 they do not illustrate the principles of the circuits themselves. Instead circuits using discrete components generally lend themselves better to illustrating the principles. RF amplifier and mixer The first stages that the signal enters after reaching the input to the receiver are the RF tuning, oscillator and mixer functions. In many portable sets a single transistor performs them all. These circuits are very cost effective to produce, requiring far fewer components than if the circuit was made up by separate circuit blocks. Naturally the perform- ance of the circuit cannot be optimized to give the best performance, but it is normally quite adequate for many of the lower cost sets available on the market. For higher performance sets high grade mixers are used. There are a number of parameters that are of importance when considering mixer performance. The first is the conversion gain or loss. This is the ratio of the signal level at the RF signal input to the output at the intermediate frequency port. The noise performance is also important as will be described later. As mixers are often located near the front end stages of the set they have a large bearing on the sensitivity and noise performance of the whole set. The isolation between ports is also specified for many ready made mixers. This figure represents the amount of signal from one port that finds its way onto another. The most important are LO to IF and LO to RF input. In a receiver the output frequency is sufficiently different to the local oscillator frequency to enable the leakage signal to be removed by filtering, but in the case of a transmitter this is not always as easy. In the case of the leakage to the RF input, this may cause an unwanted signal to be radiated by the set, and this may cause interference to other nearby users. A number of circuits can be used as mixers. Diode ring mixers are popular in many high performance applications. Although they give a conversion loss, typically of around 7 dB, other parameters in their performance are nor