This radio was previously described on the old RadioBoard forum under the title “A Simple Superhet”, and is re-presented here in more detail than previously. It is a general coverage broadcast receiver.
The radio features digital tuning and frequency readout with memory channels, a low VHF first intermediate frequency for good image rejection, and broadband circuits eliminating alignment. No unobtainium parts are used. Commercial products are used where there is an advantage to do so.
The radio is constructed modularly. Three 50 x 80mm BusBoard systems SMD prototype boards ( 200 x 100 format ) hold the RF and audio circuits. The power supplies are on simple perfboard. The completed modules and VFO are installed on a breadboard “chassis” designed on TinkerCad and printed on a simple and inexpensive 3D printer with PLA plastic, thus no complicated metal work is required. A rotary encoder is used for tuning, and directly mounted to the front panel, eliminating the tricky mechanical alignment problems associated with obsolete LC or Varactor VFO’s. The main tuning knob is also 3D printed. Volume is set by an electronic attenuator controlled by a simple DC voltage, eliminating cumbersome shielded wiring to the front panel.
The heart of the radio is a commercially produced QRP Labs VFO kit, available for about $33 USD, using the now ubiquitous Si5351 frequency synthesizer chip. The VFO can generate a tunable LO well past 200 MHz, quadrature output if required, tuning steps as fine as 1 Hz, at a square wave level sufficient to drive commercial level 7 mixers without additional amplification. A digital frequency readout and memory channels are provided. Further, an additional fixed frequency local oscillator is provided. The firmware allows the frequency displayed to account for the IF offset from the tunable oscillator. The kit is supplied with a white on blue background LCD, but 5 volt type 1602 LCD readouts are available in a wide variety of color combinations. My radio at this time uses a black LCD with red digits.
The Si5351 has many good features and one, perhaps two, significant drawbacks - there is a degree of cross talk between the oscillator channels that is inherent to the device. The output is a square wave, by definition harmonic rich. This is ideal for diode mixers. The Si5351 has a DC component on its output, so a DC blocking capacitor of 10 or 100 nF should be used in circuits where this DC component cannot be tolerated.
For up converting superheterodyne receivers, the general rule of thumb is that the first intermediate frequency should be at least 1.5 X the highest frequency to be received.
The intermediate frequencies in this receiver are 45 MHz and 450 KHz. Crystal filtering is used at the first IF, and a ceramic filter is used at the second IF. The use of a low VHF first IF, greatly simplifies the construction of the receiver, while enhancing its performance compared to older design techniques. Tunable LO injection is 45 MHz higher than the frequency to be received. The use of the low VHF IF puts the image frequency 90 MHz above the received signal, thus a simple low pass filter is satisfactory to protect a strong first mixer, while eliminating front end alignment and tracking issues.
The low pass filters and diode double balanced mixers used are commercial products by Mini Circuits. The IF amplifiers are 50 ohm broadband amplifiers. A TDA1072 Integrated Circuit handles final IF amplification, AM detection, AGC, and provides a meter output voltage that is used for signal strength indication and an additional AGC loop. Audio output is from a DA7052A BTL Integrated Circuit.
Each of the modules in the radio will be described in further detail in subsequent posts.