DDS-60 Daughtercard

A 1-60 MHz coverage VFO with built-in amplifier and variable output level from 1 to 4V p-p.

 |  Overview  |  Schematic  |  Ordering  |  Availability  |  Ways to Use  | Assembly & User Manual  |  Quick Assy Guide  |  Builder's Notes  |

Resources

Article:  "Working with Surface Mount Technology (SMT) Parts"

Tech Topic:  DDS-60 Spectral Purity Study

Tech Topic:  DDS-60 Temperature Analysis

DDS-60 Dimensions

Tech Topic:  DDS-60 Design & Application Discussion on CWTD
(Web page includes a 90-minute audio podcast and interesting videos from W2AEW and AK2B)

Overview
    A number of years ago we introduced the original DDS Daughtercard (www.njqrp.org/dds) to the QRP community, providing a low cost and modular way to add a precisely-adjustable 0-30 MHz signal generator to one's project. All you had to do was add an AD9850 DDS chip and +12V, and then any number of popular microcontrollers could control the DDS Daughtercard to have it serve as a rock-solid VFO.  Over 1,000 of these little cards were sold!

    Well, there is still a demand for this inexpensive card and an ever-growing list of uses for it so we updated the basic design and are now ready to provide the new_and_improved  DDS-60 Daughtercard. This self-contained functional module generates a good-quality RF signal from 1-60 MHz by using a small pc board to contain just the bare DDS essentials – an Analog Devices AD9851 DDS chip, a clock oscillator, a 5th-order elliptic filter and an adjustable-level RF amplifier.  Additionally, an onboard 5V regulator is provided so you only need provide a battery or power supply ranging anywhere from 8-12V DC. The three digital control lines, the power supply, and the output signal are all available on a pin header at the board edge, and the DDS-60 is pin-compatible with the original DDS Daughtercard.  The schematic is shown below on this page.

      The 8-position pin header at the board edge serves to allow DDS-60 to be plugged into and used in any project you might have on your bench, regardless of which microcontroller is employed. Just provide a single strip socket (e.g., a 16-pin IC socket split lengthwise) on the project board and plug in the DDS Daughtercard. Heck, you don’t even need a dedicated microcontroller – use a cable connected to the parallel printer port of your PC and use public domain PC software to control the DDS board!  See the Ways to Use section for a number of custom solutions for you to easily control your DDS-60 daughtercard.

      Once your controller-of-choice serially loads the 40-bit control word into the DDS, the raw waveform is presented to an elliptic filter that removes unwanted high-end frequency components, resulting in a signal of sufficient quality to serve as a local oscillator for a transceiver. We regularly see great signal quality, with harmonic content reduced to about –40 dB.

        The signal generated by the DDS chip itself is quite small so we use an AD8008 low power amplifier to provide about 18 dB of gain to boost the signal to almost 4V p-p, which is 40 mW or about +16 dBm into a 50-ohm load ... quite usable in a variety of applications. The amplifier offers unconditionally stability (k>1) and yields spectrally-clean signals with harmonics down about 40 dB from the fundamental. It is an ideal signal source for serving as a VFO in the HC908VFO, as a measurement source in the Poor Ham's Scalar Network Analyzer (PHSNA), or for making impedance measurements in the Micro908 Antenna Analyst and other demanding designs. A trimpot allows precise setting of desired output levels.

The amplified signal is then presented to P1 pin 6 on the pin header where it can be used as a 50-ohm source input signal. If not used as an input to any other component or module, the output should be terminated with a 50-ohm resistor in order for the stated specifications to be realized.

Assembled & Tested DDS-60 card uses 90-degree pinheader for P1
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