LM2889N - National Semiconductor

TL/H/5079

LM2889 TV Video Modulator

December 1994

LM2889 TV Video Modulator

General Description

The LM2889 is designed to interface audio and video sig-

nals to the antenna terminals of a TV receiver. It consists of

a sound subcarrier oscillator and FM modulator, video

clamp, and RF oscillators and modulators for two low-VHF

channels.

The LM2889 allows video information from VTRs, video disk

systems, games, test equipment, or similar sources to be

displayed on black and white or color TV receivers.

Features

YPin for pin compatible with LM1889 RF section

YLow distortion FM sound modulator (less than 1%

THD)

YVideo clamp for AC-coupled video

YLow sound oscillator harmonic levels

Y10V to 16V supply operation

YDC channel switching

YExcellent oscillator stability

YLow intermodulation products

Block and Connection Diagrams (Dual-In-Line Package)

DC Test Circuit

TL/H/5079–1

Order Number LM2889N

See NS Package Number N14A

C1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Supply Voltage 18VDC

Power Dissipation Package (Note 1) 700 mW

Operating Temperature Range 0§Ctoa

70§C

Storage Temperature Range b55§Ctoa

150§C

(V14– V13) Max g5VDC

(V12– V8) Max 7VDC

(V12– V9) Max 7VDC

Lead Temperature (Soldering, 10 seconds) 260§C

DC Electrical Characteristics

(DC test circuit, all switches normally pos. 1, VSe12V, VAe2V, VBeVCe10V)

Parameter Conditions Min Typ Max Units

Supply Current IS10 16 25 mA

Sound Oscillator Current DI13 Change VAfrom b2V to a2V 0.2 0.35 0.6 mA

Sound Oscillator Zener Current I13 0.85 mA

Sound Modulator Audio Current DI13 Change SW2 from Pos. 1 to Pos. 2 0.9 mA

Video Clamp Voltage V2

Unloaded 5.0 5.25 5.5 VDC

Loaded SW3 Pos. 3 5.1 VDC

Video Clamp Capacitor Discharge SW3 Pos. 2 20 mA

Current (VS–V2)/105

Ch. A Oscillator OFF Voltage, V6, V7 SW1 Pos. 2 2 mVDC

Ch. A Oscillator Current Level I7VBe10V, VCe11V 2.5 3.5 5.0 mA

Ch. B Oscillator OFF Voltage V4, V5 2 mVDC

Ch. B Oscillator Current Level I4SW1 Pos. 2, VBe10V, VCe11V 2.5 3.5 5.0 mA

Ch. A Modulator Conversion Ratio Measure DV9 by Changing from

DV9/(V11-V10) VBe10V, VCe11V, to VBe11V, 0.3 0.50 0.75 V/V

VCe10V; Divide by V11– V10

Ch. B Modulator Conversion Ratio SW1 Pos. 2, Measure DV8 by

DV8/(V11– V10) Changing from VBe10V, VCe11V, 0.3 0.50 0.75 V/V

to VBe11V, VCe10V; Divide by

V11– V10

AC Electrical Characteristics (AC test circuit, VSe12V)

Parameter Conditions Min Typ Max Units

Sound Carrier Oscillator Level (V13) 3.4 Vp-p

Sound Modulator Deviation Df/DVIN, SW1 Pos. 2, Change VIN from 1.4V 250 Hz/mV

to 1.0V, Measure Df at Pin 13, Divide as Shown

Ch. 3 RF Oscillator Level n6, n7 Ch. Sw. Pos. 3, fe61.25 MHz, Use FET Probe 550 mVp-p

Ch. 4 RF Oscillator Level, n4, n5, Ch. Sw. Pos. 4, fe67.25 MHz, Use FET Probe 550 mVp-p

RF Modulator Conversion Gain nOUT/(V10±V11) Ch. Sw. Pos. 3, fe61.25 MHz. (Note 2) 10 mVrms/V

Note 1: For operation in ambient temperatures above 25§C, the device must be derated based on a 150§C maximum junction temperature and a thermal resistance

of 80§C/W junction to ambient.

