This radio receiver is capable of receiving all MW and SW bandwidth, and can also operate as an SDR receiver, so with the help of PC and appropriate software we can decode many different types of signals.
It consists of several parts:
At the input we start with a potentiometer of 1 k ohm, it is the RF attenuator. The next circuit is the preselector, switchable to the MW and SW ranges. Tuning of the preselector is done with a potentiometer. The tuning voltage is stabilized by 4 LEDs but of course you can also use a 6.8 V or 8.2 V zener diode. The two transistor RF amplifier increases the sensitivity and prevents also that Local Oscillator signals from the mixer are leaking to the antenna input. This would cause a lot of hum. The mixer is a well known NE612 (or NE602, SA602, SA612).
And now a little more detail about the local oscillator. This VFO was actually the reason why I decided to build this receiver. It is based on Genius idea of T.Uebo / JF3HZB to create virtual Analog rotating dial on Colour TFT screen. For the preparation of this section is used an ESP32 microcontroller, Si5351A I2C Signal Generator module board and 1.8 inch 128x160 TFT LCD with ST7735 chip. A rotary encoder is used to change the frequency. You can download the code below or directly from autr page : https://tj-lab.org/2019/02/17/vfo5/. The code can be easily modified so that we can change scale and background colors, scale shape, the direction of rotation and rotating speed. Also can be changed Offset and Carrier frequencies as well as minimum frequeny changing step and other parameters. On the picture below you can see some visual modifications to the dial scale.
The code modifications I made in this case are:
- char f_rev = 1; //rotation direction change
- vth=0.2 // Acceleration
- Racc=0.02 // and
- Rdec=0.2 // speed change to faster
The next step is the IF amplifier and decoder. I tried several types, with integrated circuits TCA440, TDA1072, but I was challenged to do this part in the good old way with IF transformers. You really need a lot of patience to set it up, but the result is great.
Now the signal is carried to a pre-amplifier based on the 741 IC and finally an class D amplifier board with PAM8403 from Aliexpress.
And lastly to emphasize that the sensitivity and selectivity of this receiver are excellent, given that I am in a densely populated part of town and the antenna used (The PA0RDT Mini Whip) is only 3 m above the ground.