some circumstances that there will be a balance between the degree of rejection of unwanted signals and the loss of the filter. Generally the greater the rejection of unwanted signals, the more stages are required in the filter, and the greater the in-band loss. The final rejection of the unwanted signals is another important parameter. Any filter will only be able to reject signals outside the pass- band by a certain amount. Normally the figure for the final rejection is taken SO that any ripple in the stop-band does not bring the response Receivers 169 above the figure for the final rejection. When designing and constructing a filter it is necessary to ensure that there are not any leakage paths between the input and the output of the filter. Sometimes there may be a small amount of capacitance between the two connections and this can allow signals to leak across the filter, thereby reducing the level of the final rejection. Sensitivity and noise One of the most important aspects of any radio receiver is its sensitivity, or ability to pick up weak signals. Most receivers today are very sensitive and able to pick up signals which are less than a microvolt at the input to the set. Unfortunately a set cannot be made more sensitive by simply adding more stages of amplification as a number of other aspects of receiver design quickly become obvious and limit the sensitivity. The first is that if there is too much gain then some of the stages in the set become overloaded leading to other problems which will be described later. The other problem is that of noise and this is the major limiting factor. There is a certain amount of noise which is present at the input to the receiver, and each stage within the set adds a little more. Noise consists of random electrical impulses and is present at all frequencies, although the levels and sources vary with frequency. In many cases it is 'white noise' which is heard as a background hiss and it is present at all frequencies. Once it is present it cannot be removed or cancelled 