Ael Microtel, Ltd.

Abram Froese

Larry Y Onotera

Alvin Liebelt

Josef L Fikart

Muhammad J Ahmed

An APC (automatic phase control) circuit is utilized to measure the linearity of an FMO (frequency modulated oscillator). In accordance with a common APC concept a programmable divider is connected to an output of said frequency modulatable oscillator to produce a binary output signal having a predetermined pulse repetition rate. This signal is compared with a reference oscillator signal in a phase control comparator producing a DC control signal which is applied to the control input of said frequency modulated oscillator to maintain the average oscillator frequency constant. This DC control voltage gives an indication of average free running frequency of the oscillator. To measure linearity, the said binary signal at the programmable divider output is also further divided and processed to produce a low frequency square waveform which is applied via a manual ON/OFF switch and a modulation amplifier to the modulation input of the said FMO. The change in the average free running frequency of the FMO, and the corresponding change in the DC control voltage resulting from alternatively switching the low frequency waveform ON and OFF, is a measure of the FMO's linearity. This change is amplified and a zero center meter is employed to indicate the magnitude and sign of the error, and to permit adjustment to obtain a minimum error indication.

Apparatus for testing the linearity of a frequency modulated oscillator

The baseband input to a frequency modulation radio transmitter is switchably connectable to a square wave generator, the amplitude of the square wave output being adjusted to provide a predetermined deviation of the FM radio transmitter. The output frequency of a pilot oscillator is also applied to the baseband input and is subjected to amplitude modulation because of non-linearities which exist in the system and which adversely affect the signal during transmission through the system. At the receiving end of the radio system, the receiver output contains a signal comprising the pilot frequency and a superimposed amplitude modulated signal, and this signal is applied to a product detector. The output of the product detector is a pulsating DC signal comprising a basic DC component, which is representative of the amplitude of the pilot frequency, and an AC component, which is representative of the system non-linearity. The pulsating DC signal is applied to one terminal of a capacitor. The AC component appears at the other terminal and is rectified and integrated to obtain a DC signal, the amplitude of which is proportional to the slope component in the system non-linearity. This DC signal is measured by a direct current meter.

Built-in linearity testing arrangement for an FM radio system

The baseband input to a frequency modulation transmitter is switchably connectable to a square wave generator, the amplitude of the square wave output being adjusted to provide a predetermined deviation of the FM radio transmitter. The output frequency of a pilot oscillator is also applied to the baseband input and it is subjected to phase modulation because of group delay which exists in the system and which adversely affects the signal during transmission through the system. At the receiving end of the radio system, the receiver output obtains the pilot signal which has been phase modulated by the group delay slope in the system, because of the square wave test signal. This phase modulation is detected in a phase lock loop, amplified, rectified and applied to a meter so as to provide an indication of the group delay slope in the system.

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Abram Froese