Note 2: Conversion gain shown is measured with 75Xinput RF meter which makes the AC RF output load 37.5X.

Design Characteristics (AC test circuit, VSe12V)

Parameter Typ Units

Sound Modulator Audio THD at g25 kHz Deviation, VIN must be 1 kHz Source, 0.8 %

Demodulate as Shown in

Figure 1

Sound Modulator Input Impedance (Pin 1) 1.5 kX

Sound Modulator Bandwidth 100 kHz

Oscillator Supply Dependence, Sound Carrier, RF See Curves

Oscillator Temperature Dependence (IC Only)

Sound Carrier b15 ppm/§C

RF b50 ppm/§C

RF Oscillator Maximum Operating Frequency (Temperature Stability Degraded) 100 MHz

RF Modulator

Carrier Suppression (Adjust Video Bias for Minimum RF Carrier at nOUT 30 dB

and Reference to nOUT with 3V Offset at Pins 10 and 11, See Applications

Information, RF Modulation Section)

3.58 MHz Differential Gain 5 %

Differential Phase 3 degrees

2.5V Vp-p Video, 87.5% Mod

Output Harmonics below RF Carrier

2nd, 3rd b12 dB

4th and Above b20 dB

Input Impedance, Pin 10, Pin 11 1 MX//2 pF

AC Test Circuit

TL/H/5079–2

Test Circuit

TL/H/5079–3

FIGURE 1. 4.5 MHz Sound FM Demodulator

Typical Performance Characteristics (Refer to AC test circuit unless noted)

Sound Carrier Oscillator

Supply Dependence

(fOe4.5 MHz, Pin 1 Open)

RF Oscillator Frequency

Supply Dependence

(fOe67.25 MHz)

RF Modulator Common-

Mode Input Range

Pins 10, 11 (Circuit

Diagrams)

FM Sound Modulator

Dynamic Characteristics

(fMODe1 kHz)

TL/H/5079–4

Circuit Description (Refer to Circuit Diagrams)

The sound carrier oscillator is formed by differential amplifi-

er Q3, Q4 operated with positive feedback from the pin 13

tank to the base of Q4. Frequency modulation is obtained

by varying the 90 degree phase shifted current of Q9. Q14’s

emitter is a virtual ground, so the voltage at pin 1 deter-

mines the current R11, which ultimately modulates the col-

lector current of Q9.

The video clamp is comprised of devices Q58-Q60. The

clamp voltage is set by resistors R40, R41, R49, and R50.

The DVBE/R42 current sets the capacitor discharge cur-

rent. Q59 and the above mentioned resistor string help

maintain a temperature stable clamp voltage.

The channel B oscillator consists of devices Q24 and Q25

cross-coupled through level-shift zener diodes Q22 and

Q23. A current regulator consisting of devices Q17– Q21 is

used to achieve good RF stability over temperature and

supply. The channel B modulator consists of multiplier de-

vices Q28– Q31, Q34 and Q35. The top quad is coupled to

the channel B tank through isolating devices Q26 and Q27.

A DC potential between pins 10 and 11 offsets the lower

pair to produce an output RF carrier at pin 8. That carrier is

then modulated by both the sound subcarrier at pin 10 and

the composite video signal at pin 11. The channel A modu-

lator shares pin 10 and 11 buffers, Q32 and Q33, with chan-

nel B and operates in an identical manner.

The current flowing through channel B oscillator diodes

Q22, Q23 is turned around in Q36– Q38 to source current

for the channel B RF modulator. In the same manner, the

channel A oscillator Q54– Q57 uses turn-around Q49– Q51

to source the channel A modulator. One oscillator at a time

may be activated by its current turn-around, and the other

oscillator/modulator combination remains off.

Circuit Diagrams

TL/H/5079–5

Circuit Diagrams (Continued)

TL/H/5079–6

Applications Information

SOUND FM MODULATOR

Frequency deviation is determined by the Q of the tank cir-

cuit at pin 13 and the current entering the audio input, pin 1.

