LTC6948 datasheet(20/32 Pages) AD

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LTC5594

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 8 shows the high band LO input return loss for

various input TL1 transmission line lengths for LO input

frequencies from 6GHz to 9GHz. Return loss greater than

10dB can be achieved by using a 0.2pF capacitor at Cx on

the input 50Ω transmission line. The high band LO input

return loss is listed in Table 3 with no capacitor at Cx and

BAND, CF1, LF1, and CF2 registers set to 1,0,0,0.

Interstage Filter

An interstage IF filter should be used between the MIP

(MIM) and AIP (AIM) pins and the MQP (MQM) and AQP

(AQM) pins to suppress the large fRF + fLO and other mix-

ing products from the mixer outputs. Without the filter, the

linearity of the amplifier can be degraded for the desired

signal. Figure 9 shows a recommended lowpass filter.

Table 4 shows typical values used for a lowpass response

of various bandwidths.

Table 4. Component Values for Interstage Lowpass Filter

1dB BW (MHz)

L1, L2 (nH)

C9, C11 (pF)

C10, C12 (pF)

330

120

150

100

300

4.7

6.8

500

3.0

1000

8.2

1.0

3.0

It is important that the placement of C10 and C12 be

as close as possible to the amplifier inputs. Long line

lengths on the amplifier inputs can lead to instability. As

shown in Figure 10, a 50Ω common mode termination

resistance can be used to better ensure stability with long

line lengths and/or higher order filtering. The placement

of C9 and C11 should be as close as possible to the mixer

outputs for effective filtering of the 2xLO, fRF + fLO, and

other mixing products.

By adjusting the values of the capacitors in the filter, it

is possible to add or remove frequency slope of the IF

response. The RF input has a frequency slope above 2GHz

of approximately –1dB/GHz. If a high side LO (HSLO) is

Table 3. High Band LO Input Impedance

FREQUENCY

(MHz)

INPUT IMPEDANCE (Ω)

S11

MAG

ANGLE (°)

5000

87.2 + j37.9

0.436

46.4

5500

121.5 + j32.1

0.503

27.8

6000

132.1 + j5.4

0.453

4.8

6500

95.2 – j17.7

0.347

–23.8

7000

73.5 – j39.5

0.406

–58.2

7500

69.0 – j47.5

0.467

–66.0

8000

77.8 – j51.9

0.531

–59.9

8500

77.8 – j38.8

0.361

–75.4

9000

58.8 – j38.8

0.402

–122.6

Figure 7. Single-Ended LO Input Return Loss

vs BAND, CF1, LF1, and CF2

Figure 8. High Band LO Input Return Loss vs

Input Transmission Line Length Lt

9 10

–5

–10

–15

–20

–25

–30

–35

LO FREQUENCY (GHz)

Lt = 3mm

Lt = 5mm

Lt = 8mm

Lt = 12.5mm

TC = 25°C

5594 F08

–5

–10

–15

–20

–25

–30

–35

LO FREQUENCY (GHz)

0, 31, 3, 31

0, 17, 2, 31

0, 14, 1, 27

1, 21, 3, 28

1, 15, 1, 31

1, 2, 1, 10

1, 1, 0, 19

1, 0, 0, 0

TC = 25°C

EN = 0V

5594 F07

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