LT5527

13

5527fa

APPLICATIONS INFORMATION

LL

RR

CC

RR

L

X

IF

OUT

IF

IF

IF

OUT

IF

IF

12

67

1

3

,

•

,

••

=

=

=

ω

ω

ω

Compared to the lowpass/4:1 transformer matching tech-

nique, this network delivers approximately 0.8dB higher

conversion gain (since the IF transformer loss is elimi-

nated) and comparable noise figure and IIP3. At a ±15%

offset from the IF center frequency, conversion gain and

noise figure degrade about 1dB. Beyond ±15%, conver-

sion gain decreases gradually but noise figure increases

rapidly. IIP3 is less sensitive to bandwidth. Other than IF

bandwidth, the most significant difference is LO-IF leakage,

which degrades to approximately –38dBm compared to

the superior performance realized with the lowpass/4:1

transformer matching.

Discrete IF balun element values for four common IF fre-

quencies are listed in Table 5. The corresponding measured

IF output return losses are shown in Figure 11. The values

listed in Table 5 differ from the calculated values slightly

due to circuit board and component parasitics. Typical

conversion gain, IIP3 and LO-IF leakage, versus RF input

frequency, for all four IF frequency examples is shown in

Figure 12. Typical conversion gain, IIP3 and noise figure

versus IF output frequency for the same circuits are shown

in Figure 13.

IF FREQUENCY

(MHz)

L1, L2

(nH)

C6, C7

(pF)

L3

(nH)

190

120

6.8

220

240

100

4.7

220

380

56

3

68

450

47

2.7

47

For fully differential IF architectures, the IF transformer can

be eliminated. An example is shown in Figure 14, where

the mixer’s IF output is matched directly into a SAW filter.

Supply voltage to the mixer’s IF pins is applied through

Figure 11. IF Output Return Losses with Discrete Balun Matching

Figure 12. Conversion Gain, IIP3 and LO-IF Leakage vs RF Input

Frequency Using Discrete IF Balun Matching

Figure 13. Conversion Gain, IIP3 and SSB NF vs IF Output

Frequency Using Discrete IF Balun Matching

IF FREQUENCY (MHz)

–30

–20

–10

0

–25

–15

–5

150

250

350

450

5527 F11

550

100

50

200

300

400

500

190MHz

240MHz

380MHz

450MHz

RF INPUT FREQUENCY (MHz)

1700

14

18

22

26

2500

5527 F12

10

6

12

16

20

24

8

4

2

–30

–20

–10

0

–40

–50

–60

1900

2100

2300

2700

190IF

240IF

380IF

450IF

LOW SIDE LO (–3dBm)

IIP3

LO-IF

IF OUTPUT FREQUENCY (MHz)

150

12

18

20

550

5527 F13

10

8

0

250

350

450

200

300

400

500

4

26

24

22

IIP3

16

14

6

2

190IF

240IF

380IF

450IF

LOW SIDE LO (–3dBm)

SSB NF