extending range of 2ghz+ VCO

Colin,

It looks like you are using off the shelf components to do your design, but if you are working in a CMOS process and fabricating a chip, I would look into the recently developed DCO's from TI. They have written several papers on them, which use MOSFETs to 'digitize' the frequency range by roughly doubling the width in the MOSFETs (which roughly doubles the capacitance each gives). Using dithering (see their journal papers) you can cover a wide frequency range at very precise intervals.

However, this will probably not apply to you if you are using strictly BJT's, which is more common in off the shelf components, which is what I think you were mentioning here.

- zielstep

col> Hi,

Several people make stock 1-2 GHz VCOs, Emheiser for one.

What sort of laser are you using? And what's the modulation driver circuit like?

I recently discovered a VCSEL laser that modulates great at high frequencies but badly at *low* frequencies. It behaves like there's a very slow PIN diode in series with the lasing junction.

John

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Thanks, that looks interesting, I didnt find any thing like that available from usual supliers (RS, Farnell, ebay) I wonder what circuit they use ... im also curious as to what they use in wide range microwave signal generators, I gues they used to use yig in some.

Im using a cheap 5mw std red laser diode such as rlt6305g. the spec is very limited. The mod/driver circuit originaly was designed for use at 10mhz but worked fine at 100mhz, A common emitter bfg591 drives the laser, it has upto about 3vpk to pk on the collector, it needs a pull up to drive it over

strange ... I have heard that its best to avoid 100% modulation as the diode takes a finite time to start lasing properly again.

The detector circuit is an APD with a bias modulated with a LO of similar frequency.

I may be able to get away with a smaller relative bandwidth if I can get the frequency high enough wich will alow for much easier tuned stages.

Colin =^.^=

It is difficult to make a VCO cover much over one octave with continuous tuning.... it can be done with switching however... for an octave in frequency you need a 4:1 cap change...look at the hyper abrupt tuning diodes...

Mark

A good VCSEL can go from zero current to full-on with about a 100 ps risetime and well under 10 ps RMS leading-edge jitter, if you drive it with a clean, fast edge. The part I'm talking about here is apparently

John

Yes as you point out frequency ratio is sqrt of capacitance ratio, however some diodes such as the bb132 (now seemingly obsolete) has a cap ratio of 26:1 wich is quite impresive, wich makes tuning well over an octave quite easy, at least at lower frequencies however as it is listed as a vhf diode, presumably not designed to work so well at 2ghz.

many diodes have a ratio of well over 4:1 usuualy 8:1 even for uhf/microwave types, the hyper abrupt diodes make it more convenient as they use lower voltage

diode

What frequencies are we talking about ? and what does the spec say ? maybe at high frequency the junction does not have enough time to lose all of its carriers, and therefore lases quicker, at low frequencies the junction may become 'stone cold' maybe its worth adding a small bias current ?

I had assumed I would need a better LASER as the spec does not mention diode capacitance, however it seems to acheive a high modulation at 1ghz.

Colin =^.^=

Could you define please exactly what modulation signal it's required for the LASER first ? RTL6305 can be driven at 1Ghz ? (it hasn't any modulation specification on datasheet). So how did you measured that it works ? A complete datasheet of this laser diode will help us to understand what do you mean (and satisfy also our curiosity) Then check if the YIG is not too lazy (as it usualy is) for your application. Then try to see what phase noise is admitted for your signal, because

More than that, there are syntesizers up to 1Ghz without DDS (which have a poor SFDR and required good designed and manufactured filters on the output) in single chip. And there are cheap x2 multipliers (which of course are adding noise).

And BTW, you need probably some output power too...

greetings, Vasile

col> > > >

The modulation for the laser is for range finding (LIDAR), just a continous but variable frequency, the returned phase delay is used to determine range, a higher frequency allows better resolution, a variable frequency is needed for range >1 wavelength. It uses an optical hetrodyne receiver.

Ive had the system running at 1ghz so far with this diode. It hasnt worked so far at 2ghz but I think this is due to problems with layout, receiver and diode capacitance etc. I need to sweep from 1ghz to 2ghz to see where it starts to fall down.

I have a HP YIG from an old analyser, its rather large though and I would need 2, I havnt tried it as yet though. if possible I would like to use simpler varactors.

Phase noise is a problem at the moment, I look at the beet frequency of 2 VCOs 1mhz apart and its not too pretty, this might be imrpoved with a new layout with better supply/shielding etc.

My first step was to look at an old elc1043 tuner, however using a simple colpits with good layout and 5ghz transistor its quite easy to get up to

Satelitte tuners look more interesting.

I have a number of >1ghz oscillators but with very limited range, but im looking to do it with as few as possible.

Like I said,.. idealy I would like to switch in different inductive elements to just one VCO, perhaps seemlessly.

wide range tracking filters for multipliers etc is quite a hassle.

Well I have 2 stages of amplification and I just obtained a 45ghz 600ma driver transistor for the laser driver.

Colin =^.^=

That sound nice, it's homebrewed too ?

I've seen this datasheet but it's blind (for me). There is no frequency modulation specification inside...

and which is the distance range you could sense ? what accuracy ?

It hasnt worked

Did you supplied the modulator with a microstrip or stripline and adapted impedances on both ends or just a quick and dirty shielded cable ?

What can you see on the PIN control photodiode ?

Unfortunately the phase noise depends also by the VCO schematic. I've bite deeply on this problem using a PLL clock distributing circuit from TI which distroyed the phase response from -140dBc/10Khz to about

-80dBc/10Khz . I think your solution will be a fractionar/integer PLL . Take a look here:

Switching from 603 to 402 or 201 does not help too much as long they aren't microwave components. Often suppliers (like Digikey) are mixing microwave with general purpose components. The result is a mess because you don't know which is ok and which is not without having a reference. At least L and C should be verified SMD for RF purposes in Ghz range.

yes that's true, but that depends if the images are bothering or not.

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Yes I made one some time ago wich worked at 10mhz rf/455khz if

specification

Yes indeed, maybe they dont expect it to be modulated ..?

Well at 1ghz 90' phase shift is 37.5 mm, assumming a modest 1% resolution this would be .375 mm, I have no optics at the moment. 0.01 mm would be a challenging target.

and

Ive tried to avoid having to match impedances at the diode by avoiding long signal lines. the diodes are hardly well behaved 50ohm impedances so would be hard to match. Ive tried to compensate for the capacitance with a microstrip inductor.

I just use it to monitor the average power, I would gues any signal there is mostly due to crosstalk. it might be usefull to look at it if it gives a more reliable phase relationship to the output.

would

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Im using the lmx2434 as it can handle both VCOs, I tried the lmx2486 too. I think I might go back to using single PLLs to avoid cross interference. I hate those thin legless packages too. Ive managed to avoid my pulling problem I had before by mounting the VCOs on seperate boards, with good cables/smc connectors 3db matching pads at each end etc. Its all too easy to lose phase noise performance.

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Actually I hadnt thought of looking for componets specd for microwave performance, I just assumed the parasitic LRC pretty much inevitable.

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I think images could affect linearity/acuracy, but not sure to what extent.

Colin =^.^=