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參數(shù)資料
| 型號： | AN1207 |
| 廠商： | 飛思卡爾半導(dǎo)體(中國)有限公司 |
| 英文描述： | THE MC145170 in Basic HF and VHF Oscillators |
| 中文描述： | 在基本高頻和甚高頻MC145170振蕩器 |
| 文件頁數(shù)： | 5/6頁 |
| 文件大小： | 267K |
| 代理商： | AN1207 |
AN1207
5
MOTOROLA
Therefore,
CR = 2.6 =
(13)
30
ρ
2
(14)
log(2.6) =
ρ
log(15)
(15)
ρ
= log(2.6)/log(15) = 0.3528
Using the nominal capacitance of 100 pF at 4 volts:
100.3528
4 V
100 pF=
Cvmax
= 1.382
100 pF
Cvmax
(16)
Solving for Cvmax:
= 72.4 pF
100 pF
1.382
Solving for Cvmin:
2.6 =
Cvmin
49.1 pF
(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
(Cdmax + Cs)0.5
=
fmin
fmax
(18)
(Cdmin + Cs)0.5
where
fmin =
1
2
π
[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 k
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
(2
π
fmin)2(Cdmax
′
+ Cs)
The Q of the inductor should be more than 100 for best perfor-
mance.
(20)
= 19.9 nH
≈
20 nH
fmin =
1
2
π
[(19.9 nH)(69.85 pF)]0.5
(21)
= 135 MHz
fmax =
1
2
π
[(19.9 nH)(42.2 pF)]0.5
(22)
= 173 MHz
The frequency ratio is 1.5 to 1 and is impacted by the tuning
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
φ
R and
φ
V 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
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.
ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005
A
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