LTC5585

28

5585fb

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IM2 Suppression Example

IM2 adjustment circuitry can be used in a typical trans-

ceiver loop-back application as shown in Figure 27. In

this example a 2-tone SSB training source of f1 = 20MHz

and f2 = 21MHz is generated in DSP and upconverted

by the LTC5588-1 quadrature modulator to RF tones at

1970MHz and 1971MHz using an LO source at 1950MHz.

A narrowband RF filter is required to remove the IM2

component generated by the LTC5588-1. During the

loopback test these RF tones are routed through high

isolation switches and an attenuation pad to the LTC5585

demodulator input. The tones are then downconverted by

the same LO source at 1950MHz to produce two tones

at the baseband outputs of 20MHz and 21MHz plus an

IM2 impairment signal at 1MHz. After baseband chan-

nel filtering and amplification the output of the ADC is

filtered by a 1MHz bandpass filter in DSP to isolate the

IM2 tone. The power in this tone is calculated in DSP and

then a 1-D minimization algorithm is applied to calculate

the correction signal for the IP2I control voltage pin. The

1-D minimization algorithm can be implemented in many

ways such as golden-section search, backtracking or

Newton’s method.

Enable Interface

A simplified schematic of the EN pin is shown in Figure 28.

TheenablevoltagenecessarytoturnontheLTC5585is2V.

To disable or turn off the chip, this voltage should be below

0.3V. If the EN pin is not connected, the chip is disabled.

APPLICATIONS INFORMATION

Figure 27. Block Diagram for a Direct Conversion Transceiver with IM2 Adjustment. Only the I-Channel Is Shown

Figure 28. Simplified Schematic of the EN Pin Interface

Figure 29. Simplified Schematic of the EDC Pin Interface

RMS

DETECTION

1MHz BPF

DSP

DAC

ADC

IP2I

f1 = 20MHz

f2 = 21MHz

5585 F27

LNA

LTC5585

LOOPBACK

LTC5588-1

1-D

MINIMIZATION

ALGORITHM

DAC

PA

+

100k

EN

LTC5585

GND

5585 F28

100k

EN

100k

100k

EDC

LTC5585

5585 F29

GND

Figures 29 and 30 show the simplified schematics for the

EDC and EIP2 pins