wide tuning range VCO problem

[gggould]

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]

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]

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]

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]

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.

 

[VSWR]

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]

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]

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.

 

[lijulia]

how about use coarse tuning to compensate the process Temp variation?

 

[quickdraw88]

Not sure exactly what your frequency range is. I've tried to design VCOs for around 3 GHz and it's not easy. If it's a one off design, consider buying a commercial one. If you need to linearize your tuning make sure you're using a hyper abrupt varactor. You mentioned you're switching capacitors to cover the range in bands? That will guarantee your Kvco will change. A better though more costly method is to switch in two or three VCOs to cover the entire range. Regular schottky diodes can accomplish this easily though PIN diodes will have lower loss and provide higher output.

If you want to linearize the Kvco because you're worried about stability of the loop, don't. A Kvco range of 1.7 isn't bad. Pick the median Kvco and design your loop filter for 56 degrees margin at the median and you'll find you're probably still better than 45 degrees margin at either extreme.

 

[zihnisinir]

One of the solutions is to use 2 or 3 MOS varactors in parallel with different biasing points. This will help you to have a linear C-Vtune (or actually you need linear freq-Vtune) characteristic for MOS varactor tank. There are several topologies for this issue, you can check some patents. I was desinging such a VCO for 2.45 GHz ISM Band, you can find some of the figures in attached.