13

LT5557

5557fa

APPLICATIO S I FOR ATIO

These equations give a good starting point, but it is usually

necessary to adjust the component values after building

and testing the circuit. The final solution can be achieved

with less iteration by considering the parasitics of L3 in the

above calculations. Specifically, the effective parallel re-

sistance of L3 (calculated from the manufacturer’s Q data)

calculated values of L1 (=L2) and C6 (=C7). Also, the

effective parallel capacitance of L3 (taken from the manu-

facturers SRF data) must be considered, since it is in

value for L1 does not fall on a standard value for the

desired IF. In this case, a simple solution is to load the IF

output with a high-value external chip resistor in parallel

standard value.

Discrete IF balun element values for four common IF

frequencies (190MHz, 240MHz, 360MHz and 450MHz)

are listed in Table 4. The 190MHz application circuit uses

a 3.3k

Ω resistor in parallel with L3 as described above.

The corresponding measured IF output return losses are

shown in Figure 10. Typical conversion gain, IIP3 and LO-

IF leakage, versus RF input frequency, for all four ex-

amples is shown in Figure 11. Typical conversion gain,

IIP3 and noise figure versus IF output frequency is shown

in Figure 12.

Compared to the transformer-based IF matching tech-

nique, this network delivers approximately 1dB higher

conversion gain (since the IF transformer loss is elimi-

nated), though noise figure and IIP3 are degraded slightly.

The most significant performance difference, as shown in

Figure 12, is the limited IF bandwidth available from the

discrete approach. For low IF frequencies, the absolute

bandwidth is small, whereas higher IF frequencies offer

wider bandwidth.

IF FREQUENCY

(MHz)

L1, L2

C6, C7

L3

190

120nH

6.0pF

270nH || 3.3k

Ω

240

100nH

4.7pF

150nH

360

56nH

3.0pF

82nH

450

47nH

2.2pF

47nH

Figure 10. IF Output Return Losses with Discrete Balun Matching

Figure 11. Conversion Gain, IIP3 and LO-IF Leakage

vs RF Input Frequency and IF Output Frequency

(in MHz) Using Discrete IF Balun Matching

Figure 12. Conversion Gain, IIP3 and SSB NF vs IF Output

Frequency Using Discrete IF Balun Matching

IF FREQUENCY (MHz)

50

–30

–20

–10

0

150

250

350

450

5557 F10

550

190 MHz

240 MHz

360 MHz

450 MHz

IF OUTPUT FREQUENCY (MHz)

150

10

22

24

26

250

350

400

5557 F12

6

18

14

8

20

2

4

16

12

200

300

450

500

RF = 1950MHz

LOW-SIDE LO (–3dBm)

SSB NF

IIP3

190IF

240IF

360IF

450IF

RF INPUT FREQUENCY (MHz)

1700

8

22

24

26

1900

2100

5557 F11

4

18

14

12

6

20

2

16

10

–60

–10

–30

–40

–20

–70

–50

1800

2000

2200

LOW-SIDE LO (–3dBm)

LO-IF

IIP3

190IF

240IF

360IF

450IF