11

LT5521

5521f

APPLICATIO S I FOR ATIO

on the IF input frequency and the LO frequency. Smaller

C13 values have reduced impact on the LO output sup-

pression; larger values will degrade the conversion gain.

A single-ended 50

Ω source can also be matched to the

differential signal inputs of the LT5521 without an input

transformer. Figure 5 shows an example topology for a

discrete balun, and Table 4 lists component values for

several frequencies. The discrete input match is intrinsi-

cally narrowband. LO suppression to the output is de-

graded and noise figure degrades by 4dB for input

frequencies greater than 200MHz. Noise figure degrada-

tion is worse at lower input frequencies.

Operation at Reduced Supply Voltage

External resistors R1 and R7 (Figure 2) set the current

through the mixer core. For best distortion performance,

these resistors should be chosen to maintain a total of

40mA through the mixer core (20mA per side). At 5V

supply, R1 and R7 should be 110

Ω. Table 5 shows

recommended values for R1 and R7 at various supply

voltages. Caution: Using values below the recommended

resistance can adversely affect operation or damage the

part.

Table 5. Minimum External Resistor Values vs Supply Voltage

R1, R7 (

Ω)

5

110

4.5

82.5

4

54.9

3.5

38.3

3.3

23.2

Excessive mismatch between the external resistors R1

and R7 will degrade performance, particularly LO sup-

pression. Resistors with 1% mismatch are recommended

for optimum performance.

Figure 2 shows RF chokes in series with R1 and R7. These

inductors are optional. In general, the chokes improve the

conversion gain and noise figure by 2dB at 3.3V (i.e., at the

minimum values of R1 and R7). The DC resistance varia-

tion of the RF chokes must be considered in the 1% source

resistance mismatch suggested for maintaining LO sup-

pression performance.

Figure 6 indicates the typical performance of the LT5521

as the external source resistance (R1, R7) is varied while

keeping the supply current constant. Figure 6 data was

taken without the benefit of input chokes, and shows the

gradual gain degradation for smaller values of the input

resistors R1 and R7. Figure 7 shows the typical behavior

when the supply voltage is fixed and the core current is

varied by adjusting values of the external resistors R1 and

R7. Decreasing the core current decreases the power

consumption and improves noise figure but degrades

distortion performance. Figure 8 demonstrates the im-

pact of the RF chokes in series with the source resistance

at 3.3V. There is a 2dB improvement in conversion gain

and noise figure and a corresponding decrease in IIP3.

Figure 5. Alternative Transformerless Input Circuit

Using Low Cost Discrete Components

LT5521

5521 F05

L4

C14

C16

1nF

C13

R1

110

Ω

C2

82pF

R7

110

Ω

L3

50

Ω

2

3

Figure 4. Low Frequency Signal Input

R1

C2

1nF

1nF

LT5521

3

2

C13

50

Ω

5521 F04

R7

T1

2:1

Table 4. Component Values for Discrete Bridge Balun Signal

Input Matching

IF (MHz)

C14, C16 (pF)

L3, L4 (nH)

220

22

22

250

18

18

640

4.7

4.7