#279 FM/SingleStageTransmitterKit2

Build, tweak and tune another simple single-stage FM Transmitter Kit

Build

Notes

I picked up another FM transmitter kit because it was too cheap to ignore.

It turns out to have a typical 1-transistor circuit, similar to LEAP#208 SingleStageTransmitterKit.

Kit Details

Some details according to the product info available from the seller:

Frequency range: 88MHz-108MHz
Working voltage: 3V (with CR2032 button cell)
PCB materials: FR-4 glass fiber board
PCB specifications: 48*26mm thickness: 1.6mm

Tuning

After a quick build, the device was transmitting slightly high on the FM band (I could get some interference but not a good signal at the upper limits ~108MHz).

Taking a look at the air-coil inductor, seems it should have an inductance of around 66nH:

Measure	Value
diameter (d)	3.5mm
length (l)	3mm
turns (n)	5
Inductance	65.9nH per IN3OTD’s calculator

The simplified formula:

L = (d^2 n^2) / (l + 0.45d) [μH]

gets close to the same result: 66.9nH

Assuming that’s about right, the supplied C4 value of 30pF predict a resonant frequency of 113MHz, which would explain and confirm why I can’t get anything in the standard FM range.

Around 90MHz is pretty clear for me, so I tried replacing C4 with 47pF.

This worked! I’m getting a signal at 84.15MHz, from which I infer the coil inductance might be closer to 76nH.

Performance

OK, I guess you shouldn’t expect great things from such a dinky 1-transistor circuit. At least it works!

With 30cm antenna wire, reception beyond 5m was getting a bit dodgy. Frequency stability was reasonably good; once locked in I didn’t need to re-tune much.

Here’s and SDR capture of the device broadcasting on 84.15MHz. All the peaks in the waterfall are harmonic noise from the device.

broadcast_capture