DDS Amp Kit
A clean, adjustable RF amp for DDS Daughtercard owners
Description | Schematic | DDS Amp Kit Instructions | Ordering
The DDS Amp is a "grand-daughtercard" that is used in place of the MMIC amplifiers (MAR, MAV, ERA) provided previously on the DDS Daughtercard. Once you assemble this little kit and install it onto the DDS card, you will be able to ...
Adjust the output level with a small SMT trimpot on the board to get the desired output levels into your project;
Experience a very flat output signal level from the DDS card, which is required in getting consistently good calibration and measurement results in some applications; and
Have output signal spectral purity close to -40 dB.
DDS Amp mounted on DDS Daughtercard
DDS Amp (top side) DDS Amp (bottom side)
Alert: Be sure to mount positive end of C6 as shown in above (right) photo.
Banded end should be toward the inside of the board.
Although the little board is packed with 1206-size SMT components, assembly is simple and straightforward. We had initially considered us of the single-amp LT1252, but in the production version we went to using the dual version of this neat little video amp. As Jim Kortge explained during test and evaluation, the LT1253 provided a much better balance of gain/bandwidth in the circuit.
The DDS Amp circuit is based around a dual video amplifier IC (LT1253) in an 8-lead SMT-package that we fit onto a small 1" x 1/2" pc board. The board sits on the end of the DDS Daughtercard when the output amplifier components are removed and conveniently connects into the circuit with short wire jumpers. The amplifier is unconditionally stable (k>1) and yields spectrally-clean signals with harmonic energy 35-40 dB down from the fundamental, yielding the ideal signal for impedance measurement. We've added a trimpot in the feedback path to allow precise setting of +10 dBm output levels being delivered to the reflectometer for best performance. This amplifier design provides an appropriate signal using supply voltages from 16V down to 8V, thus still allowing for convenient battery operation without the need for that extra 8V regulator on the DDS Daughtercard.
All in all, the DDS Amp is a great addition to the DDSDaughtercard. It provides for outstanding measurement results from the Micro908 and also works well in the QuickieLab, MS-DDS, and IQ-VFO projects. We've already delivered DDS Amp kits to all Micro908 owners for incorporation on all these instruments in the field, and results have been superb.
Thanks to Jim Kortge, K8IQY and Joe Everhart, N2CX for the design of this circuit. It's a great improvement to the DDS Daughtercard.
DDS Amp Kit Schematic
Schematic for the DDS Amp amplifier, used in place of the MMIC-based amps originally used on the DDS Daughtercard. The 1K/1K voltage divider establishes a virtual ground that lets the DDS Amp deliver adequate signal right down to an 8V "low" battery voltage. The trimpot allows setting of the best level into the end application.
Spectrum analyzer display shows harmonic energy more than 35 dB below the 10 MHz fundamental. This is a characteristic essential for good and accurate readings to made by the Micro908 Antenna Analyst and other demanding applications.
Photo by Doug Wilson, WA0SVL
Photo of the DDS Amp in place on the DDS Daughtercard. The pcb is positioned atop the DDS Daughtercard in place of the MMIC circuitry. Electrical tape insulates the pcb from the DDS Daughtercard and wire jumpers conveniently connect and hold the Amp to the DDS card. Amplitude can be adjusted from 1.6V p-p to about 3.5V p-p using the onboard SMT trim pot.
Sorry, this kit is now retired.
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Page last updated: May 24, 2006