I'm building a simple FM transmitter to learn more about LC oscillators. I've successfully designed and built the following transmitter on a bread-board and perf-board using standard through hole components. For the 100 nH, I used this through-hole component.

enter image description here

It works adequately, not great quality, but sound comes through none the less. As an experiment, I designed a PCB using the same schematic.

enter image description here

Crucially, the conductor that I've used for my PCB is the TDK NLV25T-047J-EF. It has a Q value of 20 at 100 MHz and self resonant frequency at 1.2 GHz. I also used this 68 pF capacitor.

I should expect an oscillation around 89 MHz, however, I'm getting nothing near that on my oscilloscope.

enter image description here

Any tips on what I could change? I'm wondering if the traces are causing a lot of extra capacitance. Or perhaps, this is just the wrong type of inductor.

  • \$\begingroup\$ What 0.1 uH inductor do you refer to? You need to get rid of the 3.26 us trigger delay so we can see a cleaner signal and also add some cursors so that we can see the actual frequency. Lack of clear x and y scales makes it guesswork at the moment. \$\endgroup\$
    – Andy aka
    Commented May 12, 2023 at 19:03
  • 3
    \$\begingroup\$ How are you probing the circuit? Notice the oscilloscope probe has considerable impedance at this frequency, which changes operation. It is better to measure frequency by placing the probe near the circuit. Likewise any connected antenna or other wiring will have significant effect. \$\endgroup\$ Commented May 12, 2023 at 19:17
  • \$\begingroup\$ That oscillator doesn't look like a Colpitts or one of its derivatives. As such, it may depend on parasitic circuit elements to work. You may have taken those away going to a nice small surface-mount oscillator. I'd suggest a plain old Colpitts, and the highest-Q inductor you can fit. \$\endgroup\$
    – TimWescott
    Commented May 12, 2023 at 23:59
  • \$\begingroup\$ You can split C5 into a capacitive divider and connect the antenna wire between the capacitors for better impedance matching. \$\endgroup\$
    – Jens
    Commented May 13, 2023 at 16:13

2 Answers 2


A couple of things:

First, if you haven't already done so verify your board layout, you may have missed something, so go over it carefully and make sure all the connections are correct.

Second, you prototyped it with one value inductor and then used one with half that value in your finished design, meaning that you had to double the capacitance. The difference in the L/C ratio could be part of the problem.

Third, not absolutely necessary, but you have no means of tuning it. Usually there's a variable capacitor so you can adjust it to an unused frequency in the FM band. A good quality ceramic trimmer cap is recommended for this, cheap plastic ones will mostly just frustrate you.

I built some transmitters like this many years ago, I tried surface mount inductors similar to what you're using and for some reason had no luck at all with them. What I ended up using was a hand wound toroidal coil, it was something like 10 turns on a very small core and you have to get the right material mix for the frequency you're using. The advantages were that I could tweak the value by adding or removing a turn, and a toroid is self shielding so the frequency was barely affected by nearby fingers or metal objects the way a solenoid coil is. I think I have an image of it, will post it if I can find it.


I ran an LTspice simulation of your circuit, although I added a 10k resistor R3 to provide a better bias point and a higher output voltage. Frequency is about 85 MHz as expected. The 10 kHz input signal seems to modulate the RF a bit.

RF Oscillator

Here is the modulation with a 100 kHz input signal, which shows a more visible effect because of the higher voltage through C1:

100 kHz modulation


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.