Practical VCO

i am working on 950MHz center frequency,using LC resonator

i am still simulating using aglient ADS software.

What are you using to look at the waveform? Assuming it is a scope that has that kind of bandwidth, what kind of probe are you using? At these frequencys, probe issues become paramount. Also, what are you doing with the DC control line during your test? Unless it is terminated in a *very* low noise circuit, you will see a lot of garbage on the signal. How is the circuit laid out? Is it on an etched PC board, or a protoboard or what? Again at these kinds of frequencys, layout is

My experience with vco is a lot has to do with the frequency in question, the higher the frequency the more drift and instability and location of components effecting the frequency becomes more critical. Dave

Why don't you just get a free sample from Maxim and see how that compares. The MAX2624 covers 947 to 998 MHz

so, how does anyone design a vco? if the equations of the resonator is for ideal conditions, the parasitic capacitance has a large effect on the resonator; how to take into account the parasitic capacitances effect?

ok, i am going to put a varactor diode instead of one of the capacitors, should i try to tweak the frequency of the resonator without the varactor first and then add the varactor and tweak again, will it be easier and more accurate or it doesn't matter put a varactor and then tweak.

Forget *anything* from Maxim; most of what they advertise is vaporware.

ok, what about the reflection amplifier should i put it also then start tuning or it is better to put it after the resonator outputs the wanted frequency and which analysis technique is better and easier negative resistance or feedback?

regards

Ah. At theses frequencies, a coils definitely is not just a coil anymore and a cap is not just a cap anymore. You should take the effort to actuallt measure the parts before simulation. Plus, the unspecific waveform can be described in therms of harmonics. How far down is the second, how far down is the third ? A scope is not the right tool for this job.

Rene

Well, either by simulating with actual parts, meaning including parasitics, or by tweaking the circuit afterwards. I suggest to do a simulation with actual parts "to come close" and tweaking afterwards. Don't forget to spend 1nH per mm of copper trace.

IMO, the gain from getting a cheaper VCO than available as actual part has its price that ought to be paid back by the numbers of units sold.

Rene

You're going to a have a varactor anyway, how else would you get a voltage dependent frequency ? Depending on your requirements in terms of harmonics, one varactor may not be sufficient. A twin varactor arrangement has lower harmonics since the waveform itself changes the capacitance of just one diode. Twin diodes can be arranges such that the selfinfluence is cancelled out.

Rene

While I did recently read a lot about that subject, I tended to purchase VCOs for 20$ or so in single unit quantities.

Rene

I'm just after building a VCO myself but on 100Mhz. I used the design and analysis procedures layed out in the book "Complete wireless design" by Cotter w. Sayer. It covers VCO and Amp design from HF to UHF.

Kev>