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XR8038ACP

Precision Waveform Generator

Exar Corporation 

XR-8038A

SELECTING TIMING COMPONENTS

For any given output frequency, there is a wide range of R

and C combinations that will work. However, certain

constraints are placed upon the magnitude of the

charging current for optimum performance. At the low

end, currents of less than 0.1mA are undesirable because

circuit leakages will contribute significant errors at high

temperatures. At higher currents (1 > 5mA), transistor

betas and saturation voltages will contribute increasingly

large errors. Optimum performance will be obtained for

charging currents of 1mA to 1mA. If pins 7 and 8 are

shorted together, the magnitude of the charging current

due to RA can be calculated from:

I

+

R1·VCC

(R1 ) R2)

·

1

RA

+

VCC

5RA

A similar calculation holds for RB.

When the duty cycle is greater than 60%, the device may

not oscillate every time, unless:

1. The rise times of the V+ are 10X times slower than

RA@CT.

2. A 0.1mF capacitor is tied from pin 7 and 8 to ground.

NOTE:

- This is only needed if the duty cycle is powered up with

RA >>RB.

SINGLE-SUPPLY AND SPLIT-SUPPLY OPERATION

The waveform generator can be operated either from a

single power supply (10V to 30V) or a dual power supply

(+5V to +15V). With a single power supply the average

levels of the triangle and sine wave are at exactly one half

of the supply voltage, while the square wave alternates

between +VCC and ground. A split power supply has the

advantage that all waveforms move symmetrically about

ground.

The square wave output is not committed. A load resistor

can be connected to a different power supply, as long as

the applied voltage remains within the breakdown

capability of the waveform generator (30V). In this way,

the square wave output will be TTL compatible (load

resistor connected to +5V) while the waveform generator

itself is powered from a higher supply voltage.

FREQUENCY MODULATION AND SWEEP

The frequency of the waveform generator is an inverse

function of the dc voltage at pin 8 (measured from +VCC).

By altering this voltage, frequency modulation is

performed.

For small deviations (e.g., +10%), the modulating signal

can be applied to pin 8 by merely providing ac coupling

with a capacitor, as shown in Figure 8. An external

resistor between pins 7 and 8 is not necessary, but it can

be used to increase input impedance. Without it (i.e. pins

7 and 8 connected together), the input impedance is

8KW); with it, this impedance increases to (R // 8KW).

For larger FM deviations or for frequency sweeping, the

modulating signal is applied between the positive supply

voltage and pin 8 (Figure 9.) In this way the entire bias for

the current sources is created by the modulating signal

and a very large (e.g. 1000:1) sweep range is obtained

(f=0 at VSWEEP=0). Care must be taken, however, to

regulate the supply voltage; in this configuration the

charge current is no longer a function of the supply

voltage (yet the trigger thresholds still are) and thus the

frequency becomes dependent on the supply voltage.

The potential on pin 8 may be swept from VCC to 2/3

VCC-2V.

Rev. 2.01

10

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