AD8302 datasheet(19/24 Pages) AD

19 / 24 page

REV. A

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 forced 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 dynamics 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, directional

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 referenced to 50

Ω,

which corresponds to –43 dBV. To achieve this, the combination

of coupling factor and attenuation are given by:

OPT

−

(15)

(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

measurement 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.

GAIN

NOM

OPT

−

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