LT5578

13

5578f

TYPICAL APPLICATIONS

The following examples illustrate the implementation and

performance of the LT5578 in some selected applications.

These circuits were evaluated using the board layout

shown in Figure 12.

450MHz Application

In this case, the LT5578 was evaluated for an application

with an IF input at 70MHz, an RF output of 450MHz and

a high side LO. The LO port is tuned for high side LO in-

jection at 520MHz. The matching networks for the three

ports are shown in Figure 8.

At the IF input, the 560pF capacitors are used mainly as

DC blocks, but also help tune out the parasitic inductance

of the transformer. The 82pF differential capacitor and

3.3nH chip inductors provide an impedance transforma-

tion between the IF input pins and the transformer. The

relatively low input frequency requires the use of chip

inductors instead of the short transmission lines that are

shown in Figure 2. The measured IF port return loss is

included in Figure 3.

The RF port impedance match is realized with a shunt 12pF

capacitor and a series 18nH inductor. The return loss with

this configuration is better than 12dB from about 430MHz

to 505MHz and is plotted in Figure 7.

To tune the LO port, a series 6.8pF and shunt 4.7pF ca-

pacitor are used as shown. This combination provides a

10dB, or better, return loss from 435MHz to 580MHz as

shown in Figure 5. The series capacitor also provides DC

decoupling for the internal transformer at the LO input.

Figure 9 shows measured conversion gain, noise figure

and OIP3 as a function of RF output frequency. At 450MHz,

the gain is –2.1dB with a NF of 9.3dB and an OIP3 of

23.8dBm.

82pF

6.8pF

12pF

560pF

13.7Ω

100nH

100nH

TC4-1W+

4:1

560pF

3.3nH

3.3nH

IF

70MHz

RF

450MHz

LO

520MHz

18nH

13.7Ω

5578 F08

4.7pF

Figure 8. Schematic for 450MHz RF Application with 70MHz IF and 520MHz LO

RF OUTPUT FREQUENCY (MHz)

420

4

6

8

500

5578 F09

2

0

–4

440

460

480

–2

12

10

24

26

28

22

20

16

18

32

SSB NF

OIP3

GAIN

30

Figure 9. Gain, Noise Figure and OIP3 vs RF Frequency

in the 450MHz Application

2600MHz Application

For this application, the impedance match of the RF port is

optimized at 2600MHz and has a good return loss over the

range of 2200MHz to 2900MHz. The component values are

listed in Table 5 and typical output return loss is shown in

Figure 7. The IF input is matched at 240MHz as described

in Table 2. The LO port requires no external matching for

this band as its return loss is good for frequencies above

1.5GHz.