I did some further experiments with AM modulators. First I tried to use a NE612 as modulator. It works quite well, but requires some adjustments. Volker, SM5ZBS, describes such an approach on his German website.
As alternative approach one can make use of a Dual-Gate FET such as the BF961. J. Roschy has proposed such an AM modulator. It is working really nice, has an excellent audio quality, is simple to build and requires only one simple adjustment. Here is my version of this AM modulator::
The 4060 has an internal oscillator circuit and a counter. The crystal's frequency of 12 MHz is divided by 16 which results in a stable carrier frequency of 750 kHz.
The carrier signal is connected to the BF 961's gate G1, the audio signal to gate G2. The modulated output signal is taken from the drain. An inductor in the drain blocks the modulated 750 kHz RF signal from the power supply. The inductivity is not critical. Any value in the range of 1-5 mH will work. I used a Neosid fixed power inductor with 2.2 mH.
With the 10k trimmer the drain current is adjusted to 1-2 mA for best audio qualtiy. One can provide the audio directly from the headphone output of a mobile phone. The 150p capacitor is optional, depending on the coupling to the radio. When using a little peace of wire the ouput signal is increased by the capacitor.
As power supply a 9 volt battery might be used, but also a 5 volts stabilized power supply will do. My test setup consists of the following components
- a Raspberry PI 3 running Windows IoT and an Webradio App.
- the AM modular as described above
- and old Philetta AM tube radio
The AM modulator is powered by the Raspberry PI's 5 volts taken from the GPIO port and the audio signal from the Raspberry PI's 3.5 mm audio jack. The RF signal is fed by a simple wire to the Piletta's antenna input.
Please note that the AM modulator's output signal has some harmonics that one might want to remove by a low pass filter.