1.  Field of the Invention
This invention relates to a receiver and a space diversity receiver which are suitably applicable to a transmission and reception system of a wireless microphone used for news gathering for television and in a station studio. 
2.  Description of Related Art
A receiver of a transmission and reception system using a wireless microphone, which is used for television news gathering or in a station studio, is frequently used in such a condition that radio wave propagation is bad because it is used for television news gathering or in a station studio.  In such a case, a diversity receiver, particularly a space diversity receiver has been more frequently used as a receiver for a wireless microphone.  This is because the space diversity receiver can receive audio signals stably even if it is used under a relatively-bad radio wave propagation condition. 
Therefore, for receivers (containing a space diversity receiver) which are used in the above case, at least the following two points are required to be achieved in function:
(1) for a single receiver (non-diversity receiver), the variation in reception sensitivity in accordance with an used frequency band is required to be depressed, and
(2) for a space diversity receiver, even if there occurs dispersion in reception sensitivity between plural reception blocks which are used for diversity, it is required that no error occurs in selection of a reception block. 
The condition (1) is based on the assumption that a squelch circuit is built in the receiver, and it is used to enable a squelch operation to be operated at the same input level at all times irrespective of an input frequency. 
The condition (2) is required so that the reception block having the highest frequency input level of the plural reception blocks can be accurately selected at all times irrespective of an input frequency even if there is any dispersion in sensitivity between the reception blocks. 
The condition (1) will be first described hereunder. 
FIG.  1 shows a pass-band characteristic of a front-end portion comprising a high-frequency band-pass filter and a high-frequency amplifier in a receiver (which corresponds to each block if the receiver is a space diversity receiver, and a reception block A in this embodiment).  The abscissa represents a frequency, and its unit is "MHz" while the ordinate represents a pass gain, and its unit is "dB". 
In order to simplify the description, an extreme example is illustrated as the pass-band characteristic of this embodiment.  When the reception frequency band ranges from 794 MHz to 810 MHz, the loss of a high-frequency input signal having the lowest center frequency (carrier frequency) f1=794 MHz of high-frequency input signals is smaller than a high-frequency input signal having center frequency f2=802 MHz by 1.5 dB. On the other hand, the loss of a high-frequency input signal having the highest center frequency f3=810 MHz is larger than the high-frequency input signal having center frequency f2=802 MHz by 1.0 dB.
Accordingly, it is apparent from the above fact that the high-frequency level output from the front-end portion is varied in accordance with the input frequency irrespective of input of the same level of high frequency (radio frequency) signals in the reception frequency band. 
This means that a DC voltage obtained by detecting a high-frequency input level (hereinafter referred to as RSSI ("Receiving Signal Strength Indicator") voltage is varied in accordance with variation of the input frequency, so that the squelch operation level is dispersed.  This is easily understood from FIG.  2.  FIG.  2 is a graph showing a characteristic when the high-frequency input level is 15 dB.mu.,  and the ordinate thereof represents an RSSI voltage whose unit is (V) while the abscissa thereof represents a frequency whose unit is (MHz). 
The graph of FIG.  2 represents the case where the high-frequency input level, that is, a desired squelch operation level is set to 15 dB.mu.,  and the RSSI voltage is equal to 1.80V for f1=794 MHz, 1.50V for f2=802 MH and 1.30V for f3=810 MHz.  Accordingly, the RSSI voltage is varied in accordance with the frequency input level. 
The condition (2) will be next described hereunder. 
FIG.  3 is a graph showing variation of RSSI voltage in accordance with a reception input level for a pair of reception blocks (A) and (B) having the same construction. 
The relationship between the high-frequency input level and the RSSI voltage in the reception blocks A and B are as follows due to constituent parts and dispersion in adjustment.  That is, as shown in FIG.  7, the reception block A shows 1.25V for 10 dB.mu.,  2.25V for 30 dB.mu.  and 4.70V for 80 dB.mu.,  and the reception block B shows 1.45V for 10 dB.mu.,  2.45V for 35 dB.mu.  and 4.75V for 80 dB.mu..
As is apparent from the graph as shown in FIG.  3, dispersion occurs in slope of the RSSI voltage curve for the high-frequency input level below 35 dB.mu.. Therefore, it is apparent from this fact that a diversity switching operation cannot be accurately carried out for the high-frequency input level below 35 dB.mu.  by merely comparing the RSSI voltages of the reception blocks A and B when there is any dispersion in reception sensitivity between the reception blocks. 
As described above, if the above condition (1) is not satisfied, not only in the space diversity receiver, but also in a general receiver, the squelch operation is dispersed in accordance with the frequency.  In other words, this means that a reach distance (usable distance) between a wireless microphone and a receiver is varied in accordance with an used frequency band when the wireless wire is used for news gathering for television.  The variation of the reach distance confuses a user. 
With respect to the condition (2), the function of selecting a reception block having higher reception input level, which is inherent to the space diversity receiver, cannot be obtained.