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PDF AN1207 Data sheet ( Hoja de datos )
| Número de pieza | AN1207 | |
| Descripción | Basic HF and VHF Oscillators | |
| Fabricantes | Motorola Semiconductors | |
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MOTOROLASEMICONDUCTOR TECHNICAL DATA
Order this document
by AN1207/D
The MC14h5e1e7t04Uin.cBomasic HF and VHF Oscillators AN1207Prepared by: David Babin and Mark Clark
taSPhase–locked loop (PLL) frequency synthesizers are com-
amonly found in communication gear today. The carrier oscilla-
tor in a transmitter and local oscillator (LO) in a receiver are
.Dwhere PLL frequency synthesizers are utilized. In some cellu-
wlar phones, a synthesizer can also be used to generate 90
wMHz for an offset loop. In addition, synthesizers can be used
in computers and other digital systems to create different
w clocks which are synchronized to a master clock.
mThe MC145170 is available to address some of these
applications. The frequency capability of the newest version,
othe MC145170–2, is very broad — from a few hertz to
.c185 MHz.
ADVANTAGES
UFrequency synthesizers, such as the MC145170, use digi-
t4tal dividers which can be placed under MCU control. Usually,
all that is required to change frequencies is to change the di-
vide ratio of the N Counter. Tuning in less than a millisecond
eis achievable.
The MC145170 can generate many frequencies based on
ethe accuracy of a single reference source. For example, the
reference can be a low–cost basic crystal oscillator or a tem-
hperature–compensated crystal oscillator (TCXO). Therefore,
high tuning accuracies can be achieved. Boosting of the ref-
Serence frequency by 100x or more is achievable.
taELEMENTS IN THE LOOP
aThe components used in the PLL frequency synthesizer of
Figure 1 are the MC145170 PLL chip, low–pass filter, and
voltage–controlled oscillator (VCO). Sometimes a voltage–
.Dcontrolled multivibrator (VCM) is used in place of the VCO.
The output of a VCM is a square wave and is usually
integrated before being fed to other sections of the radio. The
VCM output can be directly used in computers and other digi-
tal equipment. The output of a VCO or VCM is typically buff-
ered, as shown.
As shown in Figure 2, the MC145170 contains a reference
oscillator, reference counter (R Counter), VCO/VCM counter
(N Counter), and phase detector. A more detailed block dia-
gram is shown in the data sheet.
HF SYNTHESIZER
The basic information required for designing a stable high–
frequency PLL frequency synthesizer is the frequencies
required, tuning resolution, lock time, and overshoot. For the
example design of Figure 3, the frequencies needed are
9.20 MHz to 12.19 MHz. The resolution (usually the same as
the frequency steps or channel spacing) is 230 kHz. The lock
time is 8 ms and a maximum overshoot of approximately 15%
is targeted. For purposes of this example, lock is considered
to be when the frequency is within about 1% of the final value.
HF SYNTHESIZER LOW–PASS FILTER
In this design, assume a square wave output is acceptable.
To generate a square wave, a MC1658 VCM chip is chosen.
Per the transfer characteristic given in the data sheet, the
MC1658 transfer function, KVCM, is approximately 1 x 108 ra-
dians/second/volt. The loading presented by the MC1658
control input is large; the maximum input current is 350 µA.
Therefore, an active low–pass filter is used so that loading
does not affect the filter’s response. See Figure 3. In the filter,
a 2N7002 FET is chosen because it has very high transcon-
ductance (80 mmhos) and low input leakage (100 nA).
w DIVIDE VALUE
wREFERENCE
w .comOSCILLATOR
MC145170
PLL
CHIP
LOW–PASS
FILTER
et4UBUFFER
SheOUTPUT
VCO
OR
VCM
ataFigure 1. PLL Frequency Synthesizer
.DREV 2
w1/98 TN98011500
wwM© OMoTtoOroRla,OInLc.A1998
REFERENCE
OSCILLATOR
FROM
VCO/VCM
REFERENCE
COUNTER
(R COUNTER)
VCO/VCM
COUNTER
(N COUNTER)
fR
PHASE
DETECTOR
fV
TO
LOW–PASS
FILTER
MULTIPLYING VALUE
Figure 2. Detail of the MC145170
AN1207
1
1 page  
Therefore,
CR = 2.6 = 30 ρ
2
(13)
log(2.6) = ρlog(15)
(14)
ρ = log(2.6)/log(15) = 0.3528
Using the nominal capacitance of 100 pF at 4 volts:
100 pF= 10 0.3528
Cvmax 4 V
100 pF = 1.382
Cvmax
Solving for Cvmax:
100 pF = 72.4 pF
1.382
Solving for Cvmin:
2.6 = Cvmin
49.1 pF
(15)
(16)
(17)
Cvmin = (2.6)(49.1 pF)
Cvmin = 127.7 pF
THE VCO
For convenience, the MC1648 VCO is selected. The tuning
range of the VCO may be calculated as
fmax =
fmin
(Cdmax + Cs)0.5
(Cdmin + Cs)0.5
(18)
where
fmin =
1
2π[L(Cdmax + Cs)]0.5
(19)
As shown in Figure 8 of the data sheet, the VCO tank circuit
is comprised of two varactors and an inductor. Typically, a
single varactor might be used in either a series or parallel
configuration. However, the second varactor has a two–fold
purpose. First, if the 10 kΩ isolating impedance is left in place,
the varactors add in series for a smaller capacitance. Se-
cond, the added varactor acts to eliminate distortion due to
the tank voltage changing.