This current is set by the input voltage VIN, the device input

impedance (1.5 kX), and any impedance network connect-

ed externally. A signal of 60 mVrms at pin 1 will yield about

g25 kHz deviation when configured as shown in

Figure 2

VIDEO CLAMP

When video is not available at DC levels within the RF mod-

ulator common-mode range, or if the DC level of the video is

not temperature stable, then it should be AC-coupled as

shown in the typical applications circuit (

Figure 2

). The

clamp holds the horizontal sync pulses at 5.2V for VSe12V.

The clamp coupling capacitor is charged during every sync

pulse and discharged when video information is present.

The discharge current is approximately 20 mA. This current

and the amount of acceptable tilt over a line of video deter-

mines the value of the coupling capacitor C1. For most ap-

plications 1 mF is sufficient.

RF MODULATION

Two RF channels are available, with carrier frequencies up

to 100 MHz being determined by L-C tank circuits at pins

4/ 5 and 6/7. The signal inputs (pins 10 and 11) are com-

mon to both modulators, but removing the power supply

from an RF oscillator will also disable that modulator.

The offset between the two signal pins determines the level

of the RF carrier output. To preserve the DC content of the

video signal, amplitude modulation of the RF carrier is done

in one direction only, with increasing video (toward peak

white) decreasing the carrier level. This means the active

composite video signal at pin 11 must be offset with respect

to pin 10 and the sync pulse should produce the largest

offset.

The largest video signal (peak white) should not be able to

suppress the carrier completely, particularly if sound trans-

mission is needed. This requires that pin 10 be biased

above the largest expected video signal. Because peak

white level is often difficult to define, a good rule to follow is

to bias pin 10 at a level which is four times the sync ampli-

tude above the sync tip level at pin 11. For example, the DC

bias at pin 10 with 0.5V sync clamped to 5.2V on pin 11,

should be 5.2a(4c0.5)e7.2V.

Typical Application

TL/H/5079–7

FIGURE 2. Two Channel Video Modulator with FM Sound

Applications Information (Continued)

When the signal inputs are exactly balanced, ideally there is

no RF carrier at the output. Circuit board layout is critical to

this measurement. For optimum performance, the output

and supply decoupling circuitry should be configured as

shown in

Figure 3

TL/H/5079–8

RF decouple supply directly to output ground.

FIGURE 3. Correct RF Supply Decoupling

The video clamp level is derived from a resistive divider con-

nected to supply (VS). To maintain good supply rejection,

pin 10, which is biased externally, should also be referenced

to supply (see

Figure 2

Pin Description (Refer to

Figure 2

)

Pin 1ÐAudio Input: Pin 1 is the audio input to the sound

FM generator. Frequency deviation is proportional to the

signal at this pin. A pre-emphasis network comprised of R1,

C2, and the device input impedance yields the following re-

sponse with an 80 mVrms audio input.

Pre-Emphasis Network

Response

TL/H/5079–9

Increasing R1 lowers the boost frequency, and decreases

deviation below the boost frequency. Increasing C2 only

lowers the boost frequency. C1 is a coupling capacitor, and

must be a low impedance compared to the sum of R1 and

the device input impedance (1.5 kX).

Pin 2ÐVideo Clamp: The video clamp restores the DC

component to AC-coupled video. The video is AC-coupled

to the clamp via C3. Decreasing C3 will cause a larger tilt

between vertical sync pulses in the clamped video wave-

form.

Pin 3ÐGround: Although separate on the chip level, all

ground terminate at pin 3.

Pins 4/5ÐChannel 4 Oscillator: Pins 4 and 5 are the col-

lector outputs of the channel 4 oscillator. L1 and C5 set the

oscillator frequency defined by fOe0.159/

L1C5. Increas-

ing L1 will decrease the oscillator frequency while decreas-

ing L1 will increase the oscillator frequency. Decreasing C5

will increase the oscillator frequency and lower the tank Q

causing possible drift problems. R2 and R3 are the oscillator

loads which determine the oscillator amplitude and the tank

Q. Increasing these resistors increases the Q and the oscil-

lator amplitude, possibly overdriving the RF modulator,

which will increase output RF harmonics. Decreasing R2

and R3 reduces the tank Q and may cause increased drift.

