Method of and apparatus for producing a logarithmic representation of an input voltage

Apparatus for producing a logarithmic representation of an input voltage, comprising:

• means for exponentially changing the charge of a capacitor;
• means responsive to the charge of the capacitor for producing pulses at a pulse rate which is dependent upon the time taken for the charge of the capacitor to change exponentially with a given time constant between two reference voltages;
• and
• means for producing a signal, constituting said logarithmic representation, in dependence upon the number of said pulses which occur during the time taken for the charge of the capacitor to change exponentially with said given time constant between said input voltage and a reference voltage.

Apparatus for producing a logarithmic representation of an input voltage, comprising:

• a capacitor;
• a resistor;
• means for providing first and second reference voltages between which the input voltage lies;
• means for sequentially charging the capacitor to the first reference voltage, discharging the capacitor through the resistor to at least the second reference voltage, charging the capacitor to the input voltage, and discharging the capacitor through the resistor to at least the second reference voltage;
• a pulse generator for generating pulses at a first fixed frequency;
• a programmable frequency divider arranged when enabled to frequency-divide the pulses at said first frequency to produce frequency-divided pulses;
• a counter for counting said frequency-divided pulses;
• and
• control means responsive to the charge of the capacitor for controlling the frequency divider;
• said control means being arranged to program the frequency divider, during charging of the capacitor to the first reference voltage, to frequency-divide by a factor corresponding to a logarithmic representation of the first reference voltage relative to the second reference voltage, to enable the frequency divider during discharge of the capacitor from the first reference voltage to the second reference voltage, whereby the frequency divider produces frequency-divided pulses at a second frequency which pulses are counted by the counter to produce a resultant count, to program the frequency divider, during charging of the capacitor to the input voltage, to frequency-divide by a factor equal to the resultant count and to reset the counter, and to enable the frequency divider during discharge of the capacitor from the input voltage to the second reference voltage, whereby the frequency divider produces frequency-divided pulses which are counted by the counter to produce a further resultant count which constitutes said logarithmic representation of the input voltage.

Apparatus for producing a logarithmic representation of an input voltage, comprising:

• a capacitor;
• a resistor;
• means for providing first and second reference voltages between which the input voltage lies;
• a variable frequency oscillator for generating pulses;
• a frequency divider arranged to frequency divide the generated pulses by a predetermined factor corresponding to a logarithmic representation of the first reference voltage relative to the second reference voltage to produce frequency-divided pulses;
• means for alternately charging the capacitor to the first reference voltage and discharging the capacitor through the resistor to at least the second reference voltage in dependence upon the frequency-divided pulses;
• means for controlling the frequency of the oscillator, in dependence upon the frequency-divided pulses and the discharge of the capacitor, so that the oscillator produces a number of said generated pulses equal to said predetermined factor during discharge of the capacitor from the first reference voltage to the second reference voltage;
• and
• means including a counter for counting the number of said generated pulses which are produced during discharge of the capacitor from the first reference voltage to the input voltage to produce a resultant count which constitutes said logarithmic representation of the input voltage.