VK1SV/VK2COW home page Very simple QRP transmitter The PA is a 2N7000 FET, in Class-E, producing about 1400mW from a 12V PSU. The oscillator is a standard xtal oscillator built using NOT gate (74HC04) and the output drives directly the gate of the 2N7000. The key is a door interlock switch (thanks Glen VK1XX). The LPF is 5-pole Chebyshev made with NP0 caps and moulded RF chokes. The efficiency is very good. There are two xtal sockets, one for FT-243 xtals and one for HC6. There is a capacitor inserted in the HC6 xtal socket in an attempt to lower the 7.030 xtal frequency a little bit in order to have a QSO with Grant, VK4JAZ, who was on 7.028 or so. The xtal installed is a tiny HC49 half height xtal in a FT-241 case. Oscillator This is the circuit of the oscillator. There is nothing novel about this well published circuit. Capacitor values can be adjusted for the available crystal, values close to the ones in the schematic should be a good starting point. I have not found a crystal in the 2 MHz to 20 MHz range that did not oscillate reliably in this circuit. By the way, there is nothing tuned to the 40m band in this circuit, so it can be used as the oscillator for any band you have crystals for. The IC used is a hex inverter 74HC04 (gates A1 to A6). The input capacitance Ciss of the 2N7000 is quite low and can easily be driven by a single gate (A2), so you may decide not to parallel gates A3 to A6. In the built transmitter, there is a 78L05 voltage regulator providing 5 volts to the 74HC04. 5 V seem to be just enough to drive the 2N7000 to a sensible degree, possibly a higher voltage could increase the efficiency a little bit. I have seen suggestions of driving the 74HC04 with higher than 5 V, however I found that this is not necessary. PA and LPF schematic 12 V give about 1.4 W output, after the LPF. The component values are a compromise so that standard E12 resistor and capacitor values can be used. L4 is a high current inductor (1 A) purchased from eBay, however a ferrite toroid and enough turns to give about 68 uH inductance, will also work very well. C7 is a bypass capacitor. L1, L2 and L3 are standard RF chokes on ferrite cores. Higher power can be obtained by increasing the voltage, however ensure that the Vds does not exceed 60 V (the limit of the 2N7000) and also the heat dissipation of the little FET is within limits. By the way, the LPF can be used in any 40m TX project. Here is the frequency response as computed by LTSpice: Keying I found that the oscillator starts very quickly and there is no problem if both the PA and the oscillator are keyed at the same time. While there is no measures taken in this simple circuit to reduce key clicks, given the low power, the result is acceptable. 73 de VK2COW, Dimitris
