LTC5585_15 Datasheet, PDF (27/38 Pages) Linear Technology – Wideband IQ Demodulator with IIP2 and DC Offset Control

LTC5585
APPLICATIONS INFORMATION
to adjust the IM2 level. The IM2 level can be effectively
minimized over a large range of the baseband bandwidth.
The circuitry has an effective baseband frequency upper
limit of about 200MHz. Any IM2 component that falls in this
frequency range can be minimized. Beyond this frequency,
the gain of the IM2 correction amplifier falls off appreciably
and the circuit no longer improves IP2 performance. The
lower baseband frequency limit of the IM2 adjustment
circuitry is set by the common mode reference decoupling
capacitor at the CMI and CMQ pins. Below this frequency
the circuit can not minimize the IM2 component.
Figure 24 shows the CMI (and identical CMQ) pin interface.
These pins have an internal 40pF decoupling capacitance
to V_CC, to provide a reference for the IP2 adjustment cir-
cuitry. The lower 3dB frequency limit, f_C, of the circuitry
is set by the following equation:
f_C = [2π • 500(40pF + C_CM(EXT))]^–1
Without any external capacitor on the CMI or CMQ pin the
lower limit is 8MHz. By adding a 0.1μF capacitor, C_CM(EXT),
between the CMI and CMQ pins to V_CAP, the lower –3dB
frequency corner can be reduced to 3kHz. Figure 25 shows
IIP2 as a function of RF frequency spacing versus common
mode decoupling capacitance values of 0.1µF and 1500pF.
There is effectively no limit on the size of this capacitor,
other than the impact it has on enable time for the IM2
circuitry to be operational. When the chip is disabled, there
is no current in the I or Q mixers, so the common mode
V_CAP
CMI OR CMQ
V_CC
LTC5585
40pF
5585 F24
GND
Figure 24. Equivalent Circuit of the CMI and CMQ Pin Interfaces
output voltage will be equal to V_CC (if no DC common mode
current is being drawn by external baseband circuitry such
as a baseband amplifier). When the chip is enabled, the
off-chip common mode decoupling capacitor must charge
up through a 500Ω resistor. The time constant for this
is essentially 500Ω times the common mode decoupling
capacitance value. For example, with a 0.01µF capacitor
this wait time is approximately 30μs. Figure 26 shows
the pulsed enable response of the common-mode output
voltage with 0.01µF on the CMI and CMQ pins.
130
0.1µF (UNCALIBRATED)
120
0.1µF (NULLED IP2I = 0.1V)
1500pF (UNCALIBRATED)
110
1500pF (NULLED IP2I = 0.15V)
100 T_C = 25°C
90
f_RF1 = 2150MHz
f_LO = 2100MHz
80
70
60
50
40
30
0.01
0.1
1
10
RF FREQUENCY SPACING (MHz)
5585 F25
Figure 25. IIP2 vs Common Mode Decoupling Capacitance
8
10
T_C = 25°C
C_CMI,Q = 0.01µF
7 _EN
PULSE
EN PULSE ON
5
OFF
6
0
5
–5
CMI, CMQ
4
–10
BASEBAND OUTPUTS
3
–15
0 10 20 30 40 50 60 70 80 90 100
TIME (µs)
5585 F26
Figure 26. Common Mode Output Voltage with a Pulsed Enable
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