MC1374

4

MOTOROLA ANALOG IC DEVICE DATA

AM Section

The AM modulator transfer function in Figure 3 shows that

the video input can be of either polarity (and can be applied at

either input). When the voltages on Pin 1 and Pin 11 are

equal, the RF output is theoretically zero. As the difference

between VPin 11 and VPin 1 increases, the RF output

increases linearly until all of the current from both I1 current

sources (Q8 and Q9) is flowing in one side of the modulator.

This occurs when

±(VPin11 – VPin1) = I1 RG, where I1 is

typically 1.15 mA. The peak–to–peak RF output is the 2I1 RL.

Usually the value of RL is chosen to be 75 Ω to ease the

design of the output filter and match into TV distribution

systems. The theoretical range of input voltage and RG is

quite wide, but noise and available sound level limit the useful

video (sync tip) amplitude to between 0.25 Vpk and 1.0 Vpk.

It is recommended that the value of RG be chosen so that

only about half of the dynamic range will be used at sync tip

level.

The operating window of Figure 5 shows a cross–hatched

area where Pin 1 and Pin 11 voltages must always be in order

to avoid saturation in any part of the modulator. The letter

φ

represents one diode drop, or about 0.75 V. The oscillator

Pins 6 and 7 must be biased to a level of VCC –

φ – 2I1 RL (or

lower) and the input Pins 1 and 11 must always be at least 2

φ

below that. It is permissible to operate down to 1.6 V,

saturating the current sources, but whenever possible, the

minimum should be 3

φ above ground.

The oscillator will operate dependably up to about

105 MHz with a broad range of tank circuit component

values. It is desirable to use a small L and a large C to

minimize the dependence on IC internal capacitance. An

operating Q between 10 and 20 is recommended. The values

of R1, R2 and R3 are chosen to produce the desired Q and to

set the Pin 6 and 7 dc voltage as discussed above.

Unbalanced operation, i.e., Pin 6 or 7 bypassed to ground, is

not recommended. Although the oscillator will still run, and

the modulator will produce a useable signal, this mode

causes substantial base–band video feedthrough.

Bandswitching, as Figure 1 shows, can still be accomplished

economically without using the unbalanced method.

The oscillator frequency with respect to temperature in the

test circuit shows less than

±20 kHz total shift from 0° to 50°C

as shown in Figure 7. At higher temperatures the slope

approaches 2.0 kHz/

°C. Improvement in this region would

require a temperature compensating tuning capacitor of the

N75 family.

Crystal control is feasible using the circuit shown in Figure

21. The crystal is a 3rd overtone series type, used in series

resonance. The L1, C2 resonance is adjusted well below the

crystal frequency and is sufficiently tolerant to permit fixed

values. A frequency shift versus temperature of less than

1.0 Hz/

°C can be expected from this approach. The resistors

Ra and Rb are to suppress parasitic resonances.

Coupling of output RF to wiring and components on Pins 1

and 11 can cause as much as 300 kHz shift in carrier (at

67 MHz) over the video input range. A careful layout can

keep this shift below 10 kHz. Oscillator may also be

inadvertently coupled to the RF output, with the undesired

effect of preventing a good null when V11 = V1. Reasonable

care will yield carrier rejection ratios of 36 to 40 dB below sync

tip level carrier.

In television, one of the most serious concerns is the

prevention of the intermodulation of color (3.58 MHz) and

sound (4.5 MHz) frequencies, which causes a 920 kHz signal

to appear in the spectrum. Very little (3rd order) nonlinearity is

needed to cause this problem. The results in Figure 6 are

unsatisfactory, and demonstrate that too much of the

available dynamic range of the MC1374 has been used.

Figures 8 and 10 show that by either reducing standard

signal level, or reducing gain, acceptable results may be

obtained.

At VHF frequencies, small imbalances within the device

introduce substantial amounts of 2nd harmonic in the RF

output. At 67 MHz, the 2nd harmonic is only 6 to 8 dB below

the maximum fundamental. For this reason, a double pi low

pass filter is shown in the test circuit of Figure 3 and works

well for Channel 3 and 4 lab work. For a fully commercial

application, a vestigial sideband filter will be required. The

general form and approximate values are shown in Figure 19.

It must be exactly aligned to the particular channel.

2I1RL

–I1RG

0

+I1RG

Differential Input, V11–V1 (V)

Figure 3. AM Modulator Transfer Function

Figure 4. AM Test Circuit

L1

R2

470

0.001

VCC

C2 56

0.1

µH

RF

V1

10

µF

+

Video

Input

1.0k

V11

RG

22

47

22

1

11

12

13

9

8

7

6

5

470

R3

R1

470

RL

75

22

µH22µH