weird sound for AM radio

sonicx927 · Post by **sonicx927** » Fri Nov 09, 2007 3:23 pm

me again, im still doing the sciencebuddies AM radio project, and when I test the transmitter, all I hear is this weird beeping noise. Am I tunning it to the wrong frequency or did I screw up something?

staryl13 · Post by **staryl13** » Fri Nov 09, 2007 3:38 pm

Hi,
Check out this website, it seems like they might have an answer to the beeping that you are getting..
http://www.ultimaterc.com/forums/archiv ... 77379.html
Good luck!

sonicx927 · Post by **sonicx927** » Fri Nov 09, 2007 4:21 pm

that has nothing to do with my project, its an RC car controller they're talking about. I'm doing the build your own low power radio transmitter project
I guess I may have worded it wrong, I'm getting more of a flat line/TV static noise

Craig_Bridge · Post by **Craig_Bridge** » Fri Nov 09, 2007 5:20 pm

There are several possibilities for trouble.

What frequency crystal are you using? Is there a radio station near that frequency (within 10 KHz) in your area? Are you doing this during the daytime or at night when when the ionisphere raises and alows radio stations from farther away to be interfering. If there are local stations within 10 KHz of your crystal oscillator, you will have to try another crystal oscillator frequency.

On test that you should try is to unplug from the audio source and very slowly tune your receiver through the transmitter frequency. If there are no stations near your transmit frequency, your transmitter should start out as a high pitched squeal and get lower in frequency until it is silent and then restart as a low pitched squeal and increase in frequency as you tune past your transmitter frequency. Once you have heard this, tune your receiver to the "null" between the squeals and then plug in your audio source and turn up its volume.

These "squeals" are called heterodynes. Do a search to find out more. In short, heterodynes are caused by a frequency difference between the local oscillator in your receiver and your transmitter "carrier frequency".

Look up some of the words and phrases to learn more.

sonicx927 · Post by **sonicx927** » Fri Nov 09, 2007 5:56 pm

I'm using a 1.000MHZ oscillator, what frequency is that?

Craig_Bridge · Post by **Craig_Bridge** » Fri Nov 09, 2007 6:12 pm

I didn't proof read very well...

"On[e] test that you should try"

If you can't hear the squeal when you hold your radio close to the transmit antenea, the there is either a strong local station or your oscillator isn't oscillating.

Check your connections and if you still haven't figured it out you are going to need at least a volt meter. If you don't have a volt meter, it might be time to buy one, or you need to borrow one.

Measure the battery voltage just to make sure it isn't dead.

Measure between pin 14 and 7 and you should read the same as the battery voltage. If not, then something is wrong in the supply leads or transformer connections.

If you have power connected and it still isn't working, then you are going to need an oscilliscope or build a test circuit to check the oscillator output.

For an oscillator test circuit, you will need a small signal diode, a 100Kohm resistor, and two identical small capacitors (say .001 ufd or anything between 470 pfd and .005ufd). Connect one capacitor lead to the oscillator output pin 8. Connect the other end to the anode lead of the diode. Connect the cathode lead of the diode to one end of the second capacitor. Connect the other end of the second capacitor to pin 7. Connect one end of the resistor to the anode of lead of the diode. Connect the other end of the resistor to pin 7.

Using the lowest voltage scale, measure the voltage across the second capacitor. If you get any indication and it goes to zero (possibly slowly) when you disconnect the battery, then your oscillator is working.

The first capacitor in this circuit is used as coupling capacitor which transfers the AC component of the oscillator signal but blocks any DC voltage. The resistor is called a load resistor which provides a current path for this AC signal to the negative battery terminal.

The diode detects (or discriminates) the AC signal and converts it back to DC. The second capacitor integrates (accumulates) this DC signal so the meter can measures it. This is a simple "carrier signal strength detection circuit".

Hope this gives you some ideas on how to trouble shoot.

sonicx927 · Post by **sonicx927** » Fri Nov 09, 2007 6:19 pm

yeah I've done all that, I'm using new batteries,I hear the squeal, but what frequency should I be checking for a 1.000MHZ oscillator???

Craig_Bridge · Post by **Craig_Bridge** » Fri Nov 09, 2007 6:36 pm

1 MHz = 1000 KHz

But you really should look up the scientific / engineering unit relations for
Giga, Mega, Kilo, mili, micro (u), pico to understand how to use these multipliers and what they mean.

Hope you aren't near Seatle WA KOMO or Chicago IL WMVP as these are 50 kW AM stations broadcasting on 1000 KHz.

See http://www.amlogbook.com/freq/9101000.htm for other smaller stations on 1000 KHz.

tylers740 · Post by **tylers740** » Sun Nov 11, 2007 1:17 pm

Corect me if I am wrong, but I've heard that the chirping you are experincing might be backround radio waves. This thing happened when they first tryed to listen for aliens using radios. They pointed their antenias up and, pop! they heard a strange chirping! That is my theory.