Is it possible to create a QRP transmitter using little more than a CMOS logic chip?

Q Is it possible to create a QRP transmitter using little more than a CMOS logic chip? It would seem that you could get one to sustain an oscillation and generate RF at HF frequencies.

A It is indeed possible! Lew Smith, N7KSB, designed a 1/2 watt CW transmitter for 10, 15 and 20 meters and it has been featured in The ARRL Handbook. The transmitter design is based on a 74HC240 octal inverting buffer. One section acts as a crystal oscillator (with a crystal for the desired fundamental frequency) while four sections amplify the signal (see Figure 3). The 74HC240 can get fairly hot in this application, so you must attach a heat sink. The supply voltage must also be held to a maximum of 8 V to help ensure the survival of the chip. Finally, this transmitter can be a prodigious harmonic generator, so you must use an output filter composed of capacitors C1 and C2, as well as inductors L1 and L2. See Table 1 for filter component values. L1 is sensitive to lead length, so keep its leads as short as possible.



Figure 3—The 74HC240 transmitter designed by N7KSB.



Table 1 Output Filter Component Values for the 74HC240 Transmitter

From QST December 1999