Radio transmitter/receiver

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of the present invention will now be described with reference to the sole FIG.
1 which is a block diagram of part of a radio transmitter/receiver.
With reference to FIG.
1, an incoming radio frequency (RF) signal from an antenna 1 is fed via transmit/receive switch 2 and band-pass filter 3 to low noise amplifier 4.
The function of low noise amplifier 4 is to enhance the signal to noise performance of the receiver and to reduce any unwanted transmission from the receiving antenna at radio frequencies.
The amplified signal from low noise amplifier 4 is then mixed in first mixer 5 with the output from fixed frequency local oscillator 6.
The frequency of fixed frequency local oscillator 6 is fixed and the output from fixed frequency local oscillator 6 is doubled in frequency in frequency multiplier (F.
M) 7 before passing to first mixer 5, at twice the frequency of fixed frequency local oscillator 6, to mix with the incoming, filtered and amplified RF signals to produce an intermediate frequency (IF) output from first mixer 5.
Use of the frequency multiplier 7 enables a lower frequency oscillator to be used as a local oscillator which provides an easing of tolerances so as to permit discrete implementation of the oscillator.
The output from first mixer 5 is fed to an active band-pass filter (B.
P.
F) 8 forming part of a phase locked loop (PLL).
The configuration and operation of the active band-pass filter 8 and associated PLL when used as a pre-selector filter for a direct conversion receiver is the subject of our co-pending application GB 9725674.
7.
In the present invention, however, active band-pass filter 8 and associated PLL operates at IF and has been improved.
The active band-pass filter 8 is a voltage controlled band-pass filter and may be constructed as an MMIC active filter, a description of an MMIC active filter can be found in IEEE Transactions on microwave theory and techniques Volume 37 number 12 December 1989, Manfred J