Radio receiver having a preselection circuit tuned by a varactor with a variable limiter threshold circuit to attenuate large amplitude signals

DETAILED DESCRIPTION What is claimed is: 1.
A circuit for a radio receiver having an intermediate frequency amplifier, said circuit comprising a control stage having an input means for receiving an input signal; a varactor tuned preselection circuit coupled to said control stage; a variable threshold limiter circuit coupled to said preselection circuit for limiting the received signal applied to said preselection circuit; and means for deriving from said amplifier a control voltage and for applying it to said control stage and said limiter to increase said threshold as the amplitude of the intermediate frequency increases.
2.
A circuit as claimed in claim 1, wherein the control stage includes two emitter-coupled transistors, each having a base and a collector, the bases receiving said control voltage, the preselection circuit being coupled to the collector of one of the transistors, and an RC section coupled to the collector circuit of the other transistor, the limiter being coupled between the two collectors.




Description:
The invention relates to a radio receiver having a preselection circuit tuned by a varactor and a preceding control stage which is controlled by a control voltage derived from a succeeding intermediate-frequency amplifier.
When varactors are used for tuning radio receivers, in particular AM car radio receivers, there is a risk that the preselection circuit has to handle signals of a magnitude such that the varactor or varactors included in this circuit is or are overdriven.
Then the capacitance of the varactor depends not only upon the tuning voltage set but also upon the amplitude of the signal applied across the circuit.
This gives rise to circuit detuning dependent upon the signal amplitude and to spurious responses and modulation distortion.
A method of protecting the preselection circuit against excessive signals consists in providing a control stage which preceeds the said circuit and is normally controlled by a control voltage derived from the intermediate-frequency signal of the receiver