AD8362
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
2.0
ADJUSTING VTGT TO ACCOMMODATE SIGNALS
WITH VERY HIGH CREST FACTORS
VOUT CW
VOUT 64QAM
VOUT WCDMA TM1-64
VOUT QPSK
1.5
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.
VOUT 256QAM
1.0
0.5
0
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.
–0.5
–1.0
–1.5
–2.0
ERROR QPSK 4dB CF
ERROR 256QAM 8.2dB CF
ERROR CW
ERROR 64QAM 7.7dB CF
ERROR WCDMA TM1-64 10.6dB CF
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.
–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
0
5
10
PIN (dBm)
Figure 54. Transfer Function and Law Conformance for Signals with
Varying Crest Factors, VTGT = 0.625 V, CLPF = 0.1 ꢀF
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.
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.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
2.0
VOUT CW
VOUT 64QAM
VOUT WCDMA TM1-64
VOUT QPSK
1.5
ALTERING THE SLOPE
VOUT 256QAM
1.0
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.
0.5
0
–0.5
–1.0
–1.5
–2.0
ERROR QPSK 4dB CF
ERROR 256QAM 8.2dB CF
ERROR CW
ERROR 64QAM 7.7dB CF
ERROR WCDMA TM1-64 10.6dB CF
AD8362
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
COMM ACOM
CHPF
DECL
INHI
VREF
VTGT
VPOS
VOUT
VSET
–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
0
5
10
PIN (dBm)
Figure 53. Transfer Function and Law Conformance for Signals with
Varying Crest Factors, VTGT = 1.25 V
V
INLO
DECL
OUT
R1
R2
PWDN ACOM
COMM CLPF
Figure 55. External Network to Raise Slope
Rev. D | Page 22 of 32
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