catch peak voltage on MHz/GHz oscillator without oscilloscope

[John Parker]

I have an oscillator circuit and I want to know the peak voltage but I don't want to buy an expensive instrument. Multimeters with this function cannot take samples so fast but as I see oscilloscopes also cannot do it. I already killed two power MOSFETs and I don't know the Vgs was too high or too low but the exteremly high temperature killed those poor transistors.

 

[Nigel Goodwin]

Post your schematic and give more details, I see little reason to want to measure peak GHz voltages.

 

[John Parker]

Post your schematic and give more details, I see little reason to want to measure peak GHz voltages.

Actually I don't know the exact freq. Maybe it's just one or two hundred MHz but this is not important. I don't have schematic but it will be a radio transmitter and the voltage is on the antenna inducted by a coil. The coil is 16uH and the antenna is 40cm with the 20cm long coil. Maybe the antenna is too long and therefore the voltage is too low but I don't know the voltage and this is the problem.

 

[JimB]

Gigahertz?
Multimeter?
MOSFET?

Not words that I would expect to find together in such close proximity.

What are you trying to do?

1 Ghz = 10^9 hz = 1,000,000,000 hz Are you really working at such high frequencies?

JimB

 

[John Parker]

Gigahertz?
Multimeter?
MOSFET?

Not words that I would expect to find together in such close proximity.

What are you trying to do?

1 Ghz = 10^9 hz = 1,000,000,000 hz Are you really working at such high frequencies?

JimB

Hundred MHz can be enough if wooden walls cannot stop them easily. As I wrote it will be a radio transmitter and just for 100-200m with two walls at least. (Private phone between my house and the neighbour. I want wireless just for fun.)
I know multimeter cannot measure this as I wrote. But why GHz with MOSFET is problem? There is MOSFET in microwave oven too.

 

[JimB]

Actually I don't know the exact freq. Maybe it's just one or two hundred MHz but this is not important. I don't have schematic but it will be a radio transmitter and the voltage is on the antenna inducted by a coil. The coil is 16uH and the antenna is 40cm with the 20cm long coil. Maybe the antenna is too long and therefore the voltage is too low but I don't know the voltage and this is the problem.

Woh!
That is seriously messed up!
You are proposing to run a radio transmitter on some random unknown frequency (which will probably drift all over the place).
This is going to seriously annoy your RF neighbours.

As I said already, what are you trying to do?

JimB

 

[John Parker]

Woh!
That is seriously messed up!
You are proposing to run a radio transmitter on some random unknown frequency (which will probably drift all over the place).
This is going to seriously annoy your RF neighbours.

As I said already, what are you trying to do?

JimB

This is just a hobby project now and with low power it cannot disturb anyone. I'm new in electronics and I learn by experiences. It's not fun to make a circuit by a schematic from the net. Everyone can do this without any knowledge but I'm interesting in the real electronics.

 

[Nigel Goodwin]

But why GHz with MOSFET is problem? There is MOSFET in microwave oven too.

No there's not, MWO's use Magnetron's.

But again, post your schematic - whatever is wrong is something you're doing BADLY wrong, with no need for what you're asking.

 

[Nigel Goodwin]

This is just a hobby project now and with low power it cannot disturb anyone.

It's still seriously illegal, with penalties depending on the country you're in - and low power can travel VAST distances.

If you really want to do this?, then find out what possible licence free frequencies are available to you, and what specifications you have to meet.

 

[large_ghostman]

A private phone between neighbors?? sounds like a crack house alarm system . (I have so got to stop watching late night TV!). I have a small hand held CB (cough cough) I think it puts out around 1.5W (its crap), but it can be picked up several miles away by another CB'er. Dont underestimate the distance small amounts of power on a large antenna will go.
And some those guys with amateur radio will know how to find you (seen it on TV)

 

[ronsimpson]

I want to know the peak voltage

The coil is 16uH and the antenna is 40cm with the 20cm long coil

If you are measuring voltage at the receiving end of a RF transmitter/receiver pair, then the relationship of "peak voltage" and "average voltage" is well known. Because most receivers have a tuned LC the antenna will hand you a sign wave where peak is 1.414 that of the average. I would build or buy a "RF demodulation probe" for a meter or scope. The output of the probe will hand you a DC voltage that is approximately the average RF voltage.

