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Wideband Microwave VCO design

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radiohead

radiohead

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wideband vco

Hi all!

At the moment I am trying to design a microwave wideband VCO tuning from 2.5-4GHz making use of discrete transistors and varactors (BB857). I can realize the tuning bandwidth, but the simulated phase noise is very bad. I use ansoft designer to get this result. Can anyone give me some information which circuits are to be preferred for these sort of designs? I saw that some VCO module manufacturers realize -110dBc/Hz at 100kHz! Most information I read about oscillator design is for narrowband! Any articles/references/books should be great!

PS I found the wideband VCO article from Vidmar. I need more suggestions!
 

microwave vco

Look at the VCO made by Matiaz Vidmar on its spectrum analyzer : 2 to 4 GHz, make a search on EDAboard
 

push push vco

Dear friend,
VCO tuning range and phase noise are contradictory specifications in a VCO design. You are talking about a VCO with almost 1 octave of tuning range, what necessarily should be associated to a poor phase noise.
Wideband VCOs are used in large deviation systems or in digital systems with lots of target's margin.
You should still regard other sources of noise that will degrade the phase noise even more in a synthesizer using a wideband VCO as for example 60Hz modulation due to the power supply's ripple, leakage of reference frequency through the loop filter, etc...
You must to be sure that your system target will not be hurted in crucial specs due to a wideband VCO.
To suggest you something else to be used in place of the wideband VCO, I would need additional details of your application.

NandoPG
 

wide band vco design

Dear nandopg,

I am aware of the fact that broad bandwith and low noise are contradictory specs. However, some people are able to combine good performance on both levels (cfr Micronetics VCO's). I am wondering how they do it.

The design I'm making is for a research project, it will be used in a synthesizer. Quick calculations made out that indeed the phase noise is not so very crucial. However, the point is that when doing research, we're not talking about sufficient performance, but about best achievable performance. And comparing the specs of other VCO modules points to mine points out that a lot of improvement can be made to my designs on the area of phase noise. So I am in need of good information about this subject.
 

wideband + vco

Dear Radiohead,

To design a wideband VCO I used to follow steps bellow:

1- Use a oscillator's topology known as negative resistance oscillator in commom collector configuration.

2- Make sure the transistor keeps a negative resistance at the base terminals constant over the whole tuning range.

3- Connect the resonator at the base terminals.

4- Use a virtual ground technique to determine the loaded Q of the resonator.

5- Check using the Lesson's equation the expected phase noise.


NandoPG
 

vco design

Hi NandoPG,

I appreciate your help! I used your methodology for my designs. Question is: what about the resonator? Do you use two series connected varactor diodes with a series inductance as in the clapp oscillator? Or two series connecter varactors in parallel with an inductance, coupled with a coupling capacitor (Collpits). Do you use a tuned microstrip resonator, like Vidmar? And what's the best to optimize phase noise?

High tuning bandwidth means high resonator loading means low Q ... how to solve this? Any suggestions?
 

microwave vco design

Dear Radiohead,
I used to use a resonator made up by a lumped SMD coil in series with one hyperabrupt varicap in series with a small cap (ATC 600S series) to center the tunning range. This resonator should be connected between base and ground of the transistor.
The choice of the SMD coil is an important merit. You should find a SMD coil with enough unloaded Q at the tuning range. The High Q series from Coilcraft can be regarded for this end. Remember to regard the physical model of the resonator's components in your simulations

NandoPG

Note: I always got better results using lumped resonators than microstrip implementation.
 

matiaz vidmar

No one mention where is the specific tradeoff phase noise versus tuning range.

It is the varactor curve themselve. The varactor could have a big tuning range if the tank amplitude is small. If the tank amplitude is big the effective capacitance gets lower and tuning cap ratio goes also lower. In addition to that the tuning voltage range gets limited because of forward bias at high amplitudes. But high amplitudes are required for low phase noise. That is exact the point where the tradeoff start.

Use switched tuning instead!
 

push push vco

Greetings...

When you desing a VCO, you must maintain a tradeoff between quality factor and wideband. Basically, noise phase affects your design as well as noise signals modulate the bias point in varactor, resulting in a FM signal.
Both ring oscillator and tank LC are the most discussed topologies about VCO. However, first one can tunnig in a wide spread band, but consumes high power. Instead, the second one, has lower power comsumption, high Q, but occupies a great area, and it'd be less reproductibility.
State-of-the-art about VCO is looking for phase noise below to -120dBc/Hz around 2.4 GHz. It can reach even lower phase noise, maybe, working in sub-threshold region.

I advise you that begin read the follow paper, that will give you some directions and overview.

Oscillator Phase Noise: A Tutorial
Thomas H. Lee, Member, IEEE, and Ali Hajimiri, Member, IEEE
www.chic.caltech.edu/Publications/phase_tutor.pdf
 

site:www.edaboard.com vco

Hello radiohead!
try to find this article
Wideband Low Phase Noise Push-Push VCO by Marco Gris.
Applied Microwave and wireless , available at web archve of this jornal.

"Push-push topology reduces noise compared to a conventional design" he said.
 

varicapless vco

Hey divan,

I've been searching for the AM&W article on their website, but the archive is not opened for public. They sell a CD with all issues, but I cannot get articles for free. Is this CD available on MCU?

Thanks to all for the helpful tips! I love this webboard!
 

vidmar matiaz analyzer

Radiohead,
Find attached the article requested, although I don't think that is the way to go but you should try by yourself.

Wideband Low Phase Noise Push-Push VCO by Marco Gris.
Applied Microwave and wireless.
 

narrow band microwave vco manufacturer

Thanks nandopg,

though ... the link is dead ... can you fix it or put it elsewhere?

ADDENDUM:

got it right now: seems very promising! Thanks mate!
 

combine vco to lower phase noise

HI radiohead !
article is attached
 

low noise vco design

hello,divan----
try to find this article
Wideband Low Phase Noise Push-Push VCO by Marco Gris.
Applied Microwave and wireless , available at web archve of this jornal.


i've download this article from here,but i can't open it.
please tell me what kind of this document?
good luck!
 
wideband tuning range vco

There are topologies where the VCO changes tank circuits depending on the frequency. This is like having multiple narrowband VCOs working together to get the full tuning range and the phase noise.
 

micronetics vco

Look for Ulrich Rohde books about VCO and RF communication equipment. I've find them in Internet.
Another way to build wide band VCO is varicapless current tuning multivibrator. Possible it will be better then VCO with bb833/bb857 or smv1265
 

wideband push push vco topology

With the Leeson's phase noise formula, to realize the phase noise of -110 dBc/Hz at the frequency range of 2.5 to 4 GHz, the required loaded Q is only about 10.

While in this frequency range the microstrip resonator Qu will be about 150 with ROGERS4350B.

So the main problem of this design I think is how to optimize the circuit parameters to realize the Ql of about 10.
 

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