LT5517

10

5517f

Figure 7. I/Q Output Equivalent Circuit

APPLICATIO S I FOR ATIO

15

16

10pF

+

–

5517 F07

13

14

10pF

+

–

60

Ω

60

Ω

60

Ω

60

Ω

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

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

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