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ML13176 View Datasheet(PDF) - LANSDALE Semiconductor Inc.

Part Name
Description
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ML13176
LANSDALE
LANSDALE Semiconductor Inc. LANSDALE
ML13176 Datasheet PDF : 16 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
LANSDALE Semiconductor, Inc.
ML13175/ML13176
Legacy Applications Information
Figure 16 Shows the improved hold–in range of the loop. The fref
is moved 950 kHz with over 200 µA swing of control current for an
improved hold–in range of ±15.2 MHz or ±95.46 Mrad/sec.
fc = 0.159/RC;
For R = 1.0 k + R7 (R7 = 53 k) and C = 390 pF
fc = 7.55 kHz or ωc = 47 krad/sec
Figure 16. ML13176 Reference Oscillator
Frequency versus Oscillator Control Current
10.6
Closed Loop Response:
10.4
fo = 32 x fref
VCC = 3.0 Vdc
ICC = 38 mA
10.2
Pout = 4.8 dB
Imod = 2.0 mA
10
Vref = 500 mVp–p
9.8
9.6
9.4
–150
–100
– 50
0
50
100
I6, OSCILLATOR CONTROL CURRENT (µA)
LOCK–IN RANGE/CAPTURE RANGE
If a signal is applied to the loop not equal to free running frequency,
ff, then the loop will capture or lock–in the signal by making fs = fo
(i.e. if the initial frequency difference is not too great). The lock–in
range can be expressed as ∆ωL ~ ± 2L ωn
FM MODULATION
Noise external to the loop (phase detector input) is minimized by nar-
rowing the bandwidth. This noise if minimal in a PLL system since the
reference frequency is usually derived from a crystal oscillator. FM can
be achieved by applying a modulation current superimposed on the
control current of the CCO. The loop bandwidth must be narrow
enough to prevent the loop from responding to the modulation frequen-
cy components, thus, allowing the CCO to deviate in frequency. The
loop bandwidth is related to the natural frequency wn. In the lag–lead
design example where the natural frequency, ωn = 5.0 krad/sec and a
damping factor, L = 0.707, the loop bandwidth = 1.64 kHz.
Characterization data of the closed loop responses for both the
ML13175 and ML13176 at 320 MHz (Figures 7 and 8, respectively)
show satisfactory performance using only a simple low–pass loop filter
network. The loop filter response is strongly influenced by the high
output impedance of the push–pull current output of the phase detector.
The application example in Figure 17a of a 320 MHz FM transmitter
demonstrates the FM capabilities of the IC. A high value series resistor
(100 k) to Pin 6 sets up the current source to drive the modulation sec-
tion of the chip. Its value is dependent on the peak to peak level of the
encoding data and the maximum desired frequency deviation. The data
input is AC coupled with a large coupling capacitor which is selected
for the modulating frequency. The component placements on the circuit
side and ground side of the PC board are shown in Figures 28 and 29
respectively.
For voice application using a dynamic or an electret microphone, an op
amp is used to amplify the microphone’s low level output . The micro-
phone amplifier circuit is shown in Figure 19. Figure 17b shows an
application example for NBFM audio or direct FSK in which the refer-
ence crystal oscillator is modulated.
Figure 19. Microphone Amplifier
Data
VCC
Input
100k 120k
3.3k 3.9k 1.0
VCC
Voice
Input
Electret
Microphone
1.0k 10k
10k
MC33171
Data or
Audio
Output
LOCAL OSCILLATOR APPLICATION
To reduce internal loop noise, a relatively wide loop bandwidth is
needed so that the loop tracks out or cancels the noise. This is empha-
sized to reduce inherent CCO and divider noise or noise produced by
mechanical shock and environmental vibrations. In a local oscillator
application the CCO and divider noise should be reduced by proper
selection of the natural frequency of the loop. Additional low pass fil-
tering of the output will likely be necessary to reduce the crystal side-
band spurs to a minimal level.
Page 9 of 16
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