My New DDS MEPT

I haven't blogged lately because I've been working diligently on a new mept based around the DDS2 kit from N3ZI.  The DDS2 has been combined with a 5W amp from W8DIZ and a few other circuits to yield a QRSS mept which works from 160 thru 10 and can put out several Watts if needed.  I particularly want the extra power to use on 80 and 160 during the upcoming dx season.

Figure 1 shows the completed project which is mounted in a homebrew cabinet made from 1/4 inch (6mm) plywood.  The dimensions were chosen to fit in my wooden mini-rack which I built some years ago when I was into digital modes and had modems, controllers, etc to bring together.  The main components are:
 the DDS2 vfo,  a 20 dB preamp,  the W8DIZ 5W amp, a LED voltmeter and a lowpass filter assembly out of my original TS-440 which was killed by lightning in 1989.  The lpf filter selection is via a 12 V, 6-position switch and contains a forward/reflected power circuit which is easily read by the LED voltmeter.  The VM is based on a National LM3915 logarithmic display driver.

Figure 1.  Top View of Completed MEPT

The DDS2 was modified slightly by moving components to clear out a space around the crystal oscillator to anticipate styrofoam insulation for a temperature controller.  Also, the 7805 voltage regulator was moved off board to allow a thermal partition to isolate the heat sensitive components from the rest of the circuitry.  There are two external inputs for the DDS2, a RS232 line for programming and a keying line for fsk via the program QRS.  I am a real fan of QRS because it allows user-defined messages, has dfcw capability and has precise timing for image stacking work. The circuit recommended in the QRS help file was used to convert rs232 voltages to the open/short needed for fsk. 

The DDS2 output is about 0.25 V peak-to-peak into 200 Ohms (40 uW) and some amplification is necessary to drive the DIZ amp.  I used the simple circuit suggested by N3ZI which gives about 20 dB of gain which is more than adequate.

I added cooling fins to the final transistors of the DIZ amp, consisting of several square inches of 40 gauge copper sheet.  They get fairly warm when running 2W but I don't intend to operate above this level since the amp is intended for intermittent use.  Power level can be adjusted by a potentiometer which I moved from the board to the front of the cabinet for convenience.

The Low Pass Filter is a board from a TS-440 and has a six wire cable for switching the individual LPFs via a 6-position switch.  It also has circuitry for reading forward and reflected power.  This was certainly a boon to the project and I'm so glad I hung onto it over the years.  If you want to make you own LPF Dave, WA5DJJ,  shows you how at his web site.  His covers 160-10 and uses dip switches to select the individual filters.

One last item is a logarithmic voltmeter circuit complete with LEDs for measuring the fwd/ref power from the LPF board.  It's on a .75 x 2.5 inch board which I mounted directly on the front panel along with a spdt switch to select forward and reflected.

Figure 2 shows the front panel and mounting of controls and indicators.

Figure 2.  Front Panel and Rack Mounting

This is not intended to be a construction article but just an outline of how I assembled the parts.  If you want more detailed information send me an email.

What a pleasure it is to be able to move to any band and select a clear frequency.  The power control is way cool also, as is being able to see the power and check on reflected to make sure the antenna ain't misbehavin.  So far I haven't gotten around to adding temperature stabilization but with the thermal isolation provided by the wooden cabinet and the thermal partition it is stable enough for image stacking up to three or four hours.  My ultimate goal is to add an ocxo which should provide stability down to 0.01 Hz...such units are available on eBay for about $20.  Then again I might make my own.

You might have noticed the BNC at the lower left of the front panel.  This is an auxiliary vfo output which I want to use in rx experiments.

Finally, if you like the idea of measuring fwd/ref power  here is kit from W8DIZ  intended for QRP.  The LED display has been in my junk box for years so ur on ur own for this component.

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