Feedforward automatic gain correction
An automatic gain control circuit is provided for an input signal in the form of a dc reference level and a superposed amplitude modulated ac data signal. A feedforward AGC loop has a low pass filter for deriving the level of attenuation from the attenuated dc reference level. A multiplier value (G) is based on the reciprocal of the level of attenuation (α) and this multiplier enables an output signal to be generated comprising a constant multiple (DG) of the input signal.
1. An automatic gain control circuit, comprising:
an input configured to receive a variably attenuated input signal, the variably attenuated input signal comprising both a dc reference level and a superposed ac data signal;
a feedforward path comprising:
a low pass filter configured to derive a level of attenuation from the dc reference level; and
a unit configured to derive a multiplier value, based on a reciprocal of the level of attenuation; and
a multiplier configured to multiply the variably attenuated input signal with the multiplier value to derive an output signal comprising a constant multiple of the variably attenuated input signal.
2. The circuit as claimed in claim 1 , wherein the multiplier value comprises a desired gain multiplied by the reciprocal of the level of attenuation.
3. The circuit as claimed in claim 2 , wherein the desired gain comprises: DG=V max /(RefDC+PM max ), V max is a maximum desired output voltage, RefDC is a dc reference voltage level, and PM max is a maximum input signal resulting when a maximum positive modulation is superposed on RefDC.
4. The circuit as claimed in claim 3 , wherein the desired gain is in a range of 0.3 to 0.5.
5. The circuit as claimed in claim 2 , wherein the desired gain is substantially 0.35.
6. The circuit as claimed in claim 1 , wherein the multiplier comprises a variable gain amplifier.
7. The circuit as claimed in claim 1 , further comprising:
a controller that is configured to control the low pass filter in response to sudden changes in the variable attenuation.
8. The circuit as claimed in claim 7 , wherein the controller comprises:
an element for varying a cut-off frequency of the low pass filter.
9. The circuit as claimed in claim 7 , wherein the controller is a speed control circuit.
10. The circuit as claimed in claim 9 , wherein the speed control circuit controls a speed of the low pass filter.
11. A car radio receiver circuit comprising:
a frequency selection circuit configured to select an amplitude modulated radio frequency signal, and convert the selected signal to baseband; and
the automatic gain control circuit as claimed in claim 1 configured to derive an output signal from a variably attenuated baseband input signal.
12. The circuit as claimed in claim 1 , wherein the superposed ac data signal is amplitude modulated.
13. An automatic gain control method, comprising:
receiving a variably attenuated input signal, the variably attenuated input signal comprising a dc reference level and a superposed amplitude modulated ac data signal;
low pass filtering the variably attenuated input signal to derive a level of attenuation from the dc reference level;
deriving a multiplier value, based on a reciprocal of the level of attenuation; and
multiplying the variably attenuated input signal with the multiplier value to derive an output signal comprising a constant multiple of the variably attenuated input signal.
14. The method as claimed in claim 13 , wherein the multiplier value comprises a desired gain multiplied by the reciprocal of the level of attenuation.
15. The method as claimed in claim 14 , wherein the desired gain comprises: DG=V max /(RefDC+PMmax), V max is a maximum desired output voltage, RefDC is a dc reference voltage level, and PM max is a maximum input signal resulting when a maximum positive modulation is superposed on RefDC.
16. The method as claimed in claim 14 , wherein the multiplying comprises:
controlling a variable gain amplifier.
17. The method as claimed in claim 14 , further comprising:
controlling the low pass filtering in response to sudden changes in the variable attenuation.
18. The method as claimed in claim 17 , wherein controlling the low pass filtering comprises:
varying a cut-off frequency of the low pass filter.