MAX2606 - Anyone Used It?

Hello,

I purchased a couple of MAX2606 ICs to experiment around with and I built this circuit minus the audio input stage attached to the tune pin via the 0.47uF capacitor.

I built the circuit up fairly quick and dirty on a piece of double sided FR4 Manhattan / Dead Bug style with both sides of the FR4 acting as ground. Initially I just wanted to see if the circuit worked and I could see a signal on my SDR - which it does and I can.

However, the circuit isn't very stable and there's a lot of drift - far more than I expected.

I'm just wondering if there's anything more I can do (beyond the ideas I've had below) to improve the circuit and keep it stable on Frequency.

I've done some digging and in addition to observing best RF design practices (Separate Staging, Screening, Via Stitching, RF Filtering etc.) I found one circuit which uses a Transistor to stabilize the supply voltage which makes sense with the 2606 being a VCO so I'm going to look at adding that to the design.

I'm obviously quite new to the world of RF design so I'm still learning.

Annoyingly (in a comical way), when I've watched youtube videos of people building and demonstrating various designs of FM Transmitters, I've seen them built on breadboards, mashed together on FR4 and even Protoboard and they never seem to get the same stability problems I get when building them lol.

Anyway, what I'm hoping to do with this MAX2606 transmitter is to ultimately end up with a really low powered VHF FM Beacon for Fox Hunting. All I want it to do is put out a low powered carrier with either a tone or series of beeps every few minutes.

I've successfully built a large fox hunting transmitter using the Dorji DRA818V VHF transmitter module, a Real Time Clock and an Atmega328P which puts out between 0.5W and 1W and plays a series of tones and some morse code and that works really well.

Now I would like a very very low powered baby transmitter.
The idea being, you find the big fox transmitter first and that puts you within the range of the smaller transmitter which is harder to find.

The idea behind using the MAX2606 was to keep the component count down as much as possible and the size of the transmitter to a bare minimum.

Just a bit of fun while learning along the way

An inductor to tune the frequency is not very useful. Get a chip that digitally tunes the frequency, has stereo and some even automatically adapt to any kind of antenna.
Adafruit has the Si4713: Adafruit Stereo FM Transmitter with RDS/RBDS Breakout - Si4713 : ID 1958 : $19.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Elechouse has the TK0803K: http://www.elechouse.com/elechouse/index.php?main_page=product_info&cPath=168_170&products_id=2206

Is it legal in your country ? In Europe it is allowed to transmit 50 nW in the FM band. That is fifty nano Watt, or 0.000000050 Watt.

While is is legal to operate a small low powered FM transmitter in the FM broadcast band it is not legal to make one yourself unless you submit it to testing such that you can mark it CE.
See How far does a 50nW (legal!) FM transmitter actually reach - UK Vintage Radio Repair and Restoration Discussion Forum

Anyway fox hunting does not take place on the FM broadcast band.

The problem is that that chip is simply a free running oscillator and will drift enormously due to stray capacitance and temperature. The simple presence of you body is enough send it off and movement of your body will cause movement of the frequency.

The only real soloution I see is to make the chip part of a PLL ( phase locked loop ) locked to a stable referance like a crystal. The dead bug style of construction does not offer enough screening to that inductor.

As to other videos showing sub optimal construction these are not stable despite what they might appeared to show.