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High resolution VCO

High resolution VCO

High resolution VCO

(OP)
Please help: What VCO would replace the Agilent E4422B (or equivalent) in the 2.000-2.300 GHz range? We are looking for a comparable high-resolution. thanks.

RE: High resolution VCO

Any VCO that covers that frequency band would work.  Go see minicircuits, Zcomm, etc.  

VCOs are voltage controlled oscillators.  You would have to build a control loop to feed them the precise voltage required to achieve the frequency resolution that you need.  That control circuit is called a phase locked loop, and the entire subsystem would be a frequency synthesizer.  You would program the synthesizer's frequency with a serial digital word.  

You would need a knowledgable engineer to design it for you.  But you should determine exactly how fine a frequency resolution you need, as finer resolution means more complex circuitry.  For instance, a 1 KHz resolution is much cheaper than a 1 Hz resolution.

RE: High resolution VCO

There are VXI board solutions that are between a boxed solution like the HP you mentioned, and a uniques circuit design.  You need to know your tuning time, and the spur levels and noise floor you need.  If you do not know your real requirements and just go about the job by saying you need to match the noise and spur level of an HP synthesizer, then you may not find an alternative.  If your only requirement is resolution, and you can live with long tune times, high spur levels, and a large noise floor, then there are many options open to you.

RE: High resolution VCO

(OP)
biff44,
thanks, but what's wrong with a having a high-resolution voltage supply to control the VCO (w/ voltage resolution high enough to match the sensitivity of the VCO)?

RE: High resolution VCO

nbound.  You still will not have accuracy unless you do some periodic calibration.  Unless you reference to something stable like quartz, you will have drift.  Your DAC will probably be much much better than your VCO accuracy (resolution is not the same as accuracy).
The Agilent references to quartz.  Therefore, I thought you should address the accuracy issue.
This leaves you with tuning speed, spur levels and the noise floor.
What are your actual requirements?

RE: High resolution VCO

(OP)
VisiGoth,

I am tracking a cavity w/ a tuning speed that is most likely to be slow (seconds) for most applications. All I care is that nothing disturbs the oscillator to within 1-10 KHz (noise, spur) from the voltage set frequency,which I need set with ACCURACY to within 10-100 KHz to match the cavity normal mode.

RE: High resolution VCO

VCO's in your range that are not locked will be within 2 MHz typical even with good epriodic open loop linearization.  If you want kHz accuracy then you need a DDS (very fast, bad spurs and poor noise floor) or a phase locked VCO.  If you are just tracking you might be able to lock to your cavity.  But if you already have your architecture than a more standard approach would be to build a PLL referenced to a crystal and use the side information you must have for the cavity frequency that must be better than 1 kHz accuacy.  

There are a lot of good books on VCO PLL.  There are some practical concerns that you could get from an experienced engineer in this forum possibly.  If you can afford it, there are single board solutions, VXI and even PXI solutions. It all depends on your time frame and what your NRE costs are relative to your purchase costs.

RE: High resolution VCO

(OP)
Thanks. I am interested in a single device/board solution. I am not familiar with VXI or the PXI boards(?)(but I hope this is not getting closer to the Agilent cost)! Please recommend a manufacturer if you have any.

RE: High resolution VCO

I am only familiar with the more expensive solutions for fast tuning.  Since you are only interested in a narrow range you might find some used RF synthesizers on the market.  If you are buying hundereds of these for factory testing then a more custom solution could be found.
You could try these, but you could find lower cost if you can have a slower speed.

http://www.aeroflex.com/products/pxi/3010.cfm
http://www.elcom-tech.com/elcom_ads.pdf

RE: High resolution VCO

What is wrong with just driving a simple vco from, say, a 14 bit DAC?  Well...it depends on your application.  A typical vco can be tuned to a frequency with a stable voltage source.  You would need some feedback mechanism in your test set to tell when you were at the precise frequency that you want.  Then, over the next second or so if you can tolerate the VCO drifting a MHz or two, you are all set (the vco is open loop, so any change in power supply voltage, case temperature, load impedance, etc, will cause an instant change in VCO frequency).

If you want to set the vco to a SPECIFIC frequency from pre-knowledge of where you want to go, and without the need to use something like a frequency counter to measure it and dither the voltage up/down until frequency is close enough, then you need a closed loop system--a PLL based frequency synthesizer.  The closed loop system will NOT drift frequency with time, so once you set it on frequency, it will stay there for the next year.

RE: High resolution VCO

If you are looking for a cheap one time test equipment purchase, go here and look at the AV1485:

http://www.madelltech.com/AV1485.html

If you are planning on manufacturing hundreds of these "VCO's", I would be glad to design you a low cost solution.

RE: High resolution VCO

(OP)
I'm using  an agilent unit = to av1485, which I'm replacing with a narrow band signal source (SC).
This SC will be voltage controlled w/ sensitivity within 40 KHz/mV (some VCO's are capable of that). I'll be adding to this, a VC attenuator (to control the power output). And since my tunability speed requirements are on the order of 500 msec, I don't think I need a PLL. In fact, the feedback loop could easily go through a laptop measuring the voltage reflecting an incident/reflected power from an antenna. I appreciate your comments. thanks

RE: High resolution VCO

Sounds like you have the system figured out, and free running is adequate for you.  I would do 4 things:

1) Put a ferrite isolator between the VCO and the variable attenuator, something like Model No. PE8301, from http://www.pasternack.com/Search/SearchResults.asp?RecNo=0&Category=ISOLATORS%20%2F%20CIRCULATORS

This way the oscillator will be happier for various loads and attenuator settings.

2)  I would use a VCO like this one: ZX95-2500 from mini circuits.  It is around 90 MHz per volt sensitivity.

http://www.minicircuits.com/oscillat.shtml

3) Put it all in a box to airconditioning currents can not change the temperature of the vco easily.  

4) Use a very regulated power supply for the VCO for the +12 Volt power supply.  A few 22 uF tantalum capacitors right at the VCO power supply pin would not hurt either.

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