I LOVE LULU
I have been happily turning mountains of electronic parts into scrap for nearly 50 years. The quest for a well-performing, easy to build transmitter has greatly added to my slag heap. After much pleasure and pain, I am pleased to present to you, Dear Reader, a transmitter that satisfies both criteria. I offer you LULU… Back around 2000, Norberto, an Argentine ham, put together several existing concepts and came up with the basic LULU design. The LULU name is an affectation that came about in our builders group. It is easier than calling it “an LU8EHA variant” as it was known for awhile. We have made several changes to Norberto’s original design, but he would certainly still recognize it.
The heart of LULU is a 74HC240 octal buffer chip, which functions as an oscillator and driver. Its gates are tied together to increase its ability to sink and source current. Our active device is a MOSFET, which is turned on and off by swings of the input voltage. We will be powering our buffer chip beyond its design limits, from an 8 volt regulator. This begets a larger output voltage swing to the MOSFET than if we were playing by the manufacturer’s rules and powering the chip with 5 volts. The chip acts as a digital driver, switching the MOSFET on and off, with the RF waveform being rendered in the drain circuit. This digital method uses less parts, costs less, and generates less waste heat – advantage, LULU.
At a basic level of understanding, MOSFETS are electronic switches. If a perfect switch is turned on, zero voltage appears across it and maximum current flows through it. If turned off, maximum voltage (the supply voltage) appears across it and minimum (no) current flows through it. Unfortunately, semiconductors aren’t perfect switches. The voltage and current are always a bit out of phase, due to the MOSFET’s input and output capacitances which fight voltage changes. These phase differences cause losses which manifest as heat. Switch mode amplification has been developed to eliminate most of these losses. LULU uses Class E amplifier topology to greatly enhance signal flow through the MOSFET.
The Oscillator/Driver is the heart of LULU and you must have a properly functioning one before moving on. I use the ubiquitous Radio Shack breakout board (RS 276-159B) method, as shown above. The view is looking down on the components and jumpers side, but you “see” the copper tracing through the board, kinda like you were from the planet Krypton. You can use either the HC or HCT family of 240 chips. Keep in mind that we are running this chip very hard. I use Mil Max SIP machined pin sockets because they allow air to flow underneath the chip, keeping it cooler. This little dude will get quite warm. I suggest that you use a VOM meter to chirp test all connections before plugging the 240 chip into the socket. I usually find at least one continuity error every time I build this board. When completed, power up and check for your signal with a nearby receiver set to your frequency. If you find that the oscillator is sluggish, misfires, or won’t turn on at all, adjust trim cap C2. If there’s still no joy, try soldering some extra capacitance across the trimmer. You MUST have a well-functioning unit before moving on. If you have an oscilloscope, you should see about 12-14 Volts p-p output at C3. Be patient and do this one right. Give LULU a good heart…
Constructing LULU’s final is straightforward. The given C8 value is nominal – it will get you into the ball park. C6 is not sacrosanct. Three 0.1uF (104) discs in parallel should work fine, but keep the value in the same zip code for best results. All capacitors after the MOSFET drain (RF output and modulated supply circuits) must be at least 100V or better. All resistors can be ¼ watt. All coils are wound with #18 or 20 AWG.
If you choose not to tweak, LULU will still work for you, but in Class C mode. This means that your output power will probably be less and your final will certainly run hotter. If you operate in this fashion, you MUST heat sink well. A fan might also be needed. Without tweaking, you should still see about 10 watts at the output. However, to unlock LULU’s real potential, you’ll have to tweak. The exact value of C8 will vary for differing IRF510s. Keep in mind that in Class E operation, the class E point does not always correspond with max output. Your efficiency tester will be your finger. You should be able to comfortably keep it on the MOSFET – even for 5 minutes or more. If you can’t, then keep tweaking. I often use a Mica trimmer (5-100pF, 300V) in place of C8 and tune for max output. Then finger test. If it runs too hot, try tweaking the capacitance a bit in each direction. You’ll eventually find a spot, near max output, that will also keep your finger happy. Measure the trimmer’s capacitance and replace it with a fixed cap. If you have a dual trace oscilloscope that will render well at 7 MHz, the MOSFET’s digital drive voltage at its gate will be “on” when its drain voltage is “off” and vice versa, more or less. This shows the almost perfect switch in action – maximum current through it when it is on and maximum voltage across it when it is off. IRF510s vary widely when used for RF. I have always coaxed at least 16, and sometimes as much as 22 watts out of them in this circuit.
For CW operation: Feed the modulated voltage input from the 12 volt supply and insert the key between pin 1 of the 240 chip and ground. This will keep the oscillator gate running full time, while keying the driver gates on and off.
For VFO operation: This input is included for frequency agility. If you use an outboard frequency source, its output should be about 8Vp-p, to keep the 240 happy. If unwanted, eliminate the switch and R4, and attach pin 11 directly to the crystal/R2 junction.
A switch mode transmitter like LULU is inherently narrow banded. Operation should be limited to about 50 KHz either side of your tweaking frequency.
So build patiently and I hope you have some fun putting LULU together. I will discuss AM operation in a later post… 73!