LTC6948IUFD-3PBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6948IUFD-3PBF
Linear Technology
'LTC6948IUFD-3PBF' PDF : 36 Pages View PDF
LTC6948
Applications Information
Design and Programming Example
This programming example uses the DC1959 with the
LTC6948-2. Assume the following parameters of interest:
fREF = 100MHz at 7dBm into 50Ω
fSTEP = 50kHz
fRF = 1921.650MHz
From the Electrical Characteristics table:
fVCO = 3.080GHz to 4.910GHz
KVCO% = 4.7%Hz/V to 7%Hz/V
Determining Divider Values
Following the loop filter design algorithm, first determine
all the divider values. The maximum fPFD while in fractional
mode is less than 100MHz, so R must be greater than 1.
R=2
Then, using Equations 5 and 7, calculate the following
values:
O=2
fPFD = 50MHz
Then using Equation 6:
N
+
F
=
2
• 1921.650MHz
50MHz
=
76.866
Therefore:
N = 76
F = 0.866
Then, from Equation 4,
NUM = 0.866 • 218 = 227017
Also, from Equation 1 and Tables 8 and 9 determine B:
B = 48 and BD[3:0] = h5
A calibration cycle takes 12 to 14 clock cycles of fCAL. This
gives a VCO calibration time of approximately:
tCAL
≅
14
fCAL
= 14 •
B
fPFD
= 13.4µs
Selecting Filter Type and Loop Bandwidth
The next step in the algorithm is choosing the open loop
bandwidth. Select the minimum bandwidth resulting from
the below constraints.
1) The OSR must be at least 10 (sets absolute maximum BW).
2) The integrated phase noise due to thermal noise should
be minimized, neglecting any modulator noise.
3) The loop bandwidth must be narrow enough to ad-
equately filter the modulator’s quantization noise.
FracNWizard reports the loop bandwidths resulting from
each of the above constraints. The quantization noise con-
strained results vary according to the shape of the external
loop filter. FracNWizard reports an optimal bandwidth for
several filter types.
FracNWizard reports the thermal noise optimized loop
bandwidth is 211kHz. Filter 3 (fourth order response) has
a quantization noise constrained BW of 150kHz, making
it a good choice. Select Filter 3 and use the smaller of the
two bandwidths (150kHz) for optimal integrated phase
noise. Use Equation 11 to calculate OSR:
OSR
=
50MHz
150kHz
=
333.3
Loop Filter Component Selection
Now set loop filter resistor RZ and charge pump current
ICP. Because the KVCO varies over the VCO’s frequency
range, using the KVCO geometric mean gives good results:
KVCO = 3.843 • 109 • 0.047 • 0.07
= 220.4MHz / V
For more information www.linear.com/LTC6948
6948f
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