Level Diagram - Analog Devices AD6122 Manual

AD6122
R1
MODOPP
Z R2
IN
R1
MODOPN
Figure 28. Pad Topology
 
1
 
R 1
=
L 20 log
 
1 1
 
+
 
R 1 R 2 2 /
= +
Z 2 1 R R 2
IN
where L is the transducer loss (or loss through the pad) in dB
and Z is the desired input resistance in ohms. Using these
IN
equations, we can design the attenuator circuit to provide what-
ever amount of attenuation we require.
MODULATORS
I
500mV p-p
DIFFERENTIAL
2
LO
100mV p-p
DIFFERENTIAL
Q
500mV p-p
DIFFERENTIAL
AD6122
IFINP
RSHUNT >>R2
IFINN
(3)
(4)
–21dBm
(REFERRED TO 1k )
252.1mV p-p DIFFERENTIAL
MODOP
VCC
20dB
ATTENUATOR
Z = 1k
IN
Figure 29. Level Diagram
This circuit is very sensitive to parasitic capacitances. As a re-
sult, extra care should be taken to ensure minimum and equal
printed circuit board transmission lines. We should also try to
keep R2 small in order to minimize the effects of printed circuit
board parasitic capacitance on loading the output of the pad.
In conclusion, we have to develop a system-level ACPR budget
for our radio, and from that budget determine how much ACPR
performance we desire from the AD6122. We then need to imple-
ment the appropriate attenuation network to get that ACPR
performance.

LEVEL DIAGRAM

Figure 29 is provided to better understand the different voltage
levels you can expect to see at different points of the AD6122. It
represents the voltage and power levels expected for a maximum
input condition of 500 mV p-p at the I and Q modulator and
maximum gain in the IF amplifiers. When trying to make these
measurements, a high impedance (10 MΩ) active FET probe
(for example, the Tek P6204, from Tektronix) should be used to
minimize the effects of loading the circuit with the probe.
In order to produce these results, the attenuator is designed to
have a 1 kΩ input impedance and the output of the IF amplifiers
are loaded with 1 kΩ. The roofing filter is designed to resonate
the parasitic capacitance at the IF frequency.
–41dBm
(REFERRED TO 1k )
25.21mV p-p
IF AMPLIFIERS
DIFFERENTIAL
IFIN
VGAIN = 2.5V
GAIN = +34dB
–14–
–7dBm
(REFERRED TO 1k )
1.263V p-p DIFFERENTIAL
TRANSMIT
OUTPUT
1k
Z = 1k
OUT
REV. B