Datasheet

LTC6945
22
6945f
APPLICATIONS INFORMATION
OUTPUT PHASE NOISE DUE TO 1/f NOISE
In-band phase noise at very low offset frequencies may
be influenced by the LTC6945’s 1/f noise, depending upon
f
PFD
. Use the normalized in-band 1/f noise of –274dBc/Hz
with Equation 12 to approximate the output 1/f phase
noise at a given frequency offset f
OFFSET
:
L
M(OUT –1/f)
(f
OFFSET
) = –274 + 20 • log
10
(f
RF
)
(12)
– 10 • log
10
(f
OFFSET
)
Unlike the in-band noise floor L
M(OUT)
, the 1/f noise
L
M(OUT –1/f)
does not change with f
PFD
and is not constant
over offset frequency. See Figure 17 for an example of
in-band phase noise for f
PFD
equal to 3MHz and 100MHz.
The total phase noise will be the summation of L
M(OUT)
and L
M(OUT –1/f)
.
The inputs may be used single-ended by applying the
AC-coupled VCO frequency at VCO
+
and bypassing
VCO
–
to GND with a 100pF capacitor (270pF for frequen-
cies less than 500MHz). Measured VCO
+
s-parameters
(with VCO
–
bypassed with 100pF to GND) are shown in
Table 10 to aid in the design of external impedance match-
ing networks.
Table 10. Single-Ended VCO
+
Input Impedance
FREQUENCY (MHz) IMPEDANCE (Ω) S11 (dB)
250 118 – j78 –5.06
500 83.6 – j68.3 –5.90
1000 52.8 – j56.1 –6.38
1500 35.2 – j41.7 –6.63
2000 25.7 – j30.2 –6.35
2500 19.7 – j20.6 –5.94
3000 17.6 – j11.2 –6.00
3500 17.8 – j3.92 –6.41
4000 19.8 + j4.74 –7.20
4500 21.5 + j15.0 –7.12
5000 21.1 + j19.4 –6.52
5500 27.1 + j22.9 –7.91
6000 38.3 + j33.7 –8.47
6500 36.7 + j42.2 –6.76
7000 46.2 + j40.9 –8.11
7500 76.5 + j36.8 –9.25
8000 84.1+ j52.2 –7.27
RF OUTPUT MATCHING
The RF
±
outputs may be used in either single-ended or
differential configurations. Using both RF outputs differen-
tially will result in approximately 3dB more output power
than single-ended. Impedance matching to an external
load in both cases requires external chokes tied to V
RF
+
.
VCO INPUT MATCHING
The VCO
±
inputs may be used differentially or single-ended.
Each input provides a single-ended 121Ω resistance to aid
in impedance matching at high frequencies. The inputs are
self-biased and must be AC-coupled using a 100pF capaci-
tors (or 270pF for VCO frequencies less than 500MHz).
Figure 17. Theoretical In-Band Phase Noise, f
RF
= 2500MHz
OFFSET FREQUENCY (Hz)
–120
PHASE NOISE (dBc/Hz)
–90
10 100 10k 100k
6945 F17
1k
–100
–110
–130
TOTAL NOISE
f
PFD
= 3MHz
TOTAL NOISE
f
PFD
= 100MHz
1/f NOISE
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