The previous parts of the "How not to design transmitters and receivers" series have described all the design decisions and blocks which need to go together to produce a 1 Watt wideband FM transmitter. It is now time to start putting those blocks together into a full-blown design.

First up is the 1 Watt transmitter chain itself, composed of:

• a voltage controlled oscillator (VCO) as described in part 1
• a buffer amplifier which is not totally dissimilar to those described in part 6,
• a matching circuit between the buffer and the power amplifier, as described in part 3,
• a 1 Watt power amplifier, based on a device identified in part 2, and
• a low-pass filter, as described in part 9.

The circuit diagram below shows these elements put together (click on the image for a larger version).



The VCO should be set to oscillate at half the wanted output frequency. The '2f' component of this is then passed to the buffer which is formed of the 2SC3356. The 22pF capacitor between them is not critical. Other transistors which can produce around 100mW of power would be equally suitable in the buffering role, such as the MPSH10. The output of the buffer is fed into the power amplifier and then through a low pass filter. The output transistor (the MRF555) can also be replaced by different devices, however the matching network between the buffer and the output stage may require alteration.

No phase locked loop (PLL) circuitry is yet shown, this will follow in an ensuing post. As illustrated, the design is called a 'free running' oscillator meaning that its output frequency is set only by the control voltage applied to its input, the value of the centre-tapped inductor in the circuit, and the variable capacitor which sits across it. This design is relatively stable and after settling down has an accuracy of better than a few kHz at an output frequency of 100 MHz, as long as the control voltage remains constant and the temperature isn't allowed to drift too much.

In operation, the BF451's should draw around 10 mA each, the 2SC3356 (or equivalent) should draw around 15 mA and the output device in the region of 150mA. If constructing a prototype based on this schematic, ensure that good radio frequency (RF) construction principles are adhered to, in particular:

• The leads of all components should be kept as short as possible, and certainly not be allowed to exceed 1 cm in length. This is because a simple piece of wire will act as an inductor at RF and introduce unwanted reactance to the circuit.
• Ideally, the circuit should be built over a solid ground-plane which acts as the ground connection. A piece of un-etched copper-clad printed circuit board (PCB) material will do the job handsomely.

Note that the 10V supply which feeds both the VCO and the buffer should be regulated using, for example, one of the designs discussed in part 4.