Method and apparatus for reflection coefficient measurements

A method for determining the reflection coefficient of a transmission line termination comprising the steps of :

• coupling an incident signal having a phase angle and a constant amplitude to said transmission line, said incident signal being incident to said termination;
• sampling said incident signal to provide an incident signal sample having a phase angle representative of said phase angle of said incident signal;
• sampling a reflected signal having a phase angle and an amplitude reflected from said termination to provide a reflected signal sample having a phase angle and an amplitude representative of said phase angle and said amplitude of said reflected signal, respectively;
• coupling one of said incident and reflected signal samples to a voltage controlled phase shifter having phase shift control terminals for propagation therethrough to emerge at an output port thereof as a phase shifted signal having a phase angle shifted relative to said phase angle of said one signal sample in accordance with a phase shift control voltage applied to said phase shift control terminals;
• comparing said phase angle of said signal sample other than said one to said phase angle of said phase shifted signal in a phase detector, thereby performing a phase comparison, said phase detector providing a phase difference signal at an output terminal that is representative of phase difference between said signal sample other than said one and said phase shifted signal, said phase difference signal being a null when differences between phase angles of said signal sample other than said one and said phase shifted signal equal a predetermined angle;
• incrementing said phase shift control voltage when said phase difference signal is not a null and repeating said phase comparison;
• comparing a phase difference signal at said output terminal of said phase detector obtained from a phase comparison to a phase difference signal at said output terminal of said phase detector obtained from a next previous phase comparison to check for a null crossover;
• generating a phase shift increment when a null crossover is not detected;
• applying said phase shift increment voltage to said phase shift control terminals thereby providing an incremental phase shift to said one signal sample;
• repeating phase comparisons and incremental phase shifting until a null crossover is detected;
• determining a phase shift control voltage at said null crossover by interpolating between phase control voltages producing signals at said output terminal of said phase detector on either side of said null;
• and
• determining said reflection coefficient from said control voltage at said null crossover and said reflected signal sample.

A reflection coefficient measuring apparatus of the type including transmission means for signal propagation having signal input port means to receive signals for propagation, end port means adapted for coupling to devices to be tested, reflected signal sampling means coupled to the transmission means for providing samples of signals reflected from the end port means and detector means coupled to the reflected signal sampling means for determining reflected signal amplitudes further comprising

• incident signal sampling means coupled to said transmission means for providing samples of signals incident to said end port means;
• phase detector means having first input means coupled to receive samples of said input signals from said input signal sampling means and second input means for providing a null at an output terminal thereof when a predetermined phase angle relationship is achieved, a signal with a positivie polarity at said output terminal when a phase angle of one of said signals at said first and second input means exceeds a phase angle of said signal other than said one by said predetermined phase angle relationship, and a signal with a negative polarity at said output terminal when said one signal is less than a phase angle of said signal other than said one by said predetermined phase angle relationship;
• phase shift means coupled between said reflected signal sampling means and said second input means of said phase detector means having phase shift control terminals for phase angle shifting signals propagating between said reflected signal sampling means and said second input means of said phase detector means in accordance with phase angle incrementing signals coupled to said phase shift control terminals, whereby a phase relationship between said signals incident to and reflected from said end port means is determinable from a phase angle setting of said phase shift means when said predetermined phase relationship is achieved;
• processor means coupled to said output terminal of said phase detector means, said phase shift control terminals of said phase shift means, and said reflection coefficient determining means for providing said phase angle incrementing signals until a null crossover is indicated by a signal polarity change at said output terminal of said phase detector means and for providing a final phase angle increment signal representative of said phase shift means phase angle setting for a null at said output terminal of said phase detector means to said reflection coefficient determining means by interpolating for a crossover between a signal polarity change when no increment signal produces said null;
• and
• means coupled to receive said final phase angle incrementing signal from said processor means and to receive said reflected signal amplitude from said detector means for determining magnitude and phase angle of said reflection coefficient of a device coupled to said end port.