Intermodulation distortion analyzer

An RMS detector circuit comprising:

• an input terminal;
• an output terminal;
• a squaring circuit for receiving a first signal and providing a squared signal having an amplitude which is proportional to the square of the amplitude of said first signal;
• gain control means for automatically adjusting signals applied to said input signal terminal to a signal at a substantially constant level, said gain control means including feedback means for maintaining said substantially constant level an inverse function of signal amplitude at said output signal terminal;
• means for applying said signal at substantially constant level to said squaring circuit as said first signal;
• and
• low pass filter means for eliminating all but DC components in the squared output signal from said squaring circuit and applying said DC components to said output signal terminal;
• wherein said squaring circuit comprises:
• an operational transconductance amplifier having an input terminal, a bias control terminal and an output terminal which provides an output current proportional to the product of the voltage applied to said input terminal times the current applied to said bias control terminal;
• means for applying said substantially constant level to said input terminal of said operational transconductance amplifier;
• and
• means for applying a current proportional to said constant level to said bias control terminal.

An RMS detector circuit for providing an output voltage proportional to the RMS value of an input voltage, said circuit comprising:

• an input port connected to receive said input voltage;
• an output port for providing said output voltage;
• squaring circuit means responsive to application of a first signal thereto for providing a second signal having an amplitude which is proportional to the square of the amplitude of said first signal;
• gain-controlled amplifier means connected to receive said input voltage from said input port and provide said first signal;
• gain control means for adjusting the gain of said gain-controlled amplifier to maintain the amplitude of said first signal substantially constant, said gain control means including feedback means connected between said output port and said gain-controlled amplifier for controlling the gain of said amplifier as an inverse function of said output voltage;
• means applying said first signal to said squaring circuit;
• low pass filter means connected to receive said second signal from said squaring circuit for removing all AC components from said second signal, said low pass filter means being further connected to pass all DC components of said second signal to said output port;
• wherein said gain-controlled amplifier is a high gain inverting amplifier having an input terminal connected to receive said input voltage;
• and
• wherein said gain control means comprises:
• an operational transconductance amplifier having an input terminal connected to receive said first signal, a bias control terminal, and an output terminal connected to the input terminal of the high gain inverting amplifier, said transconductance amplifier being of the type which delivers an output current from its output terminal which is proportional to the product of the voltage at its input terminal times the current delivered to said bias control terminal;
• and
• voltage to current converter means connected between said output port and said bias control terminal for delivering a bias current to said bias control terminal which is proportional to the output voltage at said output port.