Datasheet
LTC6945
23
6945f
APPLICATIONS INFORMATION
Measured RF
±
s-parameters are shown below in Table
11 to aid in the design of impedance matching networks.
Table 11. Single-Ended RF Output Impedance
FREQUENCY (MHz) IMPEDANCE (Ω) S11 (dB)
500 102.8 – j49.7 –6.90
1000 70.2 – j60.1 –6.53
1500 52.4 – j56.2 –6.35
2000 43.6 – j49.2 –6.58
2500 37.9 – j39.6 –7.34
3000 32.7 – j28.2 –8.44
3500 27.9 – j17.8 –8.99
4000 24.3 – j9.4 –8.72
4500 22.2 – j3.3 –8.26
5000 21.6 + j1.9 –8.02
5500 21.8 + j6.6 –7.91
6000 23.1 + j11.4 –8.09
6500 25.7 + j16.9 –8.38
7000 29.3 + j23.0 –8.53
7500 33.5 + j28.4 –8.56
8000 37.9 + j32.6 –8.64
Single-ended impedance matching is accomplished using
the circuit of Figure 18, with component values found in
Table 12. Using smaller inductances than recommended
can cause phase noise degradation, especially at lower
center frequencies.
Table 12. Suggested Single-Ended Matching Component Values
f
RF
(MHz) L
C
(nH) C
S
(pF)
350 to 1500 180nH 270pF
1000 to 5800 68nH 100pF
Return loss measured on the DC1649 using the above
component values is shown in Figure 19. 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 12. L
C
should be a wirewound
inductor selected for maximum Q factor and SRF, such as
the Coilcraft HP series of chip inductors.
Figure 19. Single-Ended Return Loss
Figure 18. Single-Ended Output Matching Schematic
The LTC6945’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,
RF
+(–)
L
C
C
S
50Ω
TO 50Ω
LOAD
V
RF
+
RF
–(+)
L
C
C
S
6945 F18
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
6945 F19
–10
–16
0
–8
–12
–14
68nH, 100pF
180nH, 270pF