Abstract A simple radio receiver for FSK signals is described in which the signals from the aerial are mixed in two high-gain mixers respectively with local oscillator signals in phase quadrature and at the signals' nominal center frequency. The outputs from the mixers are low-pass filtered to get the difference frequencies. The filter outputs are amplified and one applied to the D input of a D-type flip-flop to the clock input of which the other is applied. The flip-flop output gives the FSK modulation. It is highly desirable that the local oscillator's outputs, called Io and Qo should be in accurate phase quadrature, and the circuit described herein is intended to achieve this. To do this, the Io and Qo signals are applied to a quadrature phase sensitive detector (10) which, if the phase relation is wrong, gives an error signal. This is applied to an adjustable phase shifter (12) in one of the oscillator outputs, in this case the Qo output. Alternatively, the phase adjustment can be applied to both Io and Qo lines. The detector can be analogue, or digital, in which case it could be based on an EXCLUSIVE OR circuit.
A radio receiver for frequency shift keyed signals on an RF carrier, which comprises first and second paths to which the received radio signals are applied, each said signal path including a mixer circuit followed by a low-pass filter and a limiting amplifier stage, a local oscillator running at the carrier frequency, a first connection from the local oscillator via which the output thereof is applied directly to the mixer circuits of the first of said paths, a second connection from the local oscillator to a quadrature phase shifter the output of which is applied to the mixer circuit of the second of said paths, so that the second mixer circuit receives the local oscillator output with a quadrature phase shift with respect to the first path, a quadrature phase sensitive detector to whose inputs are applied the signal on the first connection from the local oscillator and the output of the quadrature phase shifter, the output from the detector being an error signal if the phase relation is incorrect, which error signal represents the magnitude and sign of the error in phase relation, phase adjusting means responsive to the error signal to adjust the phase of one or both of the locally generated inputs to the mixers in a manner appropriate to the restoration of the correct phase relation, and a D-type clocked flip-flop to the D input of which is applied the output of one of the limiting amplifier stages while the output of the other limiting amplifiers is applied to the clock input of the flip-flop, the output of the flip-flop being the signal modulator in frequency shift keyed manner on to the carrier.
Claim 2
Dependent Claim
277 words
A radio receiver for frequency shift keyed signals on an RF carrier, which comprises first and second paths to which the received radio signals are applied, each said signal path including a mixer circuit followed by a low-pass filter and a limiting amplifier stage, a local oscillator running at the carrier frequency, a first connection from the local oscillator via which the output thereof is applied directly to the mixer circuits of the first of said paths, a second connection from the local oscillator to a quadrature phase shifter the output of which is applied to the mixer circuit of the second of said paths, so that the second mixer circuit receives the local oscillator output with a quadrature phase shift with respect to the first path, a quadrature phase sensitive detector to whose inputs are applied the signal on the first connection from the local oscillator and the output of the quadrature phase shifter, the output from the detector being an error signal if the phase relation is incorrect, which error signal represents the magnitude and sign of the error in phase relation, phase adjusting means responsive to the error signal to adjust the phase of one or both of the locally generated inputs to the mixers in a manner appropriate to the restoration of the correct phase relation, and a D-type clocked flip-flop to the D input of which is applied the output of one of the limiting amplifier stages while the output of the other limiting amplifiers is applied to the clock input of the flip-flop, the output of the flip-flop being the signal modulator in frequency shift keyed manner on to the carrier.