Here is where I played with learning the previously mentioned testing protocol for determining the resonate frequency of a coil = Pix3.
First, I DIY’d the LED light indicator = Pix1/Blue board = One of my 1206 breakout boards, with the needed protection resistor and the two LEDs in polar opposition.
Second, I DIY’d a random coil = Pix1/White cylinder = 24ga magnet wire wound on a nylon cylinder.
Third, I set up the test rig on a breadboard = Pix1 = Notice that I did not use a Near Field Probe. More on this, later.
Forth, I manually swept the bandwidth. I immediately noted a large amount of noise and added a 0.1uF cap across the LEDs = Pix1/Yellow Cap. This greatly reduced the noise and increased the output voltage amplitude. More on this, later.
![](https://www.eevblog.com/forum/testgear/siglent-1104x-e-automatically-capture-frequency-of-max-voltage-of-sweep-of-coil/?action=dlattach;attach=1683040)
Results
The coil’s resonate frequency was determined to be approximately 19.23MHz = Pix4. The input voltage amplitude was 2.3Vpp = Pix2. The output voltage amplitude was 12.4Vpp (Pix4,) being a ratio of 12.4/2.3 = 5.4:1.
Conclusions
A breadboard always adds unknown and undesirable influences.
The test rig was highly sensitive to ambient influences. Even touching the scope controls added capacitance and greatly changed the output voltage amplitude reading. Increasing the scope’s voltage amplitude sensitivity exacerbated the problem. A more precise reading of the resonate frequency would likely require lab isolation equipment beyond my hobbyist level.
Adding the cap across the coil may be a violation of testing protocol. Meaning, I may have created a different type of circuit, entirely. Might this be called a LC Tank Circuit? If so, the test results may be wholly useless. It comes to mind that changing the cap value might have changed the resonate frequency. Thoughts?
You may have noted that in Pix1 the LEDs are not illuminated. The circuit was running, but my frequency generator can only output 2.3Vpp at 19.23MHz. That is not enough voltage to illuminate the LEDs. As the LEDs do not provide visual feedback, they will be removed for future testing. This will also remove any influence that they might contribute.
I forgot to use a Near Field Probe. This surely influenced the test results, likely to the point of wholly negating them. I plan to test again with the a NFP. I will let you know of my future findings.
Anyway, I am learning and having fun. Cheers.