Example Application

4 of 6

The circuit of Figure 1 below shows how the

ELM304 would typically be connected. This is all that

is needed to generate an NTSC signal for 525 line

systems that use a nominal horizontal rate of 15734

Hz, and a vertical field rate of 59.94 Hz. The output

signal is an interlaced type, so 262.5 lines are

displayed during each field, for a total of 525 lines per

frame. Equalization pulses, vertical retrace blanking,

etc. are all generated by the ELM304.

Figure 1 does not show a power supply, as the

type used depends mostly on the application. For

portable use, this circuit can easily be powered by two

‘AA’ cells (the average current required is only about

10 mA at 3V, and the peak current is almost double

that). Since the output level depends directly on the

supply voltage however, you may wish to consider a

higher voltage supply and a 5V regulator connected to

keep the supply voltage constant. For the following

discussion, we are assuming that the supply voltage is

regulated at 5 volts.

Once the power supply voltage is determined, the

values of the summing resistors (R1 to R4) are then

chosen using Table 1 as a starting point. Depending

on the accuracy desired, some adjustment of these

resistor values may be required, but for many

applications, you may find the results to be acceptable.

The oscilloscope traces shown in Figures 2 and 4

were obtained from a circuit that used standard 5%

carbon resistors with values as shown in Table 1

(there was no trimming of resistor values, or the

oscillator frequency).

Figure 1 shows a switch connected to pin 4, but if

you only require one type of output signal, pin 4 can be

One element of such a circuit that should be

considered if you are building it, is the use of short

ELM304

ELM304DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

Figure 1. Typical Circuit Configuration

Table 1 Typical Resistor Values

R1

R2

R3

R4

330

Ω

330

Ω

220

Ω

470

Ω

680

Ω

680

Ω

430

Ω

150

Ω

3V

5V

0.1µF

27pF

3.58MHz

27pF

R3

R4

R2

R1

75

Ω (unbalanced)

output signal

Switch closed for

gray bars, and open

for a white screen

1

2

3

4

8

7

6

5