Working mechanism of the LC cross-coupled VCO

[wccheng]

Dear all,

I have some problems about the working mechanism of the LC cross-coupled VCO. As I see the schematic of the LC tank cross-coupled VCO, I will find that one side of the transistor output will be always stack at HIGH and stack at LOW on the other side of the transistor output. Therefore, how could it oscillate the differential output? Is it some tracks to explain the oscillation effect?

Thanks

wccheng

 

[DJ]

There is a feedback. The phase between B to C (or G to D) is 180 deg. The feedback is as follow: B of Q1 to C of Q1, then to B of Q2 and then to C of Q2 and coming back to B of Q1. There is total 360 deg and gain (B to C).
The tank parallel resonator shunt any signal out of thr frewuency band. In the oscillation range the tank presents high impedance but low out of the range. So the resonator prevent any feedback outof rsnge.
Other way to look is a negative resistance at the C (tank input). The cap at the E and ind at the B presents negative resistance that is loaed by the resonator.

 

[wccheng]

Dear all,

I have read the book - "Design of Analog CMOS Integrated Circuits" on page 499. It said that the LC crossed-coupled oscillator does not latch up because its low frequency gain is very small. Actually, I know that why the gain is low at the low frequency. However, why the low frequency is easier to latch up the circuit (as memory to store 1 or 0)? Could anyone answer me? It is because I am still trouble in the working mechanism about the cross-coupled oscillator. Actually, I know the DJ said that it is need to fulfill the Barkhausen criteria. However, how could I explain why this circuit will not stack at high or low?

thanks