Therefore, with the two varactors in series, Cdmax′ =
Cdmax/2. The shunt capacitance (input plus external capaci-
tance) is symbolized by Cs.
Therefore, solving for the inductance:
L
=
1
(2πfmin)2(Cdmax′
+
=
Cs)
19.9
nH
≈
20
nH
(20)
The Q of the inductor should be more than 100 for best perfor-
mance.
fmin
=
2π[(19.9
1
nH)(69.85
pF)]0.5
=
135
MHz
(21)
MOTOROLA
fmax
=
2π[(19.9
1
nH)(42.2
pF)]0.5
=
173
MHz
(22)
The frequency ratio is 1.5 to 1 and is impacted by the tuning
range of the MV2115 varactor diode used in the tank circuit.
Therefore, the required range of 140 to 160 MHz is not limited
by this VCO design.
A pc board should be used to obtain favorable results with
this VHF circuit. The lead lengths in the tank circuit should be
kept short to minimize parasitic inductance. The length of the
trace from the VCO output to the PLL input should be kept as
short as possible. In addition, use of surface–mount compo-
nents is recommended to help minimize strays.
VHF SYNTHESIZER PROGRAMMING
Again, programming the three registers of the MC145170
is straightforward. Also, usually both the C and the R Regis-
ters are programmed only once, after power up.
The C Register configures the device and is programmed
with $80 (1 byte). This sets the phase detector to the correct
polarity and activates the φR and φV outputs while turning off
the other outputs. Like the HF oscillator, the phase detector
polarity is determined by how the filter is hooked up and the
VCO.
The R Register is programmed for a divide value that
delivers the proper frequency at the phase detector reference
input. In this case, 100 kHz is needed. Therefore, with the
1 MHz crystal shown, the R Register needs a value of
$00000A (3 bytes, 10 in decimal).
The N Register determines the frequency tuned. To tune
140 MHz, the value required for N to multiply up the reference
of 100 kHz to 140 MHz is 1400 decimal. For 160 MHz, the
value is 1600 decimal. To tune over the range, simply change
the value in the N Register with a 2–byte transfer.
ADVANCED CONSIDERATIONS
The circuit of Figure 5 may not function at very–high tem-
perature. The reason is that the MC145170 is guaranteed to
a maximum frequency of 160 MHz at 85°C. Therefore, there
is no margin for overshoot (reference Figure 4) at high tem-
perature. There are two possible solutions: (1) use the
MC145170–1 or MC145170–2 which are rated to 185 MHz,
or (2) limit the tuning to less than 160 MHz.
Operational amplifiers are usually too noisy for critical ap-
plications. Therefore, if an active element is required in the in-
tegrator, one or more discrete transistors are utilized. These
may be FETs or bipolar devices. However, active filter ele-
ments are not needed if the VCO loading is not severe, such
as is encountered with most discrete VCO designs. Because
active elements add noise, some performance parameters
are improved if they are not used. On the other hand, an ac-
tive filter can be used to scale up the VCO control voltage. For
example, to tune a wide range, the control voltage may have
to range up to 10 V. For a 5 V PLL output, this would be scaled
by 2x via use of active elements.
Some applications have requirements that must be met in
the areas of phase noise and reference suppression. These
parameters are in conflict with fast lock times. That is, as lock
times are reduced, reference suppression becomes more dif-
ficult. Both reference suppression and phase noise are ad-
vanced areas that are covered in several publications. As an
example, consider that the VCO input voltage range for
the above VHF loop was merely picked to be 8 V. Advanced
AN1207
5
5 Page | ||
| Páginas | Total 6 Páginas | |
| PDF Descargar | [ Datasheet AN1207.PDF ] | |
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