Direct Conversion Receiver on 80m Band Design3

Hello again, my friends. This is Razi Naji, an amateur radio operator with the call sign YI1RZ. Today's video is about an 80-meter band receiver using the Direct Conversion technique. It is not very different from the previous design that I published about five months ago, titled "Direct Conversion Receiver on 80m Band Design 2." I simply made several improvements to the circuit to make the receiver more stable, sensitive, and efficient. 1. I added an RC low-pass filter (LPF) directly after the antenna and before the resonant circuit of the RF preamplifier. Its purpose is to attenuate strong shortwave broadcast signals around 6 MHz, 7 MHz, and higher frequencies, helping to reduce interference from broadcast stations. I know that an RC LPF is not the ideal solution for this purpose, but I wanted to keep the filter simple. 2. The RF preamplifier stage used in Design 2 was replaced with another stage that provides higher gain, greater sensitivity, and a very high input impedance. This completely isolates the antenna stage from the mixer stage. 3. Replacing the J310 transistor in the mixer stage with an MPF102 transistor, and swapping the input locations of the received signal and the VFO signal, resulted in improved mixer performance and a stronger audio output. 4. The two-stage RC low-pass filter used after the mixer in Design 2 was replaced with a single-stage RC LPF with a cutoff frequency of approximately 3.4 kHz. 5. The 2SC945 transistor in the audio preamplifier stage was replaced with a BC547B transistor because it provides higher gain. 6. A buffer amplifier stage using a J310 transistor in a Common Drain configuration was added. This ensures that the output of the audio preamplifier does not suffer from loading effects when connected directly to the power amplifier stage. It also provides complete isolation between the two stages. 7. A second-order RC low-pass filter was added after the VFO to reduce the unwanted harmonics generated by its digital circuitry. The two filter sections were isolated using a buffer amplifier stage, as explained in point 6, to prevent interaction between them. The filter was designed with a cutoff frequency of approximately 4 MHz. T1 and T2: Oscillator transformers with a red core, commonly used as oscillator coils in AM medium-wave superheterodyne radios. Although these transformers are primarily designed for medium-wave operation, I discovered by chance that their response at 80-meter band frequencies is excellent, so I decided to use them in the RF preamplifier stage. T3 and T4: 600-ohm to 600-ohm audio transformers. These transformers are available from many electronics suppliers such as AliExpress and eBay. If you prefer not to use them, you can replace each transformer with a resistor in the range of 2.2 kΩ to 4.7 kΩ and couple the stages using a capacitor. This approach also works well. Unfortunately, some of the received signals in this video are weak primarily because of poor propagation conditions during this season. I apologize for that. Thank you for watching, and best 73's. Razi Naji – YI1RZ #YI1RZ #DirectConversionReceiver #HamRadio #80mBand #AmateurRadio #RadioReceiver