Need help building FM Transmitter

  • Thread starter Shelnutt2
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  • #26
chroot
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Well, there's something wrong with your measurement equipment, then, man. There's no way your iPod is producing a 3 millivolt signal on each channel. You could never hear such a tiny signal.

Some of the things you're saying don't make any sense. The "20 MHz" marked on the cabinet of the oscilloscope means it is not intended to display any signals with frequency components above 20 MHz; in other words, you'll never be able to see your RF signal on it. I have no idea what you mean by "gives a graph of -40 to 150," though. Oscilloscopes do not have fixed numbers on their displays; they have knobs which you can use to vary the scale of the display in both axes.

When the 'scopes probe is not connected to anything, you should see a flat horizontal line that can be adjusted until it's at the center of the graph. If you set the 'scope's vertical axis to 0.5 V / div, and then connect it to your battery, you should see the flat line jump up three entire grid divisions. You should see no oscillations of any kind. Are you actually using the 0.5V/div setting, or are you accidentally using the 0.5 mV/div setting? You might looking at very tiny noise!

Are you really sure you're using the oscilloscope correctly? Do you see anything resembling an audio waveform coming from the iPod? Do you have someone there who could help you set up and use the oscilloscope? It's starting to sound like this is all just a problem with your measurement equipment.

- Warren
 
  • #27
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Sadly I know more about the oscilloscope than anyone else at my high school. I've read the manual, and I thought I had everything setup. I'm going to take pictures tomorrow of the oscilloscope, and maybe you can see if it appears that I have it set correctly?

On the little display screen, there is a grid with the Y axes labled from -40 to 150. I wish I knew someone here that could help me with this.

Well my Wavetek Meterman (28XT) reads fine on everything else. I just checked a brand new 9volt and it reads 8.9 volts. Also the multimeter at school reads .003volts too. I don't trust the one at school, thats why I came home and double checked the number today with a new stereo cable and my own multimeter. The number is right. Unless I am reading it wrong. (It is set to DC voltage also).
 
  • #28
chroot
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Sadly I know more about the oscilloscope than anyone else at my high school. I've read the manual, and I thought I had everything setup. I'm going to take pictures tomorrow of the oscilloscope, and maybe you can see if it appears that I have it set correctly?
We'd be happy to examine your pictures -- just make sure they're high enough resolution so we can see the positions of the knobs. :smile:

On the little display screen, there is a grid with the Y axes labled from -40 to 150. I wish I knew someone here that could help me with this.
That's actually pretty weird! (And no, it doesn't mean frequency.) Do you know the manufacturer and model number of the oscilloscope?

Well my Wavetek Meterman (28XT) reads fine on everything else. I just checked a brand new 9volt and it reads 8.9 volts. Also the multimeter at school reads .003volts too. I don't trust the one at school, thats why I came home and double checked the number today with a new stereo cable and my own multimeter. The number is right. Unless I am reading it wrong. (It is set to DC voltage also).
So you're putting a multimeter, set to measure DC voltage, between one audio channel and ground, and you're getting 3 millivolts? When it's set to measure DC voltage, you're only going to measure the average voltage. If ground is 0V and one audio channel is a sine wave centered around 0V, then the average voltage between them is 0V. (Half the time, the audio signal is above 0V, half the time, it's below 0V.)

Your oscilloscope should be able to display the actual waveform of the audio signal, though. It should be centered at 0V, and go as high as perhaps 1V and as low as perhaps -1V. Have you tried changing the AC/DC coupling switch on the oscilloscope?

- Warren
 
  • #29
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I'll get the model number and manufactor tomorrow. All I can tell you is they are old. The Nelsons, you probably don't know them, but they were the physics teachers at my school for 16 years and all together they've taught for over 30 years. They were awesome. They were co authors of several textbooks. The book they taught out of was one they co-authored. But alas they retired for a few reasons.

Anyway long story short I wouldn't be supprised if these oscilloscopes were 16 or more years old. I found them sitting in the storage closet on a shelf.

How does a .04v AC reading sound? That what I get from my iPod, when Is set the multimeter to AC.

I don't think I'm messed with the coupling switch. I believe the manual said to put it to AC.
 
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  • #30
chroot
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0.04 VAC is also wrong. Keep in mind that an audio signal is composed of many different frequencies added together, and your multimeter is only capable of measuring AC of a single frequency. This means your multimeter is not going to be able to give you a meaningful number -- it may give you nonsense. The oscilloscope is the right tool for the job, but it sounds like your oscilloscope is not working properly.

