2-1.           SENSITIVITY

Sensitivity is the capability of a receiver circuit to detect weak signals and the major factor in receiver sensitivity is the signal-to-noise considerations. Due to the resistance and temperature of various components, receiver noise is inherent in any receiver. In lab measurements, the amount of signal input required to produce a signal to noise ratio of 10 dB is generally used to specify the receiver sensitivity specification.

In modern transceivers, this is typically a few tenths of a microvolt to a few microvolts depending on the input frequency, mode, and bandwidth of the receiver. Another figure of merit used in determining sensitivity is the noise floor, which is another way to express the receiver noise. Typical values are  –130 dBm to –140 dBm depending on the mode, filtering and preamplifiers used.

However atmospheric and man-made noise enters into the real environment so that the minimum required sensitivity is something quite different than the lab measurement. On the lower bands the noise can be quite heavy, an S-Meter reading of S7 of noise is not uncommon. So even though the receiver has excellent sensitivity, it is unusable in the presence of atmospheric and man-made noise. Using well-designed DSP units and low noise antennas (beverages and loops) are necessary, particularly on the lower bands. Increased sensitivity is gained at the expense of dynamic range, the latter being of greater importance in today’s crowed bands and the noise, both atmospheric and person-made noise.

Discerning weak signals generally requires a signal to noise ratio of 10 dB. The noise is a combination of atmospheric noise, receiver noise and circuit design. Note that many CW operators can typically copy code at a signal to noise ratio of almost 0 dB perhaps accounting for the superiority of CW over phone under minimal signal conditions.

2-2.           DYNAMIC RANGE

Dynamic range is expressed in dB where the lower limit is the smallest discernable signal (receiver noise floor) and the upper limit is the point where intermodulation products become noticeable. It is an important specification as it gives a figure of merit for evaluating the strong signal handling characteristics of a receiver. Values of 103 dB are typical. The use of front-end attenuation and an AIP circuit  can help reduce the effects of intermodulation. The receiver noise floor can be affected by receivers using synthesized tuning schemes. This has improved considerably with the new transceiver designs.

50 pages in Chapter 2.