# How to measure the resonant frequrncy of an FM transmitter

#### specs365

Joined Mar 14, 2019
31
Hi, this might be a very simple question, but I've been trying to find something on this for a while now. I would like to know, where, on little circuits like the one below must I attach an oscilloscope to see the frequency at which the tank oscillator is oscillating (assuming a high enough bandwidth oscilloscope ) . I'm quite new at this stuff, but it would really help me to see if my circuit is actually working, besides using a radio. I also just think it would be interesting to see on a scope. Thanks!

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#### Irving

Joined Jan 30, 2016
797
Near, but not on, the antenna. You'll need a good scope with, ideally, better than 150MHz bandwidth (assuming thats supposedly oscillating at ~100MHz).

Attach a short piece of wire to the scope probe and lay it close to the antenna. Too much coupling could stop it oscillating, assuming it actually is. 0.1uH and 50pF tuning = 71MHz

#### Papabravo

Joined Feb 24, 2006
13,926
Much like quantum mechanics where observing a system changes its nature entirely, the same is true here. Putting an oscilloscope probe on the circuit will change things in a meaningful way. A typical probe has 1 MegOhm impedance with 15 pf to Ground. The cheap way to test this circuit is to have an FM receiver that you can probe.

#### Ylli

Joined Nov 13, 2015
965
With the indicated component values, it's not goin' to work......

#### specs365

Joined Mar 14, 2019
31
Near, but not on, the antenna. You'll need a good scope with, ideally, better than 150MHz bandwidth (assuming thats supposedly oscillating at ~100MHz).

Attach a short piece of wire to the scope probe and lay it close to the antenna. Too much coupling could stop it oscillating, assuming it actually is. 0.1uH and 50pF tuning = 71MHz
Thanks! So just ground and near the antenna or do I just need to get the probe near antenna without ground?

#### Irving

Joined Jan 30, 2016
797
try without ground connection initially, there should be enough coupling to get a signal of a few mV

#### specs365

Joined Mar 14, 2019
31
Thanks!

#### Irving

Joined Jan 30, 2016
797
Ylli is right, that circuit probably won't work. Try this one below, make C1 your variable capacitor and set it midway, that should give you a good signal at 92MHz approx. You can tune it maybe 10% either side, say 83 - 102MHz. I'm not sure how well the modulation will work though

#### Ylli

Joined Nov 13, 2015
965
This is the actual original circuit.
Note the '470K' actually should be 470 Ω, and the 4.7uF should actually be 4.7pF.

#### Irving

Joined Jan 30, 2016
797
That works better. 4.7pF is really too small, startup is quite unreliable, unless C1 < 20p. Increasing R1 to 1.5k gives better starting reliability. C1 @ 15p gives 101MHz.

My version gives about 20dB more output because the coupling from tank to emitter doesn't load the tank as much. Frequency stability is marginly better too, at the expense of an extra resistor.

#### Ylli

Joined Nov 13, 2015
965
That works better. 4.7pF is really too small, startup is quite unreliable, unless C1 < 20p. Increasing R1 to 1.5k gives better starting reliability. C1 @ 15p gives 101MHz.

My version gives about 20dB more output because the coupling from tank to emitter doesn't load the tank as much. Frequency stability is marginly better too, at the expense of an extra resistor.
Yup, yours works better. I was just posting what I believe was the original design after doing a google search for it.

#### Audioguru again

Joined Oct 21, 2019
1,588
Circuits Gallery.com posted a defective FM transmitter circuit. Their website does not work anymore.