Why is this BJT Schmitt trigger producing a sine wave?

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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






transistors rf oscillator schmitt-trigger







share|improve this question



















  • 2




    150MHz isn't what 74 series capable of.
    – Long Pham
    Aug 26 at 11:45






  • 1




    Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend?
    – talikarng
    Aug 26 at 11:54










  • What happens if you run the simulation at a much lower frequency? Say, 50kHz?
    – marcelm
    Aug 26 at 11:54










  • Ah... Thankyou. It worked well at 1MHz
    – talikarng
    Aug 26 at 12:13










  • @talikarng You should check "switching characteristics"
    – Long Pham
    Aug 26 at 15:16














up vote
3
down vote

favorite












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






transistors rf oscillator schmitt-trigger







share|improve this question



















  • 2




    150MHz isn't what 74 series capable of.
    – Long Pham
    Aug 26 at 11:45






  • 1




    Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend?
    – talikarng
    Aug 26 at 11:54










  • What happens if you run the simulation at a much lower frequency? Say, 50kHz?
    – marcelm
    Aug 26 at 11:54










  • Ah... Thankyou. It worked well at 1MHz
    – talikarng
    Aug 26 at 12:13










  • @talikarng You should check "switching characteristics"
    – Long Pham
    Aug 26 at 15:16












up vote
3
down vote

favorite









up vote
3
down vote

favorite











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






transistors rf oscillator schmitt-trigger







share|improve this question















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






transistors rf oscillator schmitt-trigger




transistors rf oscillator schmitt-trigger






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Aug 27 at 0:20









Peter Mortensen

1,56131422




1,56131422










asked Aug 26 at 11:35









talikarng

666




666







  • 2




    150MHz isn't what 74 series capable of.
    – Long Pham
    Aug 26 at 11:45






  • 1




    Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend?
    – talikarng
    Aug 26 at 11:54










  • What happens if you run the simulation at a much lower frequency? Say, 50kHz?
    – marcelm
    Aug 26 at 11:54










  • Ah... Thankyou. It worked well at 1MHz
    – talikarng
    Aug 26 at 12:13










  • @talikarng You should check "switching characteristics"
    – Long Pham
    Aug 26 at 15:16












  • 2




    150MHz isn't what 74 series capable of.
    – Long Pham
    Aug 26 at 11:45






  • 1




    Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend?
    – talikarng
    Aug 26 at 11:54










  • What happens if you run the simulation at a much lower frequency? Say, 50kHz?
    – marcelm
    Aug 26 at 11:54










  • Ah... Thankyou. It worked well at 1MHz
    – talikarng
    Aug 26 at 12:13










  • @talikarng You should check "switching characteristics"
    – Long Pham
    Aug 26 at 15:16







2




2




150MHz isn't what 74 series capable of.
– Long Pham
Aug 26 at 11:45




150MHz isn't what 74 series capable of.
– Long Pham
Aug 26 at 11:45




1




1




Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend?
– talikarng
Aug 26 at 11:54




Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend?
– talikarng
Aug 26 at 11:54












What happens if you run the simulation at a much lower frequency? Say, 50kHz?
– marcelm
Aug 26 at 11:54




What happens if you run the simulation at a much lower frequency? Say, 50kHz?
– marcelm
Aug 26 at 11:54












Ah... Thankyou. It worked well at 1MHz
– talikarng
Aug 26 at 12:13




Ah... Thankyou. It worked well at 1MHz
– talikarng
Aug 26 at 12:13












@talikarng You should check "switching characteristics"
– Long Pham
Aug 26 at 15:16




@talikarng You should check "switching characteristics"
– Long Pham
Aug 26 at 15:16










1 Answer
1






active

oldest

votes

















up vote
9
down vote



accepted










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.






share|improve this answer






















  • Thankyou. It worked when I ran the simulation at much lower frequencies.
    – talikarng
    Aug 26 at 12:13










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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
9
down vote



accepted










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.






share|improve this answer






















  • Thankyou. It worked when I ran the simulation at much lower frequencies.
    – talikarng
    Aug 26 at 12:13














up vote
9
down vote



accepted










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.






share|improve this answer






















  • Thankyou. It worked when I ran the simulation at much lower frequencies.
    – talikarng
    Aug 26 at 12:13












up vote
9
down vote



accepted







up vote
9
down vote



accepted






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.






share|improve this 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.







share|improve this answer














share|improve this answer



share|improve this answer








edited Aug 26 at 14:19









Transistor

74k571161




74k571161










answered Aug 26 at 12:00









user287001

8,1731415




8,1731415











  • Thankyou. It worked when I ran the simulation at much lower frequencies.
    – talikarng
    Aug 26 at 12:13
















  • Thankyou. It worked when I ran the simulation at much lower frequencies.
    – talikarng
    Aug 26 at 12:13















Thankyou. It worked when I ran the simulation at much lower frequencies.
– talikarng
Aug 26 at 12:13




Thankyou. It worked when I ran the simulation at much lower frequencies.
– talikarng
Aug 26 at 12:13

















 

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