160mW 96MHZ FM TX
by Joy Mukherji


Joy Mukherji gave me a 96MHz FM Transmitter" that gave a few mW of RF, and he added a 60mW Power Amplifier stage. This project is along the same lines. It also uses "dead-bug" or "ugly-bug" construction, but this project gives a solid 160mW or RF.

Ugly Bug construction always looks a little messy, but the results are usually surprisingly good when RF is involved. Here is Joy's 160mW project, including his pictures and technical description.

There are two schematics, one for the RF and another for the PLL section.

160mW RF section.

160mW PLL section.

Carefully adjust VC1 for PLL lock. The PLL lock indicator LED should turn on. VC2 should be adjusted for maximum output power on 96MHz.

50 Ohm dummy load for alignment.

RF power meter for tuning. Tune VC2 for maximum peak voltage.

Second and third harmonic levels are -24dBc and -29dBc respectively. This particular prototype is delivering 170mW at 12V and 210mW at 13.5V DC.

Spectral view of the finished 160mW TX.

When power is initially switched on, the PLL takes about 2 seconds to lock. Two 74AC74 (74F74) high speed flip flops have been used as divide by 16 prescaler. It is further divided by 8 in CD4040 and fed to PLL comparator CD4046 which in turn generates the control voltage to keep the 2n2219A oscillator on frequency.

CD4060 is the reference frequency oscillator and outputs 750KHz to CD4046. A BB910 varactor diode has been used. It can be replaced with a 16V 400mW zener diode but BB910 gave better sound quality.

The final stage uses a 2n3866 and operates in class B mode. It provides 10dB power gain and boosts 20mW from the oscillator stage to 160mW after the low pass filter. A three pole low pass filter consisting L5 and two 27p capacitors cleans up the output before it is fed to an antenna or external RF amplifier.

A 27p capacitor is feeding the PA stage from 1/4T tap off L1. It can be increased to 47p for 200mW power output at 12V.

Close up of L1 and quarter turn tapping from V-pos.

RFCs 1, 2 and 3 are 1.5uH or 70T of 39SWG wound on a 10k 1/4W carbon film resistor. Without RFC3, audible tone was heard from the CD4060 crystal oscillator. R8 and C9 add 50uS pre-emphasis. C9 is five 100n capacitors in parallel. C9 should be increased to 750n for 75uS pre-emphasis. It depends on which part of the world you are in.

The transmitter was built on a 4cm X 3cm copper clad board using ugly bug construction method. Keep all leads as short as possible. Use 12V hum free regulated power supply. It is advisable to house the transmitter in a small metal box.

View of assembled prototype.

Use a matched ground plane or dipole antenna mounted 20 to 30 feet above ground and clear of obstruction to obtain the maximum range from this low power WBFM PLL transmitter. A 78cm long wire antenna can be used but load the output with a 470ohm resistor and then connect the wire antenna. The 470ohm resistor will improve linearity as the wire antenna is not exactly 50ohm impedance and also protect the PA stage by keeping VSWR low.

I made some modifications and now getting almost double the power output or 360mW at 12V.

For better matching of oscillator stage to PA, I added a low inductance coil (2T 26SWG on 8mm dia. air core) between the 27p capacitor and the base of the 2n3866 final. I also changed L2 to 7T 20SWG 8mm dia. air core and VC2 with a 56p fixed capacitor. As the output power is now higher, I also increased the value of R13 feeding the 74AC74 divider to 680ohm.

This completes the modification for 2X power output.

Audio sounds good but can be a bit distorted with increased output power. I have noticed that audio sounds best when I use a colpitts, or inverted Harley oscillator. This is probbly due to low RF power on the varactor diode. In my circuit the varactor is connected between collector of the oscillator and ground.

My ugly bug construction boards are never compact and neat due to R&D and frequent changes to the circuit until I am satisfied. So there is a lot of "hacking" involved.

I want to thank Joy for yet another useful contribution to the visitors circuits. A project like this is very useful to show a working method of construction that is cheap, quick and gives excellent results.

Don't forget to visit my messageboard. If you have any questions about this project then ask Joy on my messageboard, or e-mail Joy.

Very best regards from Harry Lythall
SM0VPO, Märsta, Sweden.

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