When to have only voltage gain in an oscillator

[Averagesupernova]

If you include Impedance in that statement then it's got to be Power gain. A transformer doth not make an oscillator oscillate.
And, on the subject of gain; it's very naughty of people to talk about voltage gain in dB unless the impedances are explicitly quoted. It accounts for a load of confusion and misunderstandings when people try to follow the 'formula' for a voltage ratio and include the "20" instead of the "10" without having a clue why.

This may be a matter of semantics and wording. Of course, we cannot take a transformer and expect it to function alone as an oscillator. But to say we cannot take a transformer and use it in the feedback path to step up the voltage and get an oscillator to run would be wrong. Of course it is implied we are using a valve or transistor of some sort.
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Now as to this voltage/db thing, that's a slippery slope. Decibels are commonly used with voltage and I know it ticks some people off severely. The unit dBmV is a reference of zero dBmV is 1mV into a given load. It is commonly used in the cable TV industry. If the power goes from zero dBmV to 10 dBmV the power has gone up 10 times.
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Voltage itself is spec'd in dB increase or decrease, but I don't find it all that common. At lease not in amplifiers. One place I can think of dB is used to spec in the form of voltage is crosstalk for instance. Take a compact disk player with line level outputs. We don't really care if they are terminated at exactly 10K ohms or whatever. Many amplifiers that a CD player would plug into vary within a range. As long as both channels are terminated the same, that's fine. We measure the voltage, do the math with '20' and state the spec.

 

[sophiecentaur]

But to say we cannot take a transformer and use it in the feedback path to step up the voltage and get an oscillator to run would be wrong. Of course it is implied we are using a valve or transistor of some sort.

Of course. You need an active device in there somewhere and it provides extra Power somewhere. All a transformer can do is to change the impedance without amplification.

One place I can think of dB is used to spec in the form of voltage is crosstalk for instance.

(again) of course. You are measuring a ratio of signal levels at the same point so the impedance that you measure the crosstalk can only be the same.
Insert a 6dB amplifier or a 6dB attenuator and you can only be sure that it will do what the label says if you are using the standard impedance throughout.

Decibels are commonly used with voltage and I know it ticks some people off severely.

Yes, it does tick them off because, as you know, people are horribly confused because they don't know what's going on and you hear "Is that Voltage dBs??" The proof of the pudding and all that. So best to steer clear and describe voltage ratios in plain numbers.

 

[Averagesupernova]

So best to steer clear and describe voltage ratios in plain numbers.

Agreed. But, sometimes crosstalk or separation is over 100 db. That's a bit much to chew off in a voltage ratio. What you said about the transformer changing impedance is dead on correct. That can be said of any situation involving transformers unless isolation is the objective.

 

[DaveE]

If you include Impedance in that statement then it's got to be Power gain.

OK, go for it. There's nothing wrong with expressing everything in terms of power. Occasionally you will annoy people, but you won't be wrong. OTOH, "If you include impedance" then voltage, current, or power are all acceptable. Not necessarily equally optimum for analysis, but acceptable. In that case it clearly doesn't have to be power. You don't have to be an expert EE to know what the voltage or current is for 10dBm into 50 ohms.

I still contend that having the flexibility to choose the representation that is most clear, most standard in industry, or easiest to manipulate is the best option. There are many signal processing applications, like control systems or audio, where power is relatively insignificant. Just as there are other systems like RF receivers and transmitters where power is of utmost importance. It is myopic to insist that various engineers all use the same tools or have the same preferences.

And, on the subject of gain; it's very naughty of people to talk about voltage gain in dB unless the impedances are explicitly quoted. It accounts for a load of confusion and misunderstandings when people try to follow the 'formula' for a voltage ratio and include the "20" instead of the "10" without having a clue why.

Yes, as I previously said, there is a burden on people to communicate clearly the assumptions made or salient circuit details required to know what ratio is being discussed. Otherwise, it sounds to me like you are describing people that don't know enough about dB's to be using them at all. I agree just "following a formula" is a horrible idea in this regard.

 

[Averagesupernova]

Otherwise, it sounds to me like you are describing people that don't know enough about dB's to be using them at all.

Bingo!

 

[sophiecentaur]

the representation that is most clear, most standard in industry, or easiest to manipulate is the best option.

dBs are the way to go, of course. Couldn't do without them

In that case it clearly doesn't have to be power.

But it is Power - by definition. It's taken for granted that a system is using a common impedance throughout so that the Voltages are all proportional to the root of the Power. You know that. I know that.

people that don't know enough about dB's to be using them at all.

But those people represent all the beginners who can so easily get the wrong message through sloppy use by people who should know better. The existing club rules (notation) are probably too well established to change but, if they had been drawn up correctly in the first place, those beginners would not be suffering. (and passing it on to fellow students). No one who reads this can argue that it's not a problem for students and non-academic technical staff of all vintages. (Double negative but resolvable)

Too lumpy for general use but dBmV₆₀₀ or dBmV₅₀ would take away any inconsistency.

 

[Averagesupernova]

I can understand the need for defined impedances in amplifiers when dealing with dB. Amplifiers have input and output impedances. Not necessarily the same. However, there are some cases it is simply NOT necessary to know. If I monitor a signal that for whatever reason occasionally varies from the source I certainly won't go to the trouble to do the math and compute power each time it changes. I do the math with '20' and accept it. I've actually been in that situation.
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The way I see it is this: Sorry if someone somewhere doesn't understand everything behind decibels. I don't plan on taking personal responsibility for them. The same way if I asked a power line tech working on a line at the end of my driveway what voltage the line is and the reply is 13.5, I don't hold the tech responsible for me misunderstanding and thinking it's no higher than a car battery when in reality it's 13.5 KV. The tech assumed I was smart enough to be speaking his language.

 

[sophiecentaur]

compute power each time it changes.

"Compute"? Any change in level on the same line is just a ratio.

I don't plan on taking personal responsibility for them.

and I wouldn't hold you responsible either. But it does bother me that people get let loose without the right knowledge and expensive mistakes have been made because the dB is not taught properly. If you have 'had conversations' about levels in dB, you must have been aware of the uncertainty in many minds about what should be doing with any readings they've made.

Just browse through some of the stuff that's written on the Web. You'll see it all over the place and it self propagates.

 

[Averagesupernova]

Any change in level on the same line is just a ratio

My point exactly.

 

[Averagesupernova]

But it does bother me that people get let loose without the right knowledge

Just browse through some of the stuff that's written on the Web.

Lol. You better stay away from the internet or it will drive you crazy.

 

[sophiecentaur]

My point exactly.

It was mine too, too, too. We seems to have started to oscillate.