients, LB and LA are the

attenuation factors and GAINNOM is the nominal DUT gain. If

identical couplers are used for both ports, then the difference in

the two attenuators compensates for the nominal DUT gain. When

the actual gain is nominal, the VMAG output is 900 mV, corresponding

to 0 dB. Variations from nominal gain appear as a deviation from

900 mV or 0 dB with a 30 mV/dB scaling. Depending on the nominal

insertion phase associated with DUT, the phase measurement may

require a fixed phase shift in series with one of the channels to

bring the nominal phase difference presented to the AD8302

near the optimal 90

° point.

When the insertion phase is nominal, the VPHS output is 900 mV.

Deviations from the nominal are reported with a 10 mV/degree

scaling. Table I gives suggested component values for the mea-

surement of an amplifier with a nominal gain of 10 dB and an

input power of –10 dBm.

ATTENA

DCA

ATTENB

DCB

“BLACK BOX”

COMM

MFLT 14

INPA

VMAG

213

OFSA

MSET

312

VPOS

VREF

411

OFSB

PSET

510

INPB

VPHS

COMM

PFLT

AD8302

Figure 12. Using the AD8302 to Measure the Gain and

Insertion Phase of an Amplifier or Mixer

Table I. Component Values for Measuring a 10 dB Amplifier

with an Input Power of –10 dBm

Component

Value

Quantity

R1, R2

52.3

Ω

R5, R6

100

Ω

C1, C4, C5, C6

0.001

µF4

C2, C8

Open

100 pF

0.1

µF1

AttenA

10 dB (See Text)

AttenB

1 dB (See Text)

DCA, DCB

20 dB

The gain measurement application can also monitor gain and

phase distortion in the form of AM-AM (gain compression) and

AM-PM conversion. In this case, the nominal gain and phase

corresponds to those at low input signal levels. As the input level

is increased, output compression and excess phase shifts are

measured as deviations from the low level case. Note that the signal

levels over which the input is swept must remain within the dynamic

range of the AD8302 for proper operation.

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