The AC/DC coupling switch on the oscilloscope just determines whether or not it rejects the DC component of the signals you give it. If you put it on DC, it will show you the entire signal, with the DC offset visible. If you put it on AC, it'll just show you the AC content of the signal, centered around 0V.

If you have the 'scope set to AC coupling, then any DC source, including a battery, will just look like a 0V flat line. If you zoom really far onto it, you might see some "oscillation," which is really just microvolt noise.

For your purposes, I suggest using DC coupling for everything.

- Warren
 
  • #31
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http://img256.imageshack.us/img256/573/1005444nx3.jpg [Broken]

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Its an Instek model OS-622G

I also have a B&K Percision Model 2120 oscilloscope I can use, if the Instek one will not work for me. Although the B&K one is 20Mhz too.
 
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  • #32
chroot
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Okay, what are you measuring in those pictures? I do note that Channel 1 is set to be AC-coupled, which is going to cause you problems when you're trying to measure DC voltages.

- Warren
 
  • #33
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Okay, what are you measuring in those pictures? I do note that Channel 1 is set to be AC-coupled, which is going to cause you problems when you're trying to measure DC voltages.

- Warren
At that time, I had nothing attached. I've got a picture of when I had both my iPod and my circuit hooked up, but I took those after school, I'll get those posted up in a bit.

I put the coupling to DC when I measured the circuit. My friend had two ideas that we tried and got slightly better results. 1) Used a CD player which takes 2 AA batteries to see if it worked better than the iPod, maybe the iPod doesn't push enough amps?

Then at the same time because I only had two double AA's at school, we used a D battery. With a D battery powering the circuit we got what appears to be ~30 Mhz on the .5v/Div setting.

2xD batterys in series got us 50Mhz on the .5v setting. So maybe the fact is that the AA is not giving enough amps? The load is too much on the battery.

What do you think? Is it possible that the AA can't give enopugh amps? Although that would be in direct contrast to what the author of this circuit has said it to be...



(Also I've swapped the 5 coiled inductor out for an 8 coiled one)
 
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  • #34
chroot
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I strongly doubt that the circuit is functioning at all, much less in a way that depends upon the current capability of your battery. I strongly suspect that you are not using the oscilloscope correctly, or that it is broken. This makes all of your measurements suspect. Besides, your oscilloscope is not really capable of measuring anything beyond 20 MHz, so I do not suggest that you attempt to measure RF signals with it at all.

We need to start at the beginning. You said earlier that connecting a battery to the oscilloscope showed an "oscillation" in the MHz range. This is obviously wrong, and you need to sort it out first.

Put the scope on 0.5 V/div. The time/div does not really matter, but set it to something reasonably fast. Put channel 1 on DC coupling, and short the probe and its ground lead together. Use the vertical adjustment knob to put the horizontal line right in the middle of the display. Connect the battery to the probe, and observe that the horizontal line jumps up (or down) three divisions, and has no oscillations of any kind.

Once you have gotten this to work, let's proceed with more advanced measurements....

- Warren
 
  • #35
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I strongly doubt that the circuit is functioning at all, much less in a way that depends upon the current capability of your battery. I strongly suspect that you are not using the oscilloscope correctly, or that it is broken. This makes all of your measurements suspect. Besides, your oscilloscope is not really capable of measuring anything beyond 20 MHz, so I do not suggest that you attempt to measure RF signals with it at all.
Lets assume that the oscilloscope is bad. I tried and couldn't seem to get the oscilloscpe to jump up or down at all when I attached a AA battery. What I did was I had a multimeter reading the coltage of the battery then I hooked the oscilloscope probe up to the positive side of the battery. It didn't seem to move the flat line at all.

If I can't seem to get it to do this then something is wrong.

In the end I don't care about the oscilloscpe or its readings. I want a working FM transmitter. I'm do not know what is wrong. I don't know enough information to guess. I've followed the circuit design to a T. Maybe my circuit is just too spaced out? Would that really cause such an issue though? I mean all my voltages are correct everywhere. I get 1.5v on the whole circuit, but I can't seem to pick it up on my walkman.
 
  • #36
chroot
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Lets assume that the oscilloscope is bad. I tried and couldn't seem to get the oscilloscpe to jump up or down at all when I attached a AA battery. What I did was I had a multimeter reading the coltage of the battery then I hooked the oscilloscope probe up to the positive side of the battery. It didn't seem to move the flat line at all.
Wait, an oscilloscope probe has two connectors -- one for the signal, and one for ground. How are you grounding the oscilloscope's probe? You need to connect it to both ends of the battery. And make sure the oscilloscops is on DC coupling.

