AN1207

3

MOTOROLA

The C Register, which configures the device, is pro-

grammed with $C0 (1 byte). This sets the phase detector to

the proper polarity and activates PDout. This also turns off the

unused outputs. The phase detector polarity is determined by

the filter and the VCM. For this example, the MC1658 data

sheet shows that a higher voltage level is needed if speed is

to be increased. However, the low–pass filter inverts the sig-

nal from the phase detector (due to the active element config-

uration). Therefore, the programming of the polarity for the

phase detector means that the POL bit must be a “1.”

The R Register is programmed for a divide value that

results in the proper frequency at the phase detector refer-

ence input. In this case, 230 kHz is needed. Therefore, with

the 4.6 MHz source shown in Figure 3, the R Register needs

a value of $000014 (3 bytes, 20 in decimal).

The N Register determines the frequency tuned. Tuning

9.2 MHz requires the proper value for N to multiply up the

reference of 230 kHz to 9.2 MHz. This is 40 decimal. For

12.19 MHz, the value is 53 decimal. To tune over the range,

change the value in the N Register within the range of 40 to 53

with a 2–byte transfer. Table 1 shows the possible frequen-

cies.

Table 1. The HF Oscillator Frequencies

N Value

Frequency, MHz

40

9.20

41

9.43

42

9.66

43

9.89

44

10.12

45

10.35

46

10.58

47

10.81

48

11.04

49

11.27

50

11.50

51

11.73

52

11.96

53

12.19

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.

VHF SYNTHESIZER

The MC145170 may be used in VHF designs, also. The

range for this next example is 140 to 160 MHz in 100 kHz

increments.

VHF SYNTHESIZER LOW–PASS FILTER

To illustrate design with the doubled–ended phase detec-

tor, the

φR and φV outputs are used. This requires an opera-

tional amplifier, as shown in Figure 5. From the design

guidelines shown in the MC145170 data sheet, the following

equations are used:

ωn =

K

φKVCO

N C R1

(1)

ζ = ωn

R2C

2

(2)

damping factor

where, from the data sheet, the equation for the

φR and φV

phase detector,

K

φ =

VDD

2

π

(3)

5

2

π

=

= 0.796 V/rad

ζ = 0.707,

ωn =

2

πfR

50

(4)

2

π x 100 kHz

50

=

= 12,566 rad/s

and

KVCO =

2

π ∆ fVCO

∆ VVCO

(5)

2

π x (160 – 140 MHz)

10 – 2

=

= 1.57 x 107 rad/s/V

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 ωn are rules of thumb that are

a good design starting point. A larger

ωn 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

ωn as:

R1=

K

φKVCO

C1ωnN

(6)

(0.796 V/rad)(1.57 x 107 rad/s/V)

(4700 pF)(12,566 rad/s)2 (1500)

=

= 11.23 k

Ω

2

Therefore, chose an 11 k

Ω standard value resistor.

R2 is determined from:

R2 =

2

ζ

ωnC1

(7)

(2)(0.707)

(12,566)(4700 pF)

=

= 23.94 k

Ω or

24 k

Ω (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

Freescale Semiconductor, Inc.

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