There are four basic forms of diversity circuitry in use today, and numerous variations on these basic designs. All have advantages as well as disadvantages depending on the quality of the receiver circuitry.
This type of diversity utilizes three or more antennas in widely separated areas. In this approach there is little chance of a phase cancellation occurring at all three locations simultaneously. However, since all three antennas are connected to a single input, they can act as an array. In addition, while this approach solves phase cancellation, the sum of the three antennas is never as good as the best signal alone, so there is always a compromise in signal quality. This approach involves a lot of cable and splitters making the design time-consuming to install. While inexpensive, it is rarely used anymore due to the compromises.
This is the most common form of diversity in use today for wireless microphone systems. This approach originally came from radio use during World War II. During war time, the reliability of the communications was a constant concern, not only from a reliability of the radio signal from phase cancellations and multipath, but also from a logistical standpoint where the radio men were often placed in great danger of being blown to bits, radio and all. Radios placed in separate locations often solved both problems, so the duplication of resources became very widespread. This same principle is used today in wireless microphone receivers although the circuitry to accomplish this task has become far more so 1940s.
In contemporary wireless systems, two antennas and two complete receiver sections are used, along with a comparator circuit which monitors the receiver with the strongest signal. When one receiver begins to lose signal, the comparator switches the audio to the other receiver. Although this method can be effective, this type of design can also have problems from receivers that are not properly matched. This imbalance usually leads to noise from one of the receivers or from the switching network. These designs are expensive to produce because two receivers must be made, and consequently, the designers can compromise on important circuitry areas such as RF & IF filtering in order to keep the price competitive, especially in low cost systems. Another probable area of concern is that each receiver can change with time, and over the long run can change from the other, further compromising the performance of the system.
There are several variations of this basic design on the market today. Some of these include soft switching between receivers to eliminate the harsh transients that can occur when a poor signal is suddenly replaced by a good signal. Either type of design can be effective provided the receivers are matched, and the RF & IF filtering is not compromised.
Antenna switching diversity uses a single high-quality receiver, along with two isolated antenna inputs fed to a signal strength comparator. When the signal begins to deteriorate in the primary antenna, the comparator selects the other antenna to try to improve the signal strength. Since this diversity technique only requires one receiver, the designer will often make a receiver with superior RF & IF filtering for the same given price point as the twin receiver diversity. This single high quality receiver will often provide superior performance in sensitivity, selectivity and IM rejection, however, the antenna switching circuitry switches blindly regardless of whether or not the current signal is the strongest of the two. The result can be a switch to a weaker signal than the first which adds noise, and possible drop-outs into the signal path.
This patented type of diversity utilizes two antennas spaced an optimum distance apart, which are connected into a single high-quality receiver. The antenna signals are connected internally to microprocessor logic circuits that monitor the phase relationship between the two antennas. Both antennas are active at all creases the signal strength under normal conditions.
In the event of a signal interruption from a partial phase cancellation (multipath) or total phase cancellation (drop-out) the logic circuitry adjusts the phase of one of the antennas to a positive condition relative to the other which instantly corrects the phase can second, and continually adjusts the phase of the antenna for the most optimum signal. A similar patented technique is used in cellular telephones to insure their reliable operation. This type of diversity is very effective and less costly than switching diversity because only one receiver is built. This approach allows the manufacturer to concentrate on the more important aspects of receiver design such as filtering and audio circuits which generally yields superior performance over switching diversity designs.
This technology is patented which allows only one manufacturer to build this design. The disadvantage to this design however is that if the phase logic circuitry is not designed properly, the system can be noisy when the circuitry adjusts the phase relationship.
It's important to note that any diversity circuitry can improve the usable range of a wireless microphone system provided the antennas are properly located, and the balance of the receiver circuitry is not compromised. The diversity circuit itself does not increase the range but rather it makes the effective range more trouble free. Another important fact is that none these circuits are clairvoyant.
All forms of diversity except the antenna diversity technique must see some degradation of the signal before the circuitry will react to correct the phase cancellation. Most well designed diversity circuits regardless of the design are effective, and the actual performance of the receiver is affected far more by receiver RF & IF filter design which is discussed next. The thing to watch here is any manufacturer that tries to discredit another manufacturer's designs. In practical application, the advantages and disadvantages mentioned will generally show-up by simple range and audio quality tests.