- Warren
 
  • #37
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Wait, an oscilloscope probe has two connectors -- one for the signal, and one for ground. How are you grounding the oscilloscope's probe? You need to connect it to both ends of the battery. And make sure the oscilloscops is on DC coupling.

- Warren
There is a ground on the Oscilloscope. The manual said to connect the groudn to that. I assumed thats what I was suppose to do for everything..if thats not the case then tomorrow I'll hook it up correctly, and see if that changes anything.
 
  • #38
chroot
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Well, when you're measuring a battery, you need to connect the ground of the oscilloscope to the negative end of the battery. Otherwise you're not really measuring anything.

- Warren
 
  • #39
I think you guys were on the right track when you were discussing the battery's internal resistance, the area of focus is spelled out in the original post:

Doesn't work:
AA battery, 1.3 volts, .0010 amps
9volt battery 8.8volts .03 amps

Works:
DC transformer 1-1.5v, .0015 amps

WHY? ...
If it works on the power supply, it should work on the battery if, to the transmitter, the battery looks like the power supply. Clearly the battery does not look like the power supply. The two ways it might differ are in the output voltage and the internal resistance.

Did you look to see if the voltage drops when you connect the transmitter to the AA cell? How much did it change? If its more than 01. volt, then your AA cell is effectively dead (I guess that if internal resistance is 100 Ohms, the cell is dead.)

1.3 volts is pretty low for an AA cell, except in the case of ni-cads (in which case, it would be a little high). If its a fresh carbon-zinc, zinc-chloride or alkaline cell, the voltage should be 1.6 volts without a load.

And I really don't know how well this circuit works at 1.3 volts. Try a different cell anyway.

The question of whether you can see the audio on the scope is largely irrelevant if you could hear Meatloaf while running off the power supply. The circuit achieves frequency modulation by varying the voltage between the base and the collector of the transmitter. Since the collector voltage is constant with respect to ground, it is only necessary to vary the base voltage. Using a 2N4401 in this circuit, it only takes a few tens of millivolts peak-to-peak on the base in order to get a large enough capacitance change to appear to be a fully modulated FM signal to FM broadcast receivers (about +/- 75 kHz.)

There should be no audio signal on the collector of the transmitter. If you see one, then either the battery or the inductor have a very high resistance at audio frequencies.

My suggestion is to make the AA cell look more like the power supply. Get a fresh alkaline cell and give it a try, and let us know.

(BTW, sorry for the late reply. I was traveling without access to email.)
 
  • #40
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If it works on the power supply, it should work on the battery if, to the transmitter, the battery looks like the power supply. Clearly the battery does not look like the power supply. The two ways it might differ are in the output voltage and the internal resistance.

It did not work with the Power Supply. What I was reading on the Osciloscpe was a frequency coming from the Power Supply unit itself. I've come to distrust the oscilloscopes I ahve access too. Well and distrust myself with being able to get them to work correctly.

I will get some fresh AA tomorrow morning and post back up if I get anything.

Thanks for the reply Mr. Cappels.

Oh and I no longer have access to the oscilloscopes readily. My semester in school is done and I was unable to convince the physics department to let me take one home. I offered to sign my life over, but they said that still wouldn't pay for the Oscilloscope if something happened..so if I really need to use oen I might be able to use one during summer school. I think that they will be there for summer school.
 
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  • #41
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It would be far more convenient to just use an on-chip, voltage-controlled oscillator. You could probably get a free sample from MAXIM IC.
 
  • #42
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http://anarchy.translocal.jp/radio/micro/howtosimplestTX.html
Extract from Q&A, the end of the page:
"Q: When I connect my iPod to the audio input of this transmitter, it suddenly stops to work. What's happening?

A: The ealier model of iPod has an automatic shut-down system in it. When you pull out the plug of your headphone, it automatically stops working. When the left and right channels are short-circuited, the same thing happens. Presumably, you use a stereo cable and solderd the left (white) and right (red) lead together. Please use the left channel lead (white) only. Usually the left output is compatible with mono audio. If you, however, insist to mix the left and right output of your audio source into the transmitter, make a simple "interface" like this by two sets of 1 kiro ohm registers."

I found this article in 2010, I'm a bit late with replying :) But hope that somebody, who will find this article like I did (it's on fifth position in http://www.google.com.ua/search?hl=uk&q=build++fm+transmitter+for+headphone+jack+curcuit"), will probably get an answer
 
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