LTC5594

21

Rev A

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APPLICATIONS INFORMATION

used, the resulting IF slope will be 1dB/GHz. If a low side

LO (LSLO) is used, the resulting IF slope will be –1dB/

GHz.TheIFfiltercomponentvaluescanbeadjustedsothat

approximately 1dB of peaking or roll-off can be achieved

over the filter bandwidth to give an overall flat IF response

for the HSLO or LSLO case.

I-Channel and Q-Channel Outputs

The phase relationship between the I-channel output

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

LO input frequency is higher (or lower) than the RF input

frequency, the Q-channel outputs (IFQP, IFQM) lead (or

lag) the I-channel outputs (IFIP, IFIM) by 90°.

Figure 11 shows a simplified schematic of the IF amplifier

outputs. The current-mode outputs require a terminating

resistance to establish a common mode voltage level. The

optimum operating current is 18mA per output. A 50Ω

termination is recommended on each output for a 0.9V

common mode voltage (R5, R6). Operation at a higher

common mode voltage is possible with the addition of

a common mode termination. For example, to operate

at 1.8V, an additional common mode resistance of 25Ω

(R5 = 66.5Ω and R6 = 0Ω, or R5 = R6 = 43.2Ω) would

be used to maintain an output current of 18mA. Alterna-

tively, a 100Ω termination to ground on each output can

be used for a 1.8V common mode voltage with 6dB more

Figure 9. Simplified Schematic of the Mixer Output and IF Amplifier Input with Interstage Filter

Figure 10. Interstage IF Filter with Common Mode Termination

L2

L1

C11

C9

C12

C10

50

MIP

MIM

AIM

AIP

5594 F10

1pF

2k

42mA

42mA

1.5nH

1.5nH

0.2pF

0.2pF

L2

100

0.6pF

50

100

50

50

0.6pF

50

L1

C11

C9

C12

C10

LTC5594

AC CURRENT

SOURCE

PACKAGE

PARASITICS

1.5nH

1.5nH

0.2pF

0.2pF

PACKAGE

PARASITICS

GND

MIP

MIM

AIM

AIP

5594 F09

1pF