3
\$\begingroup\$

Here is the original circuit found from YouTube link.

Since I don't have all the parts, I modified the circuit diagram:

My modified Circuit diagram

I have edited this circuit diagram.

When I add the Zener the sound improved. The Zener is a 1N4742 (between crystal and inductor to ground.)

Circuit Shown in the YouTube video

Circuit shown in the YouTube video

What I don't understand is how the 25 MHz frequency became 100 MHz. Also I can hear a blank sound in 100 MHz tune in (digital) radio. The 20 nf capacitor is there to get output. I am not sure if it is the output. Notice that the inductor and capacitor (near collector) are completely irrelevant as shown in the video.

According to the equation,

\$R = 1/(2πFC) \$

20 nf capacitor should be enough to pass 100 MHz wave.

\$\endgroup\$
5
  • \$\begingroup\$ Do you know what harmonics are? \$\endgroup\$
    – Hearth
    Jul 12, 2021 at 5:49
  • \$\begingroup\$ No I am just a electronic hobbyist. \$\endgroup\$
    – Avon97
    Jul 12, 2021 at 5:50
  • \$\begingroup\$ Adding a zener is probably not a good idea as it has significant capacitance and you’ve put it in the rf path. The inductor in series with the crystal is to make it oscillate on a harmonic. \$\endgroup\$
    – Kartman
    Jul 12, 2021 at 6:18
  • \$\begingroup\$ @Kartman the zener may be acting as a varicap to pull the crystal slightly and produce deviation based on the audio input - a crude FM modulator. \$\endgroup\$
    – user16324
    Jul 12, 2021 at 10:32
  • \$\begingroup\$ In your spare time watch a few videos on basic electronics. They probably will not answer your question but you will better understand the answers we give you. \$\endgroup\$
    – Gil
    Jul 12, 2021 at 19:43

1 Answer 1

8
\$\begingroup\$

Someone else can surely write up a better and more detailed answer than this, but:

A quartz crystal is, basically, a little tuning fork that vibrates at a specific frequency, called the fundamental. Anything that can vibrate naturally at one frequency can also vibrate at a multiple of that frequency, called a harmonic or an overtone of the fundamental frequency. In fact, unless you're very careful, it will vibrate in a linear combination of all overtones at once, usually (but not always) with the higher ones having lower amplitude. (this is why, for instance, a violin and a trumpet sound different, even when playing the same note.)

In your case, your circuit is exciting the crystal in such a way to make it resonate at the fourth harmonic of its fundamental frequency. Some crystals are specifically designed to be used in this manner, but any crystal can be.

Basically, there is no 25 MHz frequency anywhere in your circuit, not to any significant degree--you're using a 25 MHz crystal's fourth overtone to produce a 100 MHz oscillator.

\$\endgroup\$
1
  • \$\begingroup\$ Thank you for this helpful answer I thought crystals only oscillate at their specified fundamental frequency only. \$\endgroup\$
    – Avon97
    Jul 12, 2021 at 6:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.