Freescale Semiconductor, Inc.
The C Register, which configures the device, is pro-
VHF SYNTHESIZER
grammed with $C0 (1 byte). This sets the phase detector to
The MC145170 may be used in VHF designs, also. The
the proper polarity and activates PDout. This also turns off the
range for this next example is 140 to 160 MHz in 100 kHz
unused outputs. The phase detector polarity is determined by
increments.
the filter and the VCM. For this example, the MC1658 data
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sheet shows that a higher voltage level is needed if speed is
VHF SYNTHESIZER LOW–PASS FILTER
to be increased. However, the low–pass filter inverts the sig-
To illustrate design with the doubled–ended phase detec-
nal from the phase detector (due to the active element config-
tor, the
φ
R and
φ
V outputs are used. This requires an opera-
uration). Therefore, the programming of the polarity for the
tional amplifier, as shown in Figure 5. From the design
phase detector means that the POL bit must be a “1.”
guidelines shown in the MC145170 data sheet, the following
The R Register is programmed for a divide value that
equations are used:
results in the proper frequency at the phase detector refer-
ence input. In this case, 230 kHz is needed. Therefore, with
K
φ
KVCO
the 4.6 MHz source shown in Figure 3, the R Register needs
(1)
ω
n =
N C R1
a value of $000014 (3 bytes, 20 in decimal).
The N Register determines the frequency tuned. Tuning
ω
R C
ζ
= n 2
damping factor
(2)
9.2 MHz requires the proper value for N to multiply up the
2
reference of 230 kHz to 9.2 MHz. This is 40 decimal. For
where, from the data sheet, the equation for the
φ
R and
φ
V
12.19 MHz, the value is 53 decimal. To tune over the range,
phase detector,
change the value in the N Register within the range of 40 to 53
V
5
K
φ
= DD =
= 0.796 V/rad
(3)
with a 2–byte transfer. Table 1 shows the possible frequen-
2π
2π
cies.
ζ
= 0.707,
Table 1. The HF Oscillator Frequencies
N Value
40
41
42
43
44
45
46
47
48
49
50
51
52
53
Frequency, MHz
9.20
9.43
9.66
9.89
10.12
10.35
10.58
10.81
11.04
11.27
11.50
11.73
11.96
12.19
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ω
n =
and
KVCO =
2πfR 2π x 100 kHz
=
= 12,566 rad/s
50
50
2π
∆
fVCO 2π x (160 – 140 MHz)
=
∆
VVCO
10 – 2
= 1.57 x 107 rad/s/V
(4)
(5)
EXTRA FILTERING FOR THE HF LOOP
When the HF oscillator was built, the proper frequencies
could not be tuned. The output of the MC1658 was examined
with an oscilloscope and the switching edges were discov-
ered to be “ragged.” That is, the output did not appear to be a
square wave with clean transitions.
The fin input of the MC145170 is sensitive to 500 mV p–p
signals, and the ragged edges were being amplified and
counted down by the N Counter. Therefore, the edges need-
ed cleaning up. One method would have been to add a low–
pass filter between the MC1658 and MC145170. However,
because an additional buffer was needed elsewhere in the
circuit, an MC74HCU04 inverter was used in place of the fil-
ter. This inverter’s frequency response is low enough to clean
up the ragged edges. That is, filtering of the ragged edges
occurred, and the output had smoother transitions. As men-
tioned previously, one of the elements in the inverter package
was used to buffer the output of the VCM before feeding it to
the outside world. See Figure 3.
The control voltage range on the input to the VCO is picked
to be 2 to 10 V.
The average frequency = (140 + 160)/2 = 150 MHz. There-
fore, the average N = 1500.
The above choices for
ζ
and
ω
n are rules of thumb that are
a good design starting point. A larger
ω
n value results in faster
loop lock times and higher reference frequency VCO
sidebands for similar sideband filtering. (See Advanced
Considerations.)
Choosing C1 to be 4700 pF, R1 is calculated from the
rearranged expression for
ω
n as:
K K
(0.796 V/rad)(1.57 x 107 rad/s/V)
R1=
φ
VCO =
C1
ω
2 N
(4700 pF)(12,566 rad/s)2 (1500)
n
= 11.23 kΩ
Therefore, chose an 11 kΩ standard value resistor.
R2 is determined from:
R2 =
2ζ
(2)(0.707)
=
ω
nC1
(12,566)(4700 pF)
= 23.94 kΩ or
24 kΩ (standard value)
VHF SYNTHESIZER EXTRA FILTERING
For more demanding applications, extra filtering is some-
times added. This reduces the VCO sidebands caused by a
small amount of the reference frequency feeding through the
filter. One form of this filtering consists of spitting R1 into two
resistors; each resistor is one–half the value of R1, as indi-
cated by R1/2 in Figure 5. Capacitors CC are added from the
(7)
(6)
MOTOROLA
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