Datasheet
LTC6946
25
6946fa
APPLICATIONS INFORMATION
Single-ended impedance matching is accomplished
using the circuit of Figure 16, with component values
found in Table 13. Using smaller inductances than
recommended can cause phase noise degradation,
especially at lower center frequencies.
Table 13. Suggested Single-Ended Matching Component Values
f
RF
(MHz) L
C
(nH) C
S
(pF)
350 to 1500 180 270
1000 to 5800 68 100
Return loss measured on the DC1705A using the above
component values is shown in Figure 17. A broadband
match is achieved using an (L
C
, C
S
) of either (68nH, 100pF)
or (180nH, 270pF). However, for maximum output power
and best phase noise performance, use the recommended
component values of Table 13. L
C
should be a wirewound
inductor selected for maximum Q factor and SRF, such as
the Coilcraft HP series of chip inductors.
The LTC6946’s differential RF
±
outputs may be combined
using an external balun to drive a single-ended load. The
advantages are approximately 3dB more output power than
each output individually and better 2nd order harmonic
performance.
For lower frequencies, transmission line (TL) baluns such as
the M/A-COM MABACT0065 and the TOKO #617DB-1673
provide good results. At higher frequencies, surface mount
(SMT) baluns such as those produced by TDK, Anaren,
and Johanson Technology, can be attractive alternatives.
See Table 14 for recommended balun part numbers versus
frequency range.
Figure 17. Single-Ended Return Loss
Figure 16. Single-Ended Output Matching Schematic
RF
+(–)
L
C
C
S
50Ω
TO 50Ω
LOAD
V
RF
+
RF
–(+)
L
C
C
S
6946 F16
V
RF
+
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
FREQUENCY (GHz)
S11 (dB)
–6
–4
–2
6946 F17
–10
–16
0
–8
–12
–14
68nH, 100pF
180nH, 270pF
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
V
RF
+
voltage. Figure 18 shows a surface mount balun’s
connections with a DC FEED pin.
Table 14. Suggested Baluns
f
RF
(MHz) PART NUMBER MANUFACTURER TYPE
350 to 900 #617DB-1673 TOKO TL
400 to 600 HHM1589B1 TDK SMT
600 to 1400 BD0810J50200 Anaren SMT
600 to 3000 MABACT0065 M/A-COM TL
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. Figure18
displays RF
±
connections using these baluns.