The thread related to large signal PSpice models and an emitter follower comes from the following problem:
I have an oscillator that should drive a digital part of the system. In short, what is the best way to achieve this?
My first attempt has been a common base Colpitts oscillator that gives a signal riding on the +Vcc rail. This has been AC coupled to a 74AC gate biased to the point that gives square output signals.
This works more ore less ok, but: the startup transient, which is important here, is different depending if the oscillator amplitude is sufficient to toggle the gate. This translates into an envelope that raises more ore less smoothly until the gate begins toggling, where the envelope raises more abruptly -and I guess that the instantaneous frequency changes.
I have thought of two causes for this. The first one is feedback via the DC supply: the spikes generated by the gate switching get coupled back to the oscillator. The second one could be the change in input impedance seen by the oscillator -does this make sense? The cure for #1 could be better supply bypassing. The cure for #2 a buffer stage.
So, what could be a good way to generate a digital signal from an oscillator without loading it? Ideally I would like to preserve the instantaneous frequency of the unloaded startup transient. And: power consumption should be low, say preferably (much) less than 1 mA. Operating frequency should be initially 27 MHz, but ideally scalable up to ~1 GHz.