Hello everyone, I need to design a cmos VCO with a tuning range of 45 mhz to 55 Mhz and with a pssr of min 35 db. I am confused with what type of VCO should I go for LC tank based or ring VCO?
Your help would be much appreciated

 

I'm not sure what kind of oscillator you have in mind, but you need to start with the correct units:
I think what you meant was 45-55 MHz.

1. 45 mhz is 0.045 Hz which is very very sl-o-o-o-w-w-w.
2. 55 MHz is 55,000,000 Hz. which is in the low VHF range. It also overlaps the 6M amateur Radio Band and the 48 MHz. Public Service Band

Tell me what you intend to use it for so I can figure out what approach will best suit your needs.

 

Thankyou for your reply. The frequency range is 45Mhz- 55 Mhz The purpose is to recover clock from digital signal.

 

Should I go for ring VCO or LC VCO?

 

Thankyou for your reply. The frequency range is 45Mhz- 55 Mhz The purpose is to recover clock from digital signal.

OK, that is a helpful piece of information. What you seek is a VCO, implemented with CMOS tecnology, the produces a squarewave output. Would you prefer to start with a higher frequency and divide it down to your desired range, or would you prefer to have a lower frequency reference that your VCO would be phase locked to?

 

Can it be done without a frequency divider ? I mean can it be tuned to that frequency range?

 

Can it be done without a frequency divider ? I mean can it be tuned to that frequency range?

Maybe, but the combination of frequency and bandwidth make it a pretty big ask. A VHF oscillator can be constructed and tuned with a varactor diode, but the required bandwidth could be a problem. Using a divider and a lower frequency reference or a higher frequency oscillator with a digitally programmable divider makes getting the bandwidth you require a much more reliable proposition. Perhaps somebody has already done something similar to what you have requested, but I'm just not sure where I would want to start on such a quest.

 

I have tried tuning the circuit with a varactor diode. I faced problem with the varactor diode. The varactor is not changing the capacitance with tune voltage. Could you guide me with the varactor diode how to use it from cadence library?
Or could it also be done with ring oscillator based VCO?

 

I have no familiarity with the cadence library. If the capacitance is not changing with the tune voltage then you are doing it wrong. Post a schematic of what you tried.
What active device did you use for the oscillator?
Did you sweep it on a VNA?

 

Just an idea:

Below is a popular LC VCO circuit.

Band 3 on this band switched oscillator covers from 41 to 73 MHz. And amplifier would probably be needed to amplify the control voltage and a fast limiting amplifier would be needed to square up the frequency meter output and amplify it to that needed to switch CMOS.





Frequency meter output (above).