LTC6948 datasheet(32/38 Pages) AD

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LTC6948

6948fa

For more information www.linear.com/LTC6948

applicaTions inForMaTion

Single-ended impedance matching is accomplished using

the circuit of Figure 16, with component values found in

Table 18. Using smaller inductances than recommended

can cause phase noise degradation, especially at lower

center frequencies.

Table 18. Suggested Single-Ended Matching Component Values

fRF (MHz)

LC (nH)

CS (pF)

350 to 1500

180

270

1000 to 5800

100

Return loss measured on the DC1959 using the above

component values is shown in Figure 17. A broadband

match is achieved using an {LC,CS}ofeither{68nH,100pF}

or {180nH, 270pF}. However, for maximum output power

and best phase noise performance, use the recommended

component values of Table 18. LC should be a wirewound

inductor selected for maximum Q factor and SRF, such as

the Coilcraft HP series of chip inductors.

The LTC6948’s differential RF± outputs may be combined

using an external balun to drive a single-ended load. The

advantagesareapproximately3dBmoreoutputpowerthan

each output individually and better 2nd order harmonic

performance.

Forlowerfrequencies,transmissionline(TL)balunssuchas

the M/A-COM MABACT0065 and the TOKO #617DB-1673

providegoodresults.Athigherfrequencies,surfacemount

(SMT) baluns such as those produced by TDK, Anaren,

and Johanson Technology, can be attractive alternatives.

SeeTable19forrecommendedbalunpartnumbersversus

frequency range.

The listed SMT baluns contain internal chokes to bias RF±

and also provide input-to-output DC isolation. The pin

denoted as GND or DC FEED should be connected to the

VRF+ voltage. Figure 18 shows a surface mount balun’s

connections with a DC FEED pin.

Table 19. Suggested Baluns

fRF (MHz)

PART NUMBER

MANUFACTURER

TYPE

350 to 900

#617DB-1673

TOKO

400 to 600

HHM1589B1

TDK

SMT

600 to 1400

BD0810J50200

Anaren

SMT

600 to 3000

MABACT0065

M/A-COM

1000 to 2000

HHM1518A3

TDK

SMT

1400 to 2000

HHM1541E1

TDK

SMT

1900 to 2300

2450BL15B100E

Johanson

SMT

2000 to 2700

HHM1526

TDK

SMT

3700 to 5100

HHM1583B1

TDK

SMT

4000 to 6000

HHM1570B1

TDK

SMT

The listed TL baluns do not provide input-to-output DC

isolation and must be AC-coupled at the output. Figure 19

displays RF± connections using these baluns.

Figure 17. RF Single-Ended Return Loss

FREQUENCY (GHz)

–6

–4

–2

6947 F17

–10

–16

–8

–12

–14

68nH, 100pF

180nH, 270pF

Figure 16. Single-Ended Output Matching Schematic

RF+(–)

TO 50

LOAD

VRF+

RF–(+)

6948 F16

VRF+

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