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Amplitude Modulation of the AD9850 Direct Digital Synthesizer by Richard Cushing, Applications Engineer

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Amplitude Modulation of the AD9850 Direct Digital Synthesizer by Richard Cushing, Applications Engineer AN-423 a APPLICATION NOTE ONE TECHNOLOGY WAY • P.O. BOX 9106 • NORWOOD, MASSACHUSETTS 02062-9106 • 617/329-4700 Amplitude Modulation of the AD9850 Direct Digital Synthesizer by Richard Cushing, Applications Engineer This application note will offer a method to voltage con- The voltage at the RSET pin is part of the feedback loop of trol or amplitude modulate the output current of the the (internal) control amplifier and must not be exter- AD9850 DDS using an enhancement mode MOSFET to nally altered. The RSET modification circuit, Figure 2, replace the fixed RSET resistor; and a broadband RF uses Q1 as a variable resistor and R2 as a fixed current transformer to combine the DDS DAC outputs to pro- limit resistor in case Q1 is allowed to turn on too much. duce a symmetrical AM modulation envelope. Modula- C1 inhibits noise when Q1 is operated near cutoff. R1 tion with reasonable linearity is possible at rates lowers the input impedance for additional noise preven- exceeding 50 kHz. The AD9850 DDS output current tion. The input voltage to Q1 required to fully modulate (20 mA maximum) is normally set with a fixed resistor the AD9850 output is approximately 1.5 volts p-p and from the RSET (Pin 12) input to ground. The DAC outputs is dc offset by approximately 2.3 volts, see Figure 4. are unipolar and complementary (180 degrees out of TO RSET DC OR AUDIO phase) of each other. D C1 PIN 12 INPUT Q1 510pF Use of an enhancement mode MOSFET is in keeping G 2N7000* with the single supply concept. The design is simple and R1 S 50Ω parts count minimal. Combining the IOUT and IOUTB DAC R2 *N-CHANNEL MOSFET 1.5kΩ outputs in a center-tapped broadband RF transformer DIGI-KEY PART # 2N70000DICT-ND produces a symmetrical modulation envelope as seen in Figure 1(A). Figure 1(B) shows the effect of not combin- Figure 2. RSET Modification ing the two outputs and simply observing one output— asymmetrical amplitude modulation. The same signal was used to modulate both configurations. +0.55V +1.1V GND A B –0.55V GND Figure 1. Symmetrical (A) and asymmetrical (B) amplitude modulation envelopes Figure 3 illustrates the combining of the DAC true and Presented below in Figure 4 are 10 kHz modulation en- complement outputs using a 50 kHz to 200 MHz broad- velopes and their associated input signals to the gate of band RF transformer. The transformer performs a 1:1 Q1. impedance and voltage transformation. The load resis- This method of amplitude modulation presents an easy tance on the secondary winding, 50 ohms, is reflected solution to voltage control of the AD9850 output ampli- back to the primary. Since the transformer primary is tude. Designers interested in controlling only a single center-tapped to ground, each DAC output will see a output can eliminate the transformer. Finally, those per- 25 ohm load and develop a maximum of approximately sons desiring digital control of the AD9850 DAC ampli- 0.5 volts p-p (20 mA * 25 ohms = 0.5 V). The two DAC tude may wish to consider digital potentiometers in the outputs are summed and appear at the transformer sec- 50 k ohm to 100 k ohm range. ondary as approximately 1 V p-p. To IOUT PIN 21 T1-1T* 3 4 E2713–10–12/96 2 50Ω 1 6 1:1 To IOUTB PIN 20 *MINI-CIRCUITS BROADBAND RF TRANSFORMER Figure 3. Output Modification 200mV 200mV 200mV /DIV /DIV /DIV GND GND GND +3V +3V +3V 500mV 500mV 500mV /DIV /DIV /DIV GND GND GND Figure 4. 10 kHz Modulation Envelopes PRINTED IN U.S.A. –2–.
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