Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Problem with inverters in ring oscillator

Status
Not open for further replies.

Jergosh

Newbie level 2
Joined
May 6, 2008
Messages
2
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,297
Ring oscillator

Hello,

I'm working on an assignment for my microelectronics course, which is to design ring oscillators with different number of inverters and compare their characteristics. Oscillators with 3 or more inverters work fine, but I can't seem to get a single inverter to oscillate of its own accord . Why is that? Is it just that the delay is too small for the inverter to 'notice'?

Also, I designed the inverter of which the oscillators are composed so that it has symmetric characteristics (i. e. equal low-high and high-low time delays as well as the output of 1/2 V_DD at input equal to 1/2 V_DD). However, when I measure the delays in oscillators they are different (e. g. 5 and 7 ps). I can't think of a reason for that either. Could someone please explain that as well?

I'm using Microwind 2 for the simulations.

TIA,
Greg
 

Re: Ring oscillator

To operate as an ring-oscillator, the structure must have a gain above unity with 180° additional phase shift. This requires either two integrators or three poles and sufficient gain. So a single inverter would need additional low pass filters no meet the oscillation condition (if the gain would be still high enough then).
 

    Jergosh

    Points: 2
    Helpful Answer Positive Rating
Re: Ring oscillator

FvM said:
To operate as an ring-oscillator, the structure must have a gain above unity with 180° additional phase shift. This requires either two integrators or three poles and sufficient gain. So a single inverter would need additional low pass filters no meet the oscillation condition (if the gain would be still high enough then).

Could you please elaborate on that? Where does the gain and phase shift come from when we add additional inverters?
 

Re: Ring oscillator

During oscillation startup, an inverter is basically operating as an amplifier with a dominant pole (due to miller capacitance). What would you expect for the total gain and phase shift when cascading these elements?
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top