wide tuning range VCO problem

EDAboard.com Forum Index -> RF, Microwave, Antennas and Optics -> wide tuning range VCO problem

Author

Message

gggould

Joined: 10 Apr 2004
Posts: 26

10 Apr 2004 4:43 wide tuning range VCO problem

Hi all,

I am designing a VCO with varactor and switching cap to cover 800MHz wide. But I find that the VCO gain and amplitude variations are big problems. Both Kvco and output amplitude increase with the operating frequency...

Any tips or advices?

Thank you very much...

flatulent

Joined: 19 Jul 2002
Posts: 4875
Helped: 324
Location: Middle Earth

10 Apr 2004 5:26 Re: wide tuning range VCO problem

Your best bet would be to add an AGC loop. Measure the output voltage after a buffer amplifier and use it to change the bias on the oscillator transistor. Another thing for it to control would be an attenuator.

IICCEE

Joined: 24 Dec 2001
Posts: 233
Helped: 2

10 Apr 2004 5:36 wide tuning range VCO problem

you may use **several** the varactor ,some place near the BJT-E,some near BJT-B;
and you should be carefully design the buffer in the working band,also add an AGC loop.

gggould

Joined: 10 Apr 2004
Posts: 26

11 Apr 2004 5:10 Re: wide tuning range VCO problem

Thanks for all your feedback...

But I still don't know how to get a more constant Kvco. Right now the Kvco at high band (~4GHz) is almost 1.7 times the Kvco at low band (~3GHz).

Hi IICCEE, I don't quite understand what you mean. Could you explain the concept a little bit more? Right now I am placing a pair of back-to-back varactor between cross coupled BJT-C for fine tuning.

Besides, is there anything I should look into while deigning those buffer (emitter follower)? I only know I got to check parasitic caps.

Thanks again!!!

IICCEE

Joined: 24 Dec 2001
Posts: 233
Helped: 2

11 Apr 2004 8:24 wide tuning range VCO problem

i mean not only let varactor between cross coupled BJT-C for fine tuning,but also copuled from BJT-E.and using more than two varactor. (like mini-circuits product)

and also you can using ADS designer guide to do this,and make some modify for you application.

Google
AdSense
Google Adsense

11 Apr 2004 8:24 Ads

VSWR

Joined: 07 Feb 2002
Posts: 767
Helped: 72

11 Apr 2004 21:20 Re: wide tuning range VCO problem

Your VCO is tuned over about 23 % frequency range (800 MHz/3.5 GHz). A change of VCO sensitivity Kvco of 1.7 is not unusual in such a large frequency tuning range. And change of output power, as oscillator Q changes during tuning.
Which varacor diode are you using?
How large change of tuning voltage is required for 800 MHz tuning?
What is the change in output power?

gggould

Joined: 10 Apr 2004
Posts: 26

11 Apr 2004 22:57 Re: wide tuning range VCO problem

Thank you, IICCEE. Now I got it.

I am using NMOS in N_well varactor with 0.3~2.5V Vtune.
"VCO" (not buffer) output changes from .4Vpp to .7Vpp .

Gee, the Kvco change is really a big headache to me.
How can I design the loop filter to compensate the Kvco variation.
Besides, the +- 20% varactor process variation (which will cause about 300MHz shift) also piss me off...

flatulent

Joined: 19 Jul 2002
Posts: 4875
Helped: 324
Location: Middle Earth

11 Apr 2004 23:37 Re: wide tuning range VCO problem

As far as the tuning sensitivity goes you have two choices. One is to make the loop parameters wide enough to be stable with all values. The other is to make a compensation circuit which lowers or raises the loop gain at the low or high end. This can be done with a voltage divider with series diodes and resistors in the shunt path. It can also be done with the diode-resistor networks in the feedback of an op amp.