Variable capacitor q factor for fm radio jammer

1. Jun 23, 2014

rar0308

i'm building fm radio jammer for project for my ee class in physics department.
a circuit is here.
i bought the components, soldered on PCB.
it hasn't worked.
an oscilloscope shows sine oscillation even if there is no L and C for tank circuit. i.e. connects wire of both ends which supposed to be connected with L and C.
also it shows different frequency than expected by calculating resonance frequency formula i.e. reciprocal of (2 pi root LC) to the point that it seems what L and C are connected doesn't matter.
i suspect there are some kind of innternal inductance and capacitane.
i'm wondering why mine doesn't work.
I saw that other person made the same circuit and that one worked fine.
i did everything same except i think using different kind of variable capacitor.

i used one in second picture because i can't find first one in my local electronics components store.
difference is that they have different q factor.

is it impossible to make lc oscillator making signal in frequency 100Mhz without high q factor capacitor or there can be other thing i am doing wrong?

also i am almost ignorant of how this ciurcuit works. what books do you reccommend to study about this circuit?

2. Jun 23, 2014

MrSparkle

To understand you circuit, you first need to understand that a DC signal would be worthless. Your jammer would only operate for a few microseconds when you attach the battery. Most of your circuit in fact is to create the necessary AC signal feed into the variable capacitor & inductor. This is the oscillation you are seeing when you disconnect your tank circuit.

So, if you are a sending an AC signal through your cap & inductor circuit, then congrats, your circuit should be working and jamming 'something'. It just might not be in the FM band. Get someone to show you how to do FFTs, or Frequency Transforms on the oscilloscope. With the wire loop and capacitor connected, you should see a big spike on your jamming frequency, and it should change by playing with the variable capacitor. If its not in the FM band, think about how to adjust your circuit until it is.

3. Jun 23, 2014

rar0308

An oscilloscope showing
CH1 f=155Mhz.
using FFT mode i can see two peaks. one at 155Mhz one at 344Mhz
dialing the trimmer, i see that frequency changes in range between 155Mhz to 175Mhz.
What is a cause of 344Mhz peak?

Currently i used maximum capacitance 10pF trimmer. if i use 3 time larger trimmer i.e. max 30pF, then according to formula minimum frequency decrease by 57.7% which is 89Mhz. will it work?

4. Jun 23, 2014

MrSparkle

the 344Mhz is a harmonic. getting into those is another matter entirely.

but the 155Mhz is well above the FM radio band, so you've found the problem. FM radio is from 88 to 108 Mhz, so using the larger cap should work fine.

edit: The schematic says a 6-35pF cap. How come you decided to only use a maximum 10pF cap?

edit2: VHF channel 7 starts at about 175 Mhz (in the US), so if you like, you should also be able to see some TV interference there with your circuit as is.

Last edited: Jun 23, 2014
5. Jun 23, 2014

rar0308

This is second build.
When i first build circuit, somehow frequency didn't go up over 80Mhz. so i was dire to find smallest trimmer.
soldered desoldered capacitors and inductors several tens of times before.
by the way is it correct to connect ground of a probe to - of a battery and a lead of a probe to south of the coil? not sure i'm measuring correctly.