Receiver provided with an automatic frequency control loop

A receiver provided with a frequency control loop for automatic fine tuning, consecutively incorporating a controllable oscillator tunable in frequency by a variable capacitance diode, a mixer stage to which the signal from the controllable oscillator is applied for converting an RF receiver signal into an intermediate frequency signal, a frequency voltage converter tuned to a fixed intermediate frequency for converting frequency deviations of the intermediate frequency signal relative to the fixed intermediate frequency into amplitude variations of the frequency voltage converter output signal relative to a reference value, a control signal generating circuit for deriving a control signal for the variable capacitance diode of the controllable oscillator from the output signal of the frequency voltage converter, said controllable oscillator at least partly compensating its asymmetrical capacitance-voltage characteristic, characterized in that the control signal generating circuit comprises a threshold attenuation circuit arranged between the frequency voltage converter and the variable capacitance diode, the threshold voltage of said attenuation circuit mainly corresponding to said reference value and its transfer factor decreasing by a substantially constant and finite dB value when the frequency voltage converter output signal exceeds the threshold voltage, wherein said threshold attenuation circuit comprises a voltage comparator for comparing the output voltage of the frequency voltage converter with the threshold voltage, and a voltage divider coupled to the voltage comparator and arranged between the frequency voltage converter and the variable capacitance diode, said voltage comparator increasing the dividing factor of the voltage divider when the last-mentioned output voltage exceeds the threshold voltage.

Receiver apparatus, comprising:

• means for receiving an RF information signal;
• controllable oscillator means for generating a local RF signal, said controllable oscillator means having non-linear capacitance means having a capacitance varying in accordance with a control signal applied thereto, said local RF signal having a frequency dependent upon said capacitance of said capacitance means;
• mixing means for mixing said RF information signal and said local RF signal and generating an intermediate frequency signal in response thereto;
• frequency-voltage converter means connected to said mixing means for generating a converter output signal having an output amplitude corresponding to the difference between said intermediate frequency and a reference frequency;
• and
• control signal generating means interconnected between said frequency-voltage converter means and said non-linear capacitance means for generating said control signal, said control signal generating means having a first, constant transfer factor for control signals controlling said variable capacitance means over a first capacitance range wherein said capacitance varies at a first average rate with respect to voltage applied thereto, and a second constant transfer factor less than said first transfer factor for control signals controlling said variable capacitive means of a second capacitance range wherein said capacitance varies as a second average rate greater than said first average rate.