MC44302ADW Просмотр технического описания (PDF) - Motorola => Freescale

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MC44302ADW

ADVANCED MULTI–STANDARD VIDEO/SOUND IF

Motorola => Freescale 

1.0 MHz

Square

Wave

IF Carrier

Signal

(Limited)

Regenerated

Carrier

(Limited)

MC44302A

Figure 20. Phase Detector

VCC

Q1

Q2

SW1

SW2

I + ∆I

I – ∆I

Q3

I – ∆I

I + ∆I

SW3

Q4

VCO

PLL

2∆I

19 Filter

1.0 MHz

Square

Wave

Phase errors, resulting in quadrature video distortion, can

also be caused by dc errors in the phase detector and AFT

amplifier. Most of the dc offsets are caused by mismatches in

the current mirrors of the push–pull output stage, refer to

Figure 20. Switches SW1, SW2, and SW3 are driven by a

1.0 MHz square wave with an accurate 1:1 mark/space ratio.

Switches SW1 and SW2 maintain the same sense of error

signal, while SW2 ensures errors due to the top PNP current

mirrors average to zero on the external loop filter capacitor. In

a similar way, SW3 by interchanging Q3 and Q4, cancels

errors due to the bottom NPN mirror. With phase errors

reduced to a minimum, there is no need for any external

phase adjustments. The phase detector output is filtered and

it is used to control the VCO in a corrective manner. When the

PLL establishes a locked condition, there will be a 90° phase

shift between the two phase detector inputs.

The Voltage Controlled Oscillator and Frequency Doubler

circuits are shown in Figure 21. The oscillator operates at

one half of the picture carrier frequency and is tuned by a

control bias that is applied to the reactance stage input.

Reactance tuning allows a higher Q to be maintained in the

tank circuit as opposed to a phase shift type of oscillator with

the same tuning range. The oscillator frequency is internally

doubled to picture carrier frequency by a balanced multiplier.

Note that the multiplier input signals are at 90° to each other

for frequency doubling.

Since the oscillator operates at one half of the picture

carrier frequency, radiation from the external tuned circuit

components will not desensitize the system, even if picked

up by the amplifier input leads. This significantly reduces the

possibility of a PLL push–off condition. Running the oscillator

at twice the picture carrier and dividing it down is another way

of solving the IF input radiation problem, but there are two

significant disadvantages. First and foremost, radiation into

the antenna now becomes a problem. In the U.S.A. twice the

picture carrier falls directly into the passband of channel 6,

producing a very noticeable beat. Any second order

harmonics, four times picture carrier, will fall into the

passband of channel 8. Second, it is more difficult to produce

a stable oscillator that operates at twice the IF frequency than

one that operates at one half of the IF frequency.

Figure 21. VCO and Frequency Doubler

VCC

Control

Bias

20

21

4.7 k

Bias

PLL Limiter for

Video/Sound

Demodulations

(f = Pix Carrier)

Reactance

Tuning Stage

Oscillator

Frequency Doubler

(fOSC = 0.5 Pix Carrier) Balanced Multiplier

10

MOTOROLA ANALOG IC DEVICE DATA

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