Some experimentation, looking for a good Schmitt Trigger circuit. Once again, the 2N5088 transistors prove "magical," offering very high gain, and more margin against temperature, and voltage changes. The experimental circuit works well up to about 10 kHz.
Some additional work is needed to keep the duty cycle steady at 22 kHz < fout < 33 kHz. Later testing revealed that the RC time constant product for the speedup capacitor and RA/RB voltage division resistors are causing problems. After the sawtooth goes negative, there is a corresponding positive RC spike at Q2b, and it has insufficient time to decay before the upper trip point VP is reached — this pushes up VP as frequency increases, as seen in the measurements.
This circuit design problem needs to be fixed. As well, a precision variant of the circuit using a reference diode also seems to be a good idea, as that will make the threshold voltages independent of the Vcc supply voltage.
One key application is for a 22-33 KHz sawtooth oscillator for the Harmonic VCO. The Schmitt Trigger converts the sawtooth waveform to 50% duty cycle square-waves, which can then drive a paraphase gate that in turn drives a double-balanced frequency mixer with both phases. A 32.768 KHz sine wave oscillator provides the "RF" signal. The paraphase square-waves (in-phase, and anti-phase) provide the LO as low-side injection for down-conversion frequency synthesis into audio baseband, 0-10 KHz.
Some additional work is needed to keep the duty cycle steady at 22 kHz < fout < 33 kHz. Later testing revealed that the RC time constant product for the speedup capacitor and RA/RB voltage division resistors are causing problems. After the sawtooth goes negative, there is a corresponding positive RC spike at Q2b, and it has insufficient time to decay before the upper trip point VP is reached — this pushes up VP as frequency increases, as seen in the measurements.
This circuit design problem needs to be fixed. As well, a precision variant of the circuit using a reference diode also seems to be a good idea, as that will make the threshold voltages independent of the Vcc supply voltage.
One key application is for a 22-33 KHz sawtooth oscillator for the Harmonic VCO. The Schmitt Trigger converts the sawtooth waveform to 50% duty cycle square-waves, which can then drive a paraphase gate that in turn drives a double-balanced frequency mixer with both phases. A 32.768 KHz sine wave oscillator provides the "RF" signal. The paraphase square-waves (in-phase, and anti-phase) provide the LO as low-side injection for down-conversion frequency synthesis into audio baseband, 0-10 KHz.
Notebook
Scope Fotos
basic operation at 1 kHz
ORN⇒Vi, BLU⇒Vo, VIO⇒Q2b, GRN⇒Q1e/Q2e
basic operation at 33 kHz
fast propagation delays for 33 kHz input
basic operation at 1 kHz
ORN⇒Vi, BLU⇒Vo, VIO⇒Q2b, GRN⇒Q1e/Q2e
basic operation at 33 kHz
fast propagation delays for 33 kHz input
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