Analog Devices AN-423 Application Notes

AN-423

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Amplitude Modulation of the AD9850 Direct Digital Synthesizer

by Richard Cushing, Applications Engineer

This application note will offer a method to voltage con­trol or amplitude modulate the output current of the
AD9850 DDS using an enhancement mode MOSFET to
replace the fixed R
transformer to combine the DDS DAC outputs to pro­duce a symmetrical AM modulation envelope. Modula­tion with reasonable linearity is possible at rates
exceeding 50 kHz. The AD9850 DDS output current
(20 mA maximum) is normally set with a fixed resistor
from the R

(Pin 12) input to ground. The DAC outputs

SET

are unipolar and complementary (180 degrees out of
phase) of each other.

Use of an enhancement mode MOSFET is in keeping
with the single supply concept. The design is simple and
parts count minimal. Combining the I
outputs in a center-tapped broadband RF transformer
produces a symmetrical modulation envelope as seen in
Figure 1(A). Figure 1(B) shows the effect of not combin­ing the two outputs and simply observing one output—
asymmetrical amplitude modulation. The same signal
was used to modulate both configurations.

resistor; and a broadband RF

SET

and I

OUT

OUTB

•

NORWOOD, MASSACHUSETTS 02062-9106

DAC

APPLICATION NOTE

617/329-4700

•

The voltage at the R
the (internal) control amplifier and must not be exter­nally altered. The R
uses Q1 as a variable resistor and R2 as a fixed current
limit resistor in case Q1 is allowed to turn on too much.
C1 inhibits noise when Q1 is operated near cutoff. R1
lowers the input impedance for additional noise preven­tion. The input voltage to Q1 required to fully modulate
the AD9850 output is approximately 1.5 volts p-p and
is dc offset by approximately 2.3 volts, see Figure 4.

DC OR AUDIO

INPUT

R1

50Ω

Figure 2. R

pin is part of the feedback loop of

SET

modification circuit, Figure 2,

SET

TO R

SET

C1
510pF

PIN 12

G

1.5kΩ

D

Q1
2N7000*

S

R2

*N-CHANNEL MOSFET
DIGI-KEY PART # 2N70000DICT-ND

Modification

SET

+0.55V

GND

–0.55V

A

B

+1.1V

GND

Figure 1. Symmetrical (A) and asymmetrical (B) amplitude modulation envelopes

Figure 3 illustrates the combining of the DAC true and
complement outputs using a 50 kHz to 200 MHz broad­band RF transformer. The transformer performs a 1:1
impedance and voltage transformation. The load resis­tance on the secondary winding, 50 ohms, is reflected
back to the primary. Since the transformer primary is
center-tapped to ground, each DAC output will see a
25 ohm load and develop a maximum of approximately

0.5 volts p-p (20 mA * 25 ohms = 0.5 V). The two DAC
outputs are summed and appear at the transformer sec­ondary as approximately 1 V p-p.

To I

OUT

PIN 21

To I

PIN 20

OUTB

T1-1T*

3

4
2
1

1:1

*MINI-CIRCUITS BROADBAND
RF TRANSFORMER

50Ω

6

Figure 3. Output Modification

Presented below in Figure 4 are 10 kHz modulation en­velopes and their associated input signals to the gate of
Q1.

This method of amplitude modulation presents an easy
solution to voltage control of the AD9850 output ampli­tude. Designers interested in controlling only a single
output can eliminate the transformer. Finally, those per­sons desiring digital control of the AD9850 DAC ampli­tude may wish to consider digital potentiometers in the
50 k ohm to 100 k ohm range.

E2713–10–12/96

200mV

/DIV

GND

+3V

500mV

/DIV

GND

200mV

200mV

/DIV

/DIV

GND

+3V

500mV

/DIV

GND

Figure 4. 10 kHz Modulation Envelopes

200mV

/DIV

GND

+3V

500mV

/DIV

GND

PRINTED IN U.S.A.

–2–