Notes on MDS
Minimum Discernible Signal can be defined as a signal that produces an output equal to the receiver's internally generated noise. It is often referred to as the "noise floor."

Theoretically, MDS is easy to measure (although see the caveats below). First, you connect a signal generator and attenuator to the receiver's antenna port, and a true RMS voltmeter to the receiver's AF output. Then, you measure the receiver's internal noise on the AF voltmeter. Finally, you increase the signal generator until you see a 3 dB increase on the meter. The signal generator's net input to the receiver is equal to the MDS, expressed in dBm.

MDS has two purposes: (a) as a free-standing measurement of a receiver's sensitivity, and (b) a "baseline" that is used in the calculation of other performance indicators.

The lower the MDS, the more sensitive the receiver. However, it is important not to make a horse race out of MDS, for these reasons:

MDS is based on two difficult measurements: the intrinsic noise level of a receiver (when no external signal is present), and the level of an external signal that is only 3 dB above the noise. The problem is that noise is randomly variable. This is is dramatically illustrated by watching the needle in an AC voltmeter bounce around with noise measurements. Even an experienced lab person ends up "guesstimating" an average reading.

MDS is influenced by the bandwidth of a receiver. In general, the narrower the bandwidth, the more the MDS improves. The ARRL attempts to make all MDS measurements at a bandwidth of 500 Hz, and so do we. However, many filters that are nominally 500 Hz are considerably narrower or wider. The problem is even worse with simple, lower power equipment, in which there was no attempt to build a 500 Hz filter in the first place.

Most of the time on HF, external noise is considerably higher than the internal noise of a receiver. In that case, a receiver's performance is determined not by its own sensitivity, but by atmospheric and man-made noise. In fact, it is possible for a receiver to be too sensitive (especially for 14 MHz and below), in which case it ends up excessively amplifying external noise.

MDS data for more sophisiticated receivers tell only part of the story. For example, the Yaesu FT-1000MP has three preamps, two of which are "tuned" for the specific sensitivity requirements of each HF band. The standard MDS measurements capture the performance of these preamps only at one or two frequencies.