Why the spuaring sinwave output is ZERO?
C1, 10 pf, too small. Try 0.1 uf.
Also you can try to increase the Vin just to get a value where Schmitt trigger is working. For example a 74LS14 series needs about 2.3V to switch.
Hi, you can see the result, although Vin is 1V, after RCL , the node Vin1 is more than 4V. So the amplitude should be enough to trig the gate, why not the output?
First... for time "based circuits" AC simulation is NOT used because AC simulation linearizes the circuit around DC operating point and therefore you see nothing.Initial conditions are taken into account and circuit keeps its steady state position even there is a large signal at the input...It's a mistake..Make a transient simulation to see the waveforms.
Second... Because at the input of the Schmitt Trigger there is 2.5V DC voltage and it will keep the output at constant 5VDC. The sinusoidal signal will change the AC+DC signal between 1.5V and 3.5V and Schmitt Trigger output will never drop from 5V constant DC voltage.
Third.... 10pF coupling capacitor is too small and 700nH RFC is also too small for 50MHz.Use 10nF and at least 1uH.
This is quite a neat circuit idea ((i.e. the lumped stuff)
It looks to me like the theory is that the 10pF and .7uH are series resonant at Fin. This will deliver voltage gain at the junction of the 10pF and 0.7uH.
This voltage gain is what the OP requires in order to drive a typical logic gate from a low level sine wave :)
I don't know what Schmitt trigger device you are modelling but the rest of the circuit looks like it is suited to regular 5V logic gates eg 74AC04 inverter (in place of the Schmitt which I would expect to give inferior phase noise).
I'll ignore your simulation because I am unfamiliar with the tool you are using but I will tell you why this circuit will fall over (slightly) at 60MHz with a regular logic gate.
The series resonant circuit will be spoilt by the input capacitance of a real world logic gate. This is because the gate capacitance is significant wrt your 10pF in the resonant circuit.
This topology probably works really well for lower frequencies where the resonant components are large compared to the gate capacitance.
If you bias the gate at mid voltage then you WILL need voltage gain (from the resonance) to get ample swing from a low level sine wave.
You could try this variation that uses the same principle.
sq.doc
In this case you can make the gate input capacitance work for you as you can absorb the gate capacitance into the resonant circuit.
!IMPORTANT!
Ignore D1, D2 and C2 as these are simply modelling the input gate (rather crudely)But you can see that C3 is no longer 10pF for resonance.
The 10pF required for resonance comes from C3 and the gate capacitance in parallel.
However, you won't be able to probe this circuit with a scope probe as the probe capacitance will ruin the resonance at 60MHz.
Because of the effect of the gate capacitance I can't guarantee this will work and you might have to play with the 5.6pF capacitor to get best performance.
TBH I would use a different squarer circuit than this biased gate anyway. eg a discrete design using transistors.
I changed to transient analysis, and the result is also zero.
Also I tried to decrease the input freq to 5MHz, then calculate the oscillate circuit to amplify the input to 4V, the result is also zero.
And I found one error message of ADS. ADS have two option, one is RF, and the other is digital, the schmitt trigger model is in digital, but the other is in RF. So maybe ADS can't mix RF and digital, right? Anyone have any idea for this.
If I open RF window and digital window at the same time, the simulation can run without error message, but the result is wrong. So I guess they can't be used mixing.
Finally I used a two-transistor differential amplifier to convert sinwave to square wave, the result is not so good, but it seems to work at 5M, not 60M.
the transistor I used is 2N3906, I will try to replace this transistor to get better performance.
Attached is the result of 60M using 2N3906.
PS:
I tried to optimize the RC, and can't get better performance.
Anyone can recommend a transistor?
Also I believe the schmitt triger should be work.