Vermes
Advanced Member level 4
It is a design of basic SSB receiver set with a direct conversion. The operating principle of homodyne receiver is the direct conversion of the received signal into an audio signal. The sensitivity of such a recorder depends on gain of applied low frequency levels. The selectivity depends on used in this device filters. The gain should be as great as possible (about 100dB).
A lot of similar receivers can be found on the Internet. Such receiver does not have enough gain, selectivity and too much noise.
This project consists of receiving line of own design, based on solutions and circuits found on the Internet.
The whole line was divided into three parts, mixer with the amplifier of adjustable gain, active low-pass filter, and then the signal is amplified one more time in two-stage operational amplifier with adjustable gain.
Mixer was based on NE612, which is symmetrically connected to the level made on operational amplifier OP27. It has small noise and it relatively cheap. The amplifier has adjustable voltage gain in the range of 50-100 (40dB max plus 3dB resulting from symmetric connection with the mixer). This number should be increased by a typical gain of NE612 conversion, which is 14dB/50MHz. That means the total gain here should be 57dB.
Another part is the low-pass filter. It was built on two operational amplifiers. It is a two-stage filter from Butterworth in Sallen-Key topology. It was calculated using program FilterPro. It was designed for cutoff frequency of 2,5kHz.
Picture below shows the characteristics and the system of such designed filter:
The last part of this two-stages amplifier was the microphone amplifier kit. The amplifier operates on US NE5532, which contains a double operational amplifier. Voltage gain of this stage is adjustable in range of 10-100 (20dB-40dB). The signal should be passed through potentiometer of volume control to any amplifier of low frequency for the speaker.
The total maximum gain of low frequency line of the receiver, 57dB + 40dB = 97dB. This value should be decreased of about 4dB damping by the filter, what will give the maximum gain of 93dB.
When the filter is avoided, the signal is taken directly from the output of the first amplifier. You can compare and decide if the filter is necessary or not.
Link to original thread - Podstawowy zespół odbiornika SSB o bezpośredniej przemianie.