Oscillator design, need some help, freq 40 Mhz, curr 60uA

[wangwang96]

Oscillator design help

Hi,everybody,
I want to design a oscillator and the frequency is about 40MHz,the current is not more than 50-60uA,if use ring structure,the size of transistor should be small enough because the current is too small,but the stability maybe a issue,if use a PLL,the power comsumption may be larger than expected,I am not sure which structure is more applicable?
I am a junior and have not enough experience about it,Anybody can heip me and give me some advice?
Thanks in advance

 

[FvM]

Re: Oscillator design help

Consider a one transistor crystal oscillator with low output voltage and a resonant circuit to cancel the capacitive load. What is intended as oscillator load? With pF load capacitance and resitive output, it can source only mV amplitude.

 

[wangwang96]

Re: Oscillator design help

Consider a one transistor crystal oscillator with low output voltage and a resonant circuit to cancel the capacitive load. What is intended as oscillator load? With pF load capacitance and resitive output, it can source only mV amplitude.

one transistor crystal oscillator? but we have no crystal to use

 

[renwl]

Oscillator design help

ring oscillator can meet your requirement.
it's stable too.
but the output frequency variation is large.

 

[wangwang96]

Re: Oscillator design help

ring oscillator can meet your requirement.
it's stable too.
but the output frequency variation is large.

I designed the ring oscillator before,from silicon results,the jitter of output frequency is terrible,I am not sure it is OK this time.

 

[jecyhale]

Re: Oscillator design help

Current is too small, it is not easy to design.

 

[wangwang96]

Re: Oscillator design help

because the current is too small,the size of MOS is small accordingly,from siilicon results,the jitter of output frequency is too terrible,can anyone help me to overcome the jitter of clock?

 

[cinch]

Re: Oscillator design help

You can go for RC Relaxation type oscillator. You can also implement trimming feature if its needed, very easily. The critical blocks in this are - Reference current and voltage generator (from bandgap), comparator and few digital blocks for initialization process.

cinch

 

[wangwang96]

Re: Oscillator design help

You can go for RC Relaxation type oscillator. You can also implement trimming feature if its needed, very easily. The critical blocks in this are - Reference current and voltage generator (from bandgap), comparator and few digital blocks for initialization process.

cinch

Yes ,the RC type oscillator is a good choice,but the bandwith of comparator is a issue(36MHz),if DC gain is 40dB ,then GBW is 40dBx36MHz,I think it is difficult for me to design the comparator.

 

[cinch]

Re: Oscillator design help

Bandwidth should not be an issue. Use micro-power opamp as comparator. Micro-power opamp has poles at high frequencies, so u ll get good BW. Also, keep the output swing low for the comparator. Since you use a latch for comparator output, comparator need not settle to final value. Even the transition will trigger the latch. So BW need not be very high. Only issue is comparator will consume quite a lot of current if frequency is high.