Universal DDS Unit with AD9850 or AD9851

This DDS unit construction was inspired by articles in German ham radio journal Funkamateur (A. Lindenau: VFO mit DDS, LC-Display und Mikroprocessor-steuerung, Funkamateur, 11/97, B. Kernbaum: Einfache universelle DDS-Baugruppe, Funkamateur, 12/97 a 1/98).

AD9850 and AD9851 produced by Analog Devices are integrated circuits with very high degree of integration. They uses DDS technology together with very fast 10 bit D/A converter to produce fully programmable sine wave and clock generators.

Both generate high spectral quality sine wave which is programmable both by frequency and phase. Set up is done using 40 bit tuning word. Minimum frequency step is derived from clock frequency. At 125 MHz it is 0.0291 Hz.

AD9851 differs from AD9851 by programable internal multiplier of CLOCK (by 6).

The relation between clock frequency and tuning word is defined by equation:

    TW x CLKIN
f = -----------------


  • TW is 32 bit tuning word,
  • CLKIN is clock frequency.

The structure of the tuning word is:

bite meaning
0 to 31 definition of frequency, 0 bite is lowest, 31 bite is highest
32 and 33 control bites, always 0
34 Power Down (set to 0)
35 to 39 phase definition, 35 bite is lowest, 39 bite is highest (can leave 0)

The tuning word can be sent to DDS both by parallel and serial mode. When sent by serial mode the input pin is D7. Data prepared are written by signal WORD LOAD CLOCK (W_CLKand frequency is updated by signal FREQUENCY UPDATE (FQ_UD).

The disadvantage of the circuit is 28 pin SMD chip with 0.65 mm distance of pins. The price is around 25 US$. The standard application of the circuit usually requires lowpas filter at the output of the circuit. The formulas to set up that filter are:

L = m * R / (PI * f1)
C2 = m / (2 * PI * f1 * R)
C1 = (1 - m 1)/(4 * m PI * f1 * R)
m = SQRT[ 1 - (f1/fn)] where:

  • f1 is upper frequency,
  • fn is pole frequency,
  • R is filter impedance,
  • C1 parallel capacity,
  • C2 capacity to ground,
  • L inductance of the coil.

To control DDS unit by PC it was written programme in Borland's Turbo Pascal 7. It allows to set up output frequency from 1 Hz to 1/3 of clock frequency.It is connected to parallel port by five wires cable (D0 DATA, D1 W_CLK, D2 FQ_UD, D3 Enable and ground). The programme is using configuration file containing screen colours definitions, clock frequency definition and parallel port addres (decadic value).

To control DDS unit in ham radio transceiver it was created PIC16(C)F84 universal control unit. It can use both 3 button and two optocouplers (for example from computer mouse) control.

You need also include files.

List of components:

IC's: AD9850BRS, MHB4081, 2x78L05, PIC16F(C)84-04
Resistors: 4x10k, 6x1k, 3k9, 25, 51
Condensers: 4x1n, 2x100n, 33n, 10M tantal
Clock unit: up to 125MHz, AD9851 up to 180 MHz
Low pass filter: depends on clock frequency