Transponder and interrogator

Claim 2

A transponder, comprising: means for receiving a periodically modulated interrogating signal; means for demodulating the received signal to produce a periodic baseband signal; and means for transmitting data at a rate determined by the periodicity of the baseband signal.

Claim 3

A transponder as claimed in claim 1, wherein the interrogating signal has a periodic envelope, and wherein the means for demodulating comprises means for detecting the envelope of the received signal to produce the periodic baseband signal.

Claim 4

A transponder as claimed in claim 2, wherein: the means for detecting comprises means for half wave rectifying the received signal.

Claim 5

A transponder as claimed in any preceding claim, wherein the means for transmitting data comprises: means for providing a carrier modulated by said data, the carrier having a frequency also determined by the periodicity of the baseband signal.

Claim 6

A transponder as claimed in any preceding claim, including: means for storing said data.

Claim 7

Apparatus for interrogating a transponder as claimed in any preceding claim, comprising: means responsive to a periodic reference signal, for transmitting a periodically modulated interrogating signal, the periodicity of said modulation being determined by the periodicity of the reference signal; and means for receiving a signal transmitted by the transponder as a result of the interrogating signal, to recover the data.

Claim 8

Apparatus as claimed in claim 6, wherein the interrogating signal has a periodic envelope.

Claim 9

Apparatus as claimed in claim 7, wherein the means for transmitting an interrogating signal comprises: means for generating a signal comprising the sum of two sinusoids having frequencies differing by an amount determined by the periodicity of the reference signal.

Claim 10

Apparatus as claimed in claim 8, wherein the means for generating comprises: a fixed frequency oscillator; a variable frequency oscillator; means for summing the outputs of the oscillators; means for detecting the amplitude variations in the summed outputs to produce a baseband signal; and a phase detector for detecting differences in phase between the baseband signal and the reference signal, to control the frequency of the variable frequency oscillator to reduce such phase differences.

Claim 11

Apparatus as claimed in claim 8, wherein the means for transmitting comprises: an oscillator and a balanced modulator arranged to modulate the oscillator output with a modulating signal input from the reference signal, so as to produce an output signal having two frequency components differing by twice the frequency of the modulating signal input.

Claim 12

Apparatus for interrogating a transponder which is operative to derive a clock signal from an interrogating signal and to repetitively transmit a block of data containing a start code at a data rate controlled by said clock signal, the apparatus comprising: means for generating first and second periodic reference signals, the phase of one reference signal being adjustable relative to the phase of the other; means for transmitting an interrogating signal having a phase determined by the phase of the first reference signal; means for receiving a modulated signal transmitted by the transponder as a result of the interrogating signal; means responsive to the phase of the second reference signal to demodulate the received signal to recover the data; means for identifying the start code in the demodulated signal; and means responsive to the start code not being identified in a block of the demodulated signal of equivalent length to a block of data, to cause a relative shift in phase between the first and second reference signals.

Claim 13

Apparatus for interrogating a transponder which is operative to derive a clock signal from an interrogating signal and to repetitively transmit a block of data containing a start code at a data rate controlled by said clock signal, the apparatus comprising: means for generating first and second periodic reference signals, the phase of one reference signal being adjustable relative to the phase of the other; means for transmitting an interrogating signal having a phase determined by the phase of the first reference signal; means for receiving a modulated signal transmitted by the transponder as a result of the interrogating signal; means responsive to the phase of the second reference signal to demodulate the received signal to recover the data; means for identifying the start code in the demodulated signal; and means responsive to the start code not being identified in a block of the demodulated signal of equivalent length to a block of data, to cause a relative shift in phase between the first and second reference signals.

Claim 14

A transponder comprising: means for receiving an interrogating signal; means responsive to receipt of the interrogating signal to generate a clock signal; means for storing data in the format of a modulated signal, and responsive to the clock signal to read the data; and means for transmitting the data serially and direct, without modulating a carrier.

Claim 15

A transponder comprising: means for receiving an interrogating signal; means responsive to receipt of the interrogating signal to generate a clock signal; means for storing data in the format of a modulated signal, and responsive to the clock signal to read the data; and means for transmitting the data serially and direct, without modulating a carrier.