 

[John Parker]

If you are measuring voltage at the receiving end of a RF transmitter/receiver pair, then the relationship of "peak voltage" and "average voltage" is well known. Because most receivers have a tuned LC the antenna will hand you a sign wave where peak is 1.414 that of the average. I would build or buy a "RF demodulation probe" for a meter or scope. The output of the probe will hand you a DC voltage that is approximately the average RF voltage.

Thanks but I've already measured and solved the problem. As I suspected the voltage was too low but not enough low. The transistor was open but not saturated and that's why overheated and damaged.

 

[John Parker]

A private phone between neighbors?? sounds like a crack house alarm system . (I have so got to stop watching late night TV!). I have a small hand held CB (cough cough) I think it puts out around 1.5W (its crap), but it can be picked up several miles away by another CB'er. Dont underestimate the distance small amounts of power on a large antenna will go.
And some those guys with amateur radio will know how to find you (seen it on TV)

Because my neighbour is my best friend and we do this project together I don't think it will disturb him.

 

[John Parker]

It's still seriously illegal, with penalties depending on the country you're in - and low power can travel VAST distances.

If you really want to do this?, then find out what possible licence free frequencies are available to you, and what specifications you have to meet.

You are too worried. Why you affraid of a hobby project or make mistakes? How could you learn electronics at all? This is science and not legal issue! But if I can disturb my neighbour with a 9V battery powered stuff I must claim the Nobel prize.

 

[John Parker]

No there's not, MWO's use Magnetron's.

But again, post your schematic - whatever is wrong is something you're doing BADLY wrong, with no need for what you're asking.

Sorry, my mistake because I came from the future: https://www.heise.de/newsticker/meldung/Mikrowelle-ohne-Magnetron-1624974.html
And by the way: what is in a CPU? Of course MOSFETs, many-many MOSFETs and work on many-many GHz.

 

[Nigel Goodwin]

You are too worried. Why you affraid of a hobby project or make mistakes? How could you learn electronics at all? This is science and not legal issue! But if I can disturb my neighbour with a 9V battery powered stuff I must claim the Nobel prize.

It's not a question of 'being afraid', merely informing you that you are intending doing something illegal that comes with substantial penalties when caught.

Your complete lack of understanding means you have no clue how far RF can travel - with only 1 watt at VHF providing communication easily over 100 miles, and on short wave far lower powers are commonly used for communication to the other side of the planet (try googling QRP).

This isn't science', this is a deliberate illegal act where apparently you couldn't care less about damage you mighty cause to others, including potentially disrupting Emergency services and aircraft - there are good reasons why amateur radio users have to pass exams.

 

[John Parker]

Gigahertz?
Multimeter?
MOSFET?

Not words that I would expect to find together in such close proximity.

What are you trying to do?

1 Ghz = 10^9 hz = 1,000,000,000 hz Are you really working at such high frequencies?

JimB

And by the way:

It's not a question of 'being afraid', merely informing you that you are intending doing something illegal that comes with substantial penalties when caught.

Your complete lack of understanding means you have no clue how far RF can travel - with only 1 watt at VHF providing communication easily over 100 miles, and on short wave far lower powers are commonly used for communication to the other side of the planet (try googling QRP).

This isn't science', this is a deliberate illegal act where apparently you couldn't care less about damage you mighty cause to others, including potentially disrupting Emergency services and aircraft - there are good reasons why amateur radio users have to pass exams.

1. Why you are interesting in this? I'm not your neighbour.
2. Do you remember what was the question? If you don't know the answer or don't want to help me don't post! I appreciate your worry about me and my environs but it start to be offending. You wrote a warning and after that wrote again. I already understood your warning but you keep up posting it again and again. What's the point of your comments actually?

 

[JimB]

John Parker, your comments are typical of someone who comes here to the forum asking questions about building some kind of radio transmitter.

When it is pointed out to them that they have insufficient knowledge to do this correctly, and that their actions may cause problems to others who they are totally unaware of, at considerable distance from them, they get all upset and justify their ill advised and illegal activities with comments such as:
"It is only for educational purposes"
"In my country it is not illegal"
"There is no one near me to cause problems"

So, I have heard enough, I will not help you in this venture.

JimB