Electronic – Why is this BJT Schmitt trigger producing a sine wave

oscillatorRFschmitt-triggertransistors

I am trying to build a Schmitt trigger from a pair of BJTs to convert a sine to a square wave.

The sine wave is from a Colpitts oscillator running at over 150 MHz. I want to measure the frequency using something like the SN74LV4040, which is why I want to square the sine wave somewhat.

This is the relevant part of the Schmitt trigger (with V1 representing the oscillator). Emitter current was set to 2 mA, high voltage to 3 V, low voltage to 2 V.

Schematic

simulate this circuit – Schematic created using CircuitLab

But this is the output I get in LTspice.

Sine wave output from Schmitt trigger

Why am I getting a sine wave output from the Schmitt trigger?

This is the whole schematic (with some voltages plotted), if that helps:

Enter image description here

Best Answer

Kilo-ohms and over 100 MHz! You cannot get anything rectangular looking due the time constants caused by the parasitic capacitances, which can easily be 10 pF or more.

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