C4 is an RF decoupling capacitor. Increasing C4 may result

in less effective decoupling at RF. Decreasing C4 may intro-

duce RF to supply coupling.

Pins 6/7ÐChannel 3 Oscillator; Pins 6 and 7 are the chan-

nel 3 oscillator outputs. Every component at these pins has

the same purpose and effect as those at pins 4 and 5.

Pin 8ÐChannel 4 RF Output: Pin 8 is the channel 4 RF

output and R13 is the load resistor. The RF signal is AC

coupled via C15 to the output filter which is a two channel

VSB filter. L5 is parallel resonant with the filter input capaci-

tance minimizing loss in the output network. R14 terminated

the filter output.

Pin 9ÐChannel 3 RF Output: Pin 9 is the channel 3 RF

output with all components performing the same functions

as those in the pin 8 description.

Pin 10ÐRF Modulator Sound Subcarrier Input: Pin 10 is

one of the RF modulator inputs and may be used for video

or sound. It is used as a sound subcarrier input in

Figure 2

R8, R9, and R10 set the DC bias on this pin which deter-

mines the modulation depth of the RF output (see Applica-

tion Notes). R12 and C11 AC-couple the sound subcarrier

from the sound modulator to the RF modulator. R12 and

R11 form a resistive divider that determines the level of

sound at pin 10, which in turn sets the picture carrier to

sound subcarrier ratio. Increasing the ratio of R11/R12 will

increase the sound subcarrier at the output. C10 forms an

AC ground, preventing R8, R9, and R10 from having any

effects on the circuit other than setting the DC potential at

pin 10. R11 and R12 also effect the FM sound modulator

(see pin 13 description).

Pin Description (Continued)

Pin 11ÐVideo Input: Pin 11, when configured as shown, is

the RF modulator video input. In this application, video is

coupled directly from the video clamp. Alternatively, video

could be DC-coupled directly to pin 11 if it is already within

the DC common-mode input range of the RF modulator (see

curves). In any case, the video sync tip at pin 11 must have

a constant DC level independent of video content. Because

of circuit symmetry, pins 10 and 11 may be interchanged.

Pin 12ÐRF Supply: Pin 12 is the RF supply, with C12 and

C7 serving as RF decouple capacitors. Increasing C12 or

C7 may result in less effective RF decoupling, while de-

creasing them may cause supply interaction. It is important

that C7 be grounded at the RF output ground.

Pin 13ÐSound Tank: Pin 13 is the collector output of the

sound oscillator. L3 and C13 determine the oscillating fre-

quency by the relationship fOe0.159/

L3C13. Increasing

L3 or C13 will lower the operating frequency, while decreas-

ing them will raise the frequency. L3 and C13 also help

define the Q of the tank, on which FM modulator de-

viation level depends. As C13 increases, Q increases, and

frequency deviation decreases. Likewise, decreasing C13

increases deviation. The other factor concerning Q is the

external resistance across the tank. The series combination

R11aR12 usually dominates the tank Q. Decreasing this

resistive network will decrease Q and increase deviation. It

should be noted that because the level of phase modulation

of the 4.5 MHz signal remains constant, variation in Q will

not effect distortion of the frequency modulation process if

the audio at pin 1 is left constant. The amplitude of the

sound subcarrier is directly proportional to Q, so increasing

the unloaded Q or either of the resistors mentioned above

will increase the sound subcarrier amplitude. For proper op-

eration of the frequency modulator, the sound subcarrier

amplitude should be greater than 2 Vp-p.

Pin 14ÐSound Supply: Pin 14 is the sound supply and C14

is an RF decouple capacitor. Decreasing C14 may result in

increased supply interaction.

Printed Circuit Layout

Printed circuit board layout is critical in preventing RF feed-

through. The location of RF bypass capacitors on supply is

very important.

Figure 4

shows an example of a properly

layed out circuit board. It is recommended that this layout be

used.

TL/H/5079–10

FIGURE 4. Printed Circuit Board and Component Diagram

(Component Side 1X)

LM2889 TV Video Modulator

Physical Dimensions inches (millimeters)

Molded Dual-In-Line Package (N)

Order Number LM2889N

NS Package Number N14A

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