AD8364-EVAL-500 Analog Devices, AD8364-EVAL-500 Datasheet - Page 29

• Manufacturer Part Number: AD8364-EVAL-500
• Description: LF to 2.7GHz, Dual 60dB TruPwr™ Detector; Package: EVALUATION BOARDS; No of Pins: -; Temperature Range: Commercial
• Manufacturer: Analog Devices

GAIN-STABLE TRANSMITTER/RECEIVER
There are many applications for a transmitter or receiver with a
highly accurate temperature-stable gain. For example, a
multicarrier basestation high power amplifier (HPA) using
digital predistortion has a power detector and an auxiliary
receiver. The power detector and all parts associated with it can
be removed if the auxiliary receiver has a highly accurate
temperature-stable gain. With a set gain receiver, the ADC on
the auxiliary receiver can not only determine the overall power
being transmitted but can also determine the power in each
carrier for a multicarrier HPA.
In controller mode, the AD8364 can be used to hold the
receiver gain constant over a broad input power/temperature
range. In this application, the difference outputs are used to
hold the receiver gain constant.
The RF input is connected to INPA, using a 19.1 dB coupler,
and the down converted output from our signal chain is
connected to INPB, using a 10.78 dB coupler. A 0.1 µF capacitor
is connected between FBKA and OUTP, forming an integrator.
OUTA is connected to VLVL, forcing OUTP to adjust the VGA
so that OUTB is equal to OUTA. The circuit gain is set by the
difference in the coupling values of the input and output
couplers. As noted, OUTP is used to drive the gain control of
the ADL5330 by adjusting the gain up or down as needed to
force the power at the AD8364 inputs to be equal in amplitude.
Since operating at different frequencies, the appropriate voltages
on the ADJ[A, B] pins must be supplied. Because INPA is
operating at 1880 MHz, ADJA is set to 0.75 V. Likewise, because
INPB is operating at 80 MHz, ADJB is set to 0 V.
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Because the difference in the coupler values is 8.32 dB, a fixed
gain of −8.32 dB is expected. In practice, there is a gain of
−13 dB. This is caused by the intercept shift of the AD8364 due
to its frequency response, the insertion loss of the output
coupler, and the insertion loss differences of the baluns used on
the input of the AD8364. In this configuration, approximately
33 dB of control range with 0.5 dB drift over temperature is
obtained.
Figure 66 shows a gain-stable receiver amplifier circuit using
the AD8364 to control an ADL5330 VGA and the AD8343
mixer. The input power was swept from +3 dBm to −35 dBm,
the output power was measured, and the gain was calculated at
multiple temperatures between −40°C and +85°C. Note that the
gain changed less than ±0.45 dB over this range (Figure 65).
Most of the gain change was caused by performance differences
at different frequencies.
–11.0
–11.5
–12.0
–12.5
–13.0
–13.5
–14.0
–40
Figure 65. Performance of Gain-Stable Receiver
–35
–30
–25
–20
P
IN
(dBm)
–15
+25°C
+85°C
–40°C
–10
–5
0
AD8364
5