Many radio systems employ a zero intermediate frequency (zero IF or ZIF) receiver for receiving radio frequency (RF) and intermediate frequency (IF) signals. Such receivers are also known as homodyne receivers. Such receivers translate the signal spectrum of received signals from a spectrum about a modulating carrier frequency directly to substantially 0 Hz (i.e. DC or baseband) for demodulation.
Zero IF or ZIF receivers have the advantage that they may be readily implemented in monolithic silicon integrated circuits. Such receivers do not require complicated high frequency bandpass filters. Rather, such receivers employ relatively low frequency, low pass filters which reject unwanted signal frequencies (i.e., unwanted channels), passing only the in-phase (I) and quadrature phase (Q) components of the desired radio channel. These filters can be integrated on-chip with other components of the receiver to reduce the size and cost of the receiver and the radio. ZIF receivers may also be advantageously used in consumer products, such as radiotelephone handsets for cellular and cordless radiotelephone systems, which are sensitive to cost and physical size. Moreover, ZIF receivers generally require relatively low operating power, making them ideal for battery-powered products such as radiotelephone handsets.
A disadvantage of ZIF receivers is that the input signal level must be kept within the linear operating range of the low pass filters included in the receiver. These filters generally operate in one of three modes. In cut off mode, the input signal level is too small to produce a meaningful response from the filter. In saturation, the input signal level is so large that the filter output is clipped or otherwise distorted. In the linear range of operation, input signal level is appropriate for proper filter operation.
To insure that the input signal level remains within the linear operating range of the low pass filters, ZIF receivers have included an automatic gain control (AGC) circuit. The AGC circuit regulates the gain of an input amplifier in response to the detected signal level of the received signals. In this manner, the AGC circuit keeps the input signal level within the linear operating range of the receiver filters.
A significant limitation of prior art AGC circuits is the speed with which the AGC circuit can respond to detected signals. In a typical situation, when the radio is operating in listening mode, awaiting receipt of RF signals, the receiver must scan all defined channels searching for incoming RF signals. Because a transmitter may be near or far and because channel noise is indeterminate, the receiver has no advance information of incoming signal strength or level. To detect weak signals, the receiver sets the gain of the input amplifier high, using the AGC circuit. If, instead of weak signals, the receiver detects, moderate or strong signals, the gain is automatically adjusted to accommodate the detected signal.
The necessary gain adjustment by the AGC circuit takes time, known as the acquisition time. The required time is a function of the IF modulation bandwidth of the receiver, which cannot be varied to improve performance of the receiver. During the acquisition time, when a receiver is adjusting AGC gain, the incoming signal is not being accurately detected and demodulated by the receiver. Therefore, a portion of the signal is lost, while the receiver detects the signal. Also, because the receiver is energized during this acquisition, the receiver is depleting energy from batteries which are used to power the radio.
These problems are exacerbated in a receiver operating according to a time division multiple access (TDMA) protocol. In a TDMA system, the receiver receives bursts of data during assigned time slots. At the start of each time slot, the receiver must determine appropriate gain for the input amplifier. If the receiver's automatic gain control circuit responds slowly, performance of the receiver is degraded.
Accordingly, there is a need in the art for an automatic gain control circuit which improves acquisition time for radio receivers.