Mini-circuit VCO

[ve3bbn]

Greetings I am new here so please be patient woth me if I should blunder this thread. I have purchased and used the VCOs in a few of my projects and I think they're great. I have one question of which I have been searching out where I may.
The VCO's have a 10dbm output. I believe that to be 10mw.
and into a 50 ohm load should give me an output of .707 v rms.
or 2.828 v p-p. this should be easily measured on a Tecktronic 4 channel 300 mhz scope with no problem.
In realit I see only 1.75mv and the output is a very distorted waveform.
Is my above math correct? Any suggestions from the group would ba greatly appreciated.

Cheers David ve3bbn

 

[MikeHalloran]

What's the intended frequency range?



Mike Halloran
Pembroke Pines, FL, USA

 

[biff44]

Yes, your math is right.

The VCO may be blown out. I would check for other obvious mistakes,like is the 300 MHz bandwidth high enough for the actual VCO output. Is the "bandwidth limit" button on the oscope pushed in. Are you using a X10 probe that may not have the bandwidth. Try putting some tuning voltage on the VCO and see if a volt or two makes it oscillate more. There is a small chance you need a DC blocking series cap on the VCO output. Make sure the output is well connected to the scope input--if the vco is a pin output type, make sure there is a good ground connection to the coax connector, etc.

 

[biff44]

ALso, the scope may be having trouble triggering, which sometimes looks like a small noise signal, but when triggered properly grows to a huge sine wave.

There is a small chance it is a digital output oscillator. In any event, a lot of these cheap oscillators do not look very much like a sine wave (lots of 2nd and 3rd harmonic output)

 

[ve3bbn]

Thanks guys for the quick response. The original 5 100-200 mhz
vco's had virtually no output as I explained. The new batch have 5 mv output p-p. I am heading out to the shop at this time and i will check out the scope settings (I think they're right
but will check anyway) to see if they are ok.
I don't think the operating freq. is of any importance (correct me if I'm wrong) as 10 mw is 10mw at any freq.
Reactances change with freq. but the 10dbm is still 10mw.
I really appreciate your input and will check out the scope.
The probe was a direct one.

Thanks again es cheers

David ve3bbn

 

[IRstuff]

No! if you are using an oscilloscope, then the impedance and frequency response of the oscilloscope becomes a paramount importance.

Would you expect to pass a eye exam with dirty sunglasses?

TTFN

Eng-Tips Policies FAQ731-376

 

[ve3bbn]

Greeti9ngs IRstuff

I don't know how to answer this question because I didn't mention the freq. of the scope as being of no importance.
The question was what was the freq. of the VCO?

**No! if you are using an oscilloscope, then the impedance and frequency response of the oscilloscope becomes a paramount importance.
**Would you expect to pass a eye exam with dirty sunglasses?

I see there is also arrogance on this site, I guess that comes with being a so called engineer of sorts. Also the addition of the **Eng-Tips Policies FAQ731-376**

I guess from what I read here is that you are to search out all other sources before asking a question on this site.

In my opinion that makes this site redundant. I came here to have a question answered, but I guess I had better search elsewhere first. Thanks for your time.

cheers David ve3bbn

 

[BrianG]

I am sure IRstuff does not need me to spring to his defence, but really ve3bbn you are being far too sensitive to critcism (which in fact you asked for: "I don't think the operating freq. is of any importance (correct me if I'm wrong) as 10 mw is 10mw at any freq.")

Far from being arrogant IRstuff was giving you a good analogy of why you need to avoid assumptions and take care when making any measurement. Otherwise it's possible to jump to conclusions which could erroneously condemn an item as being faulty.

 

[biff44]

"I see there is also arrogance on this site,"

What I see is there is too much coffee in some engineering wannabe. Go play with your toys somewhere else!

 

[ve3bbn]

Hi Brian, and thanks for your poste. If you look at Mike's reply (which I thank him for) He was talking about the freq. of the VCO ,not the scope.

MikeHalloran (Mechanical)
12 Mar 07 21:00
What's the intended frequency range?

Somehow this was taken by others to be the freq. of the scope, but I quess I was on the wrong wavelength.
I have been around radio for a good number of years and 10mw is 10mw at any freq.
Life is too short to have to listen to some of the garbage here. I guess your right about that Brian, maybe i have become more sensitive over the years. Oh well I guess I'll have to live with that.
This was all taken out of context and I don't have the patience to deal with some types of people. This could be a great place to give and receive knowledge but I will leave it at that.
Thanks again Brian and have a great day

cheers David ve3bbn

ps I'll go and smell the aroma of some coffee

 

[IRstuff]

life is definitely too short

TTFN

Eng-Tips Policies FAQ731-376

 

[bogeyman]

I dont wish to sound arrogant just as the others who have been kind enough to respond (for no charge) to your query dont.

Here is my twopenneth.

Is your scope realy good at 50 - 100 mhz sine waves? "Should" and "no problem" are arrogant assumptions when it comes to a shuffty scope.

Have you connected it via a 50 ohm cable with a suitable terminator of 50 ohms at the scopes input? - with no other tee connections in the output connection.

I have in the past made all sorts of mistakes in trying to measure RF signals using kit that "should be capable", you must be sure of what to expect from your test kit or your results will just confuse you.

Have you thought of getting hold of an RF power meter or spectrum analiser to make these measurements, they are much more accurate than any scope can ever be. Use the power meter to get an idea of how your scope measures the RF and then you can make someting more like a measurement with your scope. You must be carfull to correctly terminate all RF signals or the reflections will cause massive errors.

regards bogeyman

 

[nbucska]

Hi David:
I am an EE with 30+ yrs, 10+ patents etc. I could fill bookshelfs with the things I know and libraries with the things I don't.
----------------------------------------------
When I ask 4 help, I don't expect the experts who try to help me to pussyfoot around: If they repeat, what I know,
I accept it in the spirit it was given. If I need help,
I am less than expert on that field so I can as well behave as pupils should...

If they misunderstand something, I neglected to tell
all necessary details -- so it was my fault.

I submit that you can learn something from anyone --
it just takes some interest and humolity.




----------------------------
Please read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[nbucska]

I submit we don't have enough data for meaningful answer:
E.g. if the probe is not terminated it may resonate on a
harmonic. We don't know the wiring, etc .

We don't know the type of the scope or VCO, the circuit,
is it properly decoupled, etc.

If I remember correctly Mini0Circuit specifies harmonic content ? is there more ?

I would measure the VCO-s current, too -- it may
indicate if something is wrong with it.






----------------------------
Please read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[MKimagin]

ve3bbn , what model of Mini-Circuits VCO you are using?

From the spec you can get pretty good idea what the condition should be apply to get the results.
In general the Mini-Circuits VCO do no required additional coupling capacitor (usually is there).
As bogeyman mention the power meter or spectrum analyzer should be the best to do that measurement.
In general the VCOs have pretty low match so in many cases 10dB attenuator is use on output of RF for the measurement purpose to have more closer real match of 50 ohms.
The DC control and supply should be feed thru some bias tee with the input terminated with 50 ohms load, that should prevent reflection of RF signal from DC Control and supply line.

One of my first adventure with VCOs was when I build circuit and the RF signal lick all over. I did not realize that all the DC control and DC supply line from each VCO should be properly isolated before you tight one to another VCO.
Well you learn by mistake.