An apparatus ( 7 ) for switching from a first pilot signal and a second pilot signal on a system transporting a plurality of signals. A tuner ( 30 ) receives the signals and provides an output signal to a loop compensation circuit ( 26 ). The tuner includes a bandpass filter ( 22 ) for removing signals other than the selected pilot signal from the output of the tuner. The output of the bandpass filter is provided to a detector circuit ( 24, 25 ). The output of the detector is compared with a reference signal ( 28 ) by the loop compensation circuit ( 28 ). A microcontroller ( 39 ) sends a tuning command to the tuner which causes the tuner to switch from tuning to the first pilot signal to tuning to the second pilot signal and also sends a new reference voltage to the loop compensation circuit. Thus, if the input signal changes from analog to digital modulation on the pilot channel, the set-point, or target level of the reference signal, will automatically change to provide the correct value for operation of the circuit. The microcontroller may change the selected pilot signal based upon a command in the input signal or upon an operator command, such as by setting a switch (not shown).

Automatic slope and gain (ASG) detector technique including a pilot signal

A post-equalizer and pre-equalizer circuit for use in communicating between nodes in a Pulse Amplitude Modulated (PAM) digital or analog communication system is described. The post-equalizer circuit comprises a first variable zero circuit, a second variable zero circuit, and a gain shaper circuit wherein the gain and frequency location of the zeros in the zero circuits combined with the gain of the gain shaping circuit are simultaneously controlled by a control circuit which generates a control voltage which is a monotonically increasing function of cable loss. In the case of a PAM digital communication system, the control voltage generates a signal equal to the difference between the equalized signal and the original transmitted signal which is used to vary the resistance of voltage variable resistors in the form of FET's in each of the zero circuits and gain shaper circuits. In the case of an analog system, the control voltage is derived from the sealing current that is determined by the DC resistance of the cable to be equalized. If the cable loss is above a predetermined value, a pre-equalizer circuit is switched into the transmit path of the communication system and provides a gain, zero and pole at predetermined frequencies which pre-compensates for the extra loss incurred in transmission over length greater than can be equalized by the post-equalizer. Additionally, a bi-quad ACE circuit is described which provides a hyperbolic relationship between the zero frequency location and circuit gain utilizing a cascode amplifier and emitter follower circuit with a feedback loop through a voltage variable resistor.

Equalizer for digital transmission systems

A transmission system in which wide band electrical signals are transmitted along a transmission line has repeater stations located at intervals along the line for boosting and equalizing the signals. Each repeater station has an equalizer having a frequency response characteristic which generally complements that of the section of the transmission line which it covers. The frequency response characteristic of the equalizer is capable of adjustment to match the section of transmission line which feeds it. The adjustment required will vary when the section of transmission line feeding it has a length differing from that of a norm and where the line is subject to temperature changes. The repeater station incorporates a feedback chain which senses a pilot transmitted along the section of transmission line and responds to the monitored level of the pilot signal to automatically adjust the characteristic of the equalizer until it matches that of the transmission line section in question. This avoids the necessity of manually adjusting the equalizer of the repeater station at each installation.

Transmission system and repeater stations therefor

Circuitry to transmit an analog data signal over a wireless communication link includes a receiver that receives a data signal, an automatic gain control circuit that keeps a peak amplitude of the data signal substantially constant and within a predetermined linear amplification range, and a transmitter, having the predetermined linear amplification range, that receives the data signal from the automatic gain control circuit and that outputs the data signal.

Circuitry to establish a wireless communication link

Disclosed are bidirectional equalizer arrangements in which the outputs of the forward and reverse equalizers are combined using low complexity processing to yield a single output sequence. In each disclosed arrangement of the invention, the low complexity processing utilizes the equalizer error outputs of the forward and reverse equalizers, which are then zero-delay lowpass filtered to provide relative reliability estimates. In one arrangement of the invention, these reliability estimates are used to select, on a symbol-by-symbol basis, either the forward or the reverse equalizer output. In another arrangement of the invention, a single linear lowpass filter is used to filter the difference of the equalizer error signals. In another arrangement of the invention, the relative reliability estimates are used to form a weighted average of the forward and reverse equalizer pre-decision signals. This weighted-average complex-valued signal then acts as a new equalized signal, and is fed to a constellation decision device to form a symbol decision. The arrangements of the invention can provide the majority of the gain offered by schemes that optimally (in a maximum-likelihood sense) consider all possible sequences formed from the bidirectional equalizers, while avoiding the optimal scheme's complexity which exhibits an exponential dependence on the duration of the channel impulse response.

Low-complexity bidirectional equalizer

An adaptive equalizer to compensate for the effects of frequency-selective fading on a microwave radio system. The equalizer includes two Bode-type bump networks and an AGC amplifier whose slope and gain are respectively controlled by pilot tones added to the message load. Importantly, the frequency characteristics of the two Bode networks are non-overlapping, thus ensuring stability.

Mclean D I

Papers

Pilot tone controlled adaptive amplitude equalizer