Title: HB TX from OE3WHB Post by: Steve - K4HX on July 12, 2015, 01:42:26 PM Willi sent me the details on his homebrew transmitter. It's a 12BY7 modulated by a EL84/6BQ5 and uses RF-derived negative feedback to lower audio distortion.
AM 6m-Transmitter with 4W This transmitter is a crystal controlled 2 stage device with a 12BY7 in the PA. It is AM modulated with a Heising-circuit driven by a tube EL84. The Mike-amplifier is based on Transistors and OP amplifiers. The input stage of the audio amplifier is automatic gain controlled and has an output voltage of approx. 0.28Vrms. The next 2 stages are build by OP amplifiers. The first OP amplifier has a summing node to mix the mike signal with a negative feedback signal (= Gegenkopplung, GK, in German) which stems from a RF-detector on the transmitter output port (M). The second OP amplifier builds up the driving level for the EL84. This feature improves significantly the modulation distortion you will find without negative feedback. The mike amplifier puts out a constant level no matter how loud you talk into the microphone. Depending on the sensitivity of the mike it can be useful to reduce the gain of the second transistor. This can be done by inserting a resistor of 0 to 1k in the emitter circuit. The final value has to be tested in a QSO and to some degree it is also a question of personal taste. Proper adjustment of the negative feedback: 1) the feedback loop has to be opened (Rgk removed) and the gain of the second OP amplifier is adjusted with Rv for 95% modulation at full output of the audio preamplifier. 2) Rv has to be changed to a 10 to 15 times greater value and the feedback loop must be closed by connecting a variable resistor of 500k from the RF-detector M to the GK-input (Rgk). Now adjust Rgk for 95% modulation degree. Thats all. You will notice a significant improvement of the shape of the RF envelope when you feed in a 1kHz sine signal to the mike input. Please notice, the RF detector (M) works with a load resistor of 12k and a carging C of 2.7nF. It is important that the time constant of these elements will NOT exceed 33µs otherwise the cap will not discharge fully during the negative half-period at the higher audio frequencies and some distortion arises. Title: Re: HB TX from OE3WHB Post by: Steve - K4HX on July 12, 2015, 01:51:07 PM Scope modulation envelope displays with and without feedback.
Title: Re: HB TX from OE3WHB Post by: Opcom on July 12, 2015, 07:13:15 PM 10 to 15 times, so it is in a range of 10+dB for NFB possibly? Very nice.
Title: Re: HB TX from OE3WHB Post by: W7TFO on July 13, 2015, 03:20:03 AM Nice design/execution. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
Western Electric used that feedback topology in their Doherty final type broadcast transmitters from the late 30's. It was amazing to pull the RF rectifier tube and hear the program distortion rise. 73DG |