Output of VCO module - need verification

[watertreader]

Output of VCO

Hi,

I have tried to power out a VCO module(mini circuits ZOS -200) that I have bought with a control voltage of 9 V


Is this the correct output from a VCO... the signal does not look too clean??

Anyway to improve them? (does regulating the power supply helps?)

Thanks

 

[wpchan05]

Re: Output of VCO

The amp is very small! Is it supposed to be like this range?

Also, you've got 53 MHz in your vco. Is it supposed to be?

 

[watertreader]

Re: Output of VCO

it doesn't really fit my control input....

 

[biff44]

Re: Output of VCO

No, that does not look right. It should look, more or less, like a sine wave. ZOS-200 requires a +12 volt supply, are you giving it that? Tuning voltage should be positive on the center pin of the tune port.

Set your scope to 1 ns/div, to make sure you are not aliasing the signal. You should be around 150 MHz at 9 volt tuning voltage. Also, make sure the scope input is set to 50 ohms in. You could try AC coupling it, just in case the VCO does not have a blocking cap inside (but is should). Also, make sure you are not lowpass filtering the scope input.

If none of that helps, try putting a 0.1 uF ceramic capacitor in parallel with something bigger (say 4.7 uF) between the power supply pin and vco ground--you could have some weird power supply oscillation going on. Keep lead length short. If the power supply is the trouble, you should see oscillations on the power supply line.

 

[watertreader]

Re: Output of VCO

Hi,

I have tried your method but it doesn't seem to work. I have tried to vary the control voltage and found that in low voltage about 1-2V there is perfect sinusoid. By tuning to the higher control voltage... the signal corrupts to the form that is attached above

The output voltage frequency is at about 50-60MHz. It seem that the VCO seems to behave like an low pass filter(or low pass filtered) or either that there is some problem with my osciloscope.

Do you have any similar occurrence with VCO?

Thanks

 

[biff44]

Re: Output of VCO

Well, 60 MHz is pretty low according to the data sheet. Do you have another one lying around that you have not yet hooked up?

It sounds like it is blown up, and my guess would be that the varactor diode is fried by accidental application of the wrong polarity tune voltage. Happens all the time, especially if you are driving it from an op amp. The varactor diode is a silicon junction diode, and if you reverse bias it it acts like a variable capacitor, but if you accidently forward bias it, it can conduct current. If it conducts too much current, like .5A, it can fry itself.

In hooking up oscillators like this, in most cases I like to put a 220 ohm resistor in series with the center conductor of the tune port connector. That way, no matter how I drive it, or how I sequency the power supplies, I am prevented from blowing up the diode (the 220 ohm series resistor will limit the worst case current thru the varactor if improperly biased).

If you do not have a new VCO to try, put a limiting resistor (1K ohm) in series with the tune port, and use a power supply to tune from 0 to positive 16 volts. There should be no appreciable voltage drop across the resistor, since the varactor is reverse biased for that range. If there is a big voltage drop, then the varactor diode is breaking down where it shouldn't.

If that worked, then reverse the tune voltage power supply (again with the 1K series protection resistor), and set the external tune voltage to around -3 V. The varactor should be forward biased in this case, so you should read around -.9V at the tune port center conector, and -3 volts on the tune voltage power supply. If so, this is good--is says the varactor diode is still in there and acting like a diode should.

 

[watertreader]

Re: Output of VCO

thanks for the help... think i have managed to learn a great deal from you... managed to solve the problem... think there is some problem with the cabling

once again thanks