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AD8362

Data Sheet

Rev. E | Page 22 of 32

ADJUSTING VTGT TO ACCOMMODATE SIGNALS

WITH VERY HIGH CREST FACTORS

An external direct connection between VREF (1.25 V) and VTGT

sets up the internal target voltage, which is the rms voltage that

must be provided by the VGA to balance the AGC feedback loop.

In the default scheme, the VREF of 1.25 V positions this target

to 0.06 × 1.25 V = 75 mV. In principle, however, VTGT can be

driven by voltages that are larger or smaller than 75 mV. This

technique can be used to move the intercept, which increases

or decreases the input sensitivity of the device, or to improve

the accuracy when measuring signals with large crest factors.

For example, if this pin is supplied from VREF via a simple

resistive attenuator of 1 kΩ:1 kΩ, the output required from the

VGA is halved to 37.5 mV rms. Under these conditions, the

effective headroom in the signal path that drives the squaring

cell is doubled. In principle, this doubles the peak crest factor

that can be handled by the system.

Figure 53 and Figure 54 show the effect of varying VTGT on

measurement accuracy when the AD8362 is swept with a series

of signals with different crest factors, varying from CW with a

crest factor of 3 dB, to a W-CDMA carrier (Test Model 1-64)

with a crest factor of 10.6 dB. The crest factors of each signal

are listed in the plots. In Figure 53, VTGT is set to its nominal

value of 1.25 V, while in Figure 54, it is reduced to 0.625 V.

PIN (dBm)

–2.0

–65

2.0

1.5

1.0

0.5

–0.5

–1.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

–1.5

–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

VOUT CW

VOUT 64QAM

VOUT WCDMA TM1-64

VOUT QPSK

VOUT 256QAM

ERROR QPSK 4dB CF

ERROR 256QAM 8.2dB CF

ERROR CW

ERROR 64QAM 7.7dB CF

ERROR WCDMA TM1-64 10.6dB CF

Figure 53. Transfer Function and Law Conformance for Signals with

Varying Crest Factors, VTGT = 1.25 V

PIN (dBm)

–2.0

–65

2.0

1.5

1.0

0.5

–0.5

–1.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

–1.5

–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

VOUT CW

VOUT 64QAM

VOUT WCDMA TM1-64

VOUT QPSK

VOUT 256QAM

ERROR QPSK 4dB CF

ERROR 256QAM 8.2dB CF

ERROR CW

ERROR 64QAM 7.7dB CF

ERROR WCDMA TM1-64 10.6dB CF

Figure 54. Transfer Function and Law Conformance for Signals with

Varying Crest Factors, VTGT = 0.625 V, CLPF = 0.1 μF

Reducing VTGT also reduces the intercept. More significant in

this case, however, is the behavior of the error curves. Note that

in Figure 54 all of the error curves sit on one another, while in

Figure 53, there is some vertical spreading. This suggests that

VTGT should be reduced in those applications where a wide

range of input crest factors are expected. As noted, VTGT can

also be increased above its nominal level of 1.25 V. While this

can be used to increase the intercept, it would have the undesir-

able effect of degrading measurement accuracy in situations

where the crest factor of the signal being measured varies

significantly.

ALTERING THE SLOPE

None of the changes in operating conditions discussed so far

affects the logarithmic slope (VSLP) in Equation 10. This can

readily be altered by controlling the fraction of VOUT that is

fed back to the setpoint interface at the VSET pin. When the

full signal from VOUT is applied to VSET, the slope assumes

its nominal value of 50 mV/dB. It can be increased by including

a voltage divider between these pins, as shown in Figure 55.

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

AD8362

VOUT

Figure 55. External Network to Raise Slope

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