LT5517 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT5517

40MHz to 900MHz Quadrature Demodulator

Linear Technology 

LT5517

APPLICATIO S I FOR ATIO

approach is to terminate each output with a shunt capaci-

tor. The capacitor value can be optimized depending upon

the operating frequency and the specific PCB layout.

The phase relationship between the I-channel output sig-

nal and the Q-channel output signal is fixed. When the LO

input frequency is higher than the RF input frequency, then

the Q-channel outputs (QOUT+, QOUT–) lead the I-channel

outputs (IOUT+, IOUT–) by 90°.

When the LO input frequency is lower than the RF input

frequency, then the Q-channel outputs lag the I-channel

outputs by 90°. Note that the phase relationship of the I-

and Q-channel outputs relative to the LO can vary by 180°,

depending on start-up conditions. This is the nature of a

frequency divider-based quadrature phase generator.

When AC output coupling is used, the resulting highpass

filter’s –3dB roll-off frequency is defined by the R-C

constant of the blocking capacitor and RLOAD, assuming

RLOAD > 600Ω.

Care should be taken when the demodulator’s outputs are

DC coupled to the external load to make sure that the I/Q

mixers are biased properly. If the current drain from the

outputs exceeds 6mA, there can be significant degrada-

tion of the linearity performance. Each output can sink no

more than 13mA when connected to an external load with

a DC voltage higher than VCC – 0.78V.

VCC

60Ω 60Ω 60Ω 60Ω

10pF

10pF

IOUT+ 16

IOUT–

15

QOUT+

14

QOUT–

13

5517 F07

Figure 7. I/Q Output Equivalent Circuit

5517f

10

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