A Clean, Adjustable RF AMP for DDS Daughtercard Owners or for Standalone Use!
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.
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.
Why provide the DDS Amp?
Some of the original MMICs used in the DDS Daughtercard kits have not been able to produce enough output level to drive the mixers or other circuits needing the VFO signal. Further, some users of the later DDS Daughtercard kits containing the ERA-3 had experienced problems with parasitic and UHF oscillations occurring in the ERA-3 circuits. Although many daughtercards proved to be stable, some in the field could not be stabilized enough, even by adding capacitive or inductive filtering. The extraordinarily-high frequency response of the ERA-3 device (0-3 GHz) offered too much opportunity for unintended oscillation, and when this occurs it takes away from the power of the fundamental frequency and introduces massive harmonic energy to the test signal being generated by the DDS Daughtercard. The transmitters and reflectometers using this DDS signal could not tolerate this level of instability.
Got Good Results Already? … No Need for DDS Amp!
If you are currently experiencing good results with your DDS Daughtercard and are happy with its performance, especially when using the original MAR-1 or the MAV-11 devices, there is no compelling reason to purchase and use the DDS Amp Kit.
Expected Signal Level Rolloff
The expected difference in signal levels between 1 MHz and 30 MHz for the AD9850 DDS chip is about 1.8 dB, which is attributed to sin(x)/x sampling theory. It is unavoidable (and uncompensated for) in the inexpensive AD9850.
To get optimum performance from the DDS Amp circuit (i.e., a “flat response” with only 1.8 dB drop off from min-to-max freq), the gain needs to be properly balanced between the two amplifier sections. Ideally this happens when the feedback resistors are set at 620 ohms for both sections, as shown in the Bode Plots done by Jim Kortge, K8IQY, located on the DDS Amp Analysis web page.
Nearly all the DDS Amp grand-daughtercards have been shipping with two resistors changed from the original design, thus putting the optimum input signal into the DDS Amp card and having the right amount of gain in the Amp sections. With the DDS card’s R1=5.6K and the DDS Amp’s R3=620 ohms, and the trimpot adjusted to about 620 ohms, the measured/documented signal level drop off from the DDS Amp matches the theoretical 1.8 dB.
If you have a lower level going into the DDS Amp (like when using a 10K resistor for R1), and you crank the timpot way up to get maximum voltage out before distortion occurs (you do check for distortion, right?), the gain-bandwidth product for the two-stage amplifier will not be optimum and you will experience a greater rolloff in signal level at the upper end That is, the amps will not be able to respond as well as frequency increases.
If you don’t have the 4.7K and 620-ohm SMT resistors, please let us know and we’ll get them to you. But for most applications, the signal level rolloff can be tolerated without experiencing any problems. (An exception is with the reflectometer in the Micro908 Antenna Analyst. The AA908 uses the DDS card and amp and needs the flatest possible response, so that’s when those two resistors are really needed.)
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 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.