LTC5556

23

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 16. Test Circuit and Measured Conversion Gain Using 200Ω Output Matching with a 4:1 IF Transformer to

Realize a 50Ω Single-Ended IF Output

RF to IF Conv Gain vs IF Frequency

200Ω Output with 4:1 Balun

5556 F16a

½ LTC5556

680nH

680nH

LO

RF1

RF1

(50)

IF1

(50)

1nF

1nF

18nH

18nH

680nH

10nH

10nH

680nH

10nF

10nF

T1

4:1

1pF

IF

Single-Ended IF Outputs Using a Balun

The LTC5556 evaluation board has differential IF outputs,

but can be modified for single-ended operation by insert-

ing a 4:1 balun, as shown in Figure 16. The 10nH series

inductors at the differential IF output compensate for the

IF amplifier’s output capacitance, producing a 200Ω dif-

ferential output up to approximately 500MHz. The 4:1

balun then converts the 200Ω differential output to 50Ω

single-ended. For applications with IF frequency less than

250MHz, the series 10nH inductors are not needed.

Figure 16 shows the measured conversion gain vs IF out-

put frequency, using a Mini-Circuits TCM4-19+ balun. The

RF input was swept from 3.35GHz to 3.85GHz using a

fixed 3.33GHz LO, producing an IF output ranging from

20MHz to 520MHz. Measured conversion gain for the

standard 100Ω differential output matching is also shown

on the same graph for comparison, highlighting the inser-

tion loss of the balun.

0dB

3dB

6dB

9dB

12dB

15dB

RF = 3.35GHz to 3.85GHz

LO = 3.33GHz

200Ω OUT WITH 4:1 BALUN

100Ω DIFF OUT

IF OUTPUT FREQUENCY (MHz)

20 70 120 170 220 270 320 370 420 470 520

–12

–9

–6

–3

0

3

6

9

12

5556 F16b