General Coverage double conversion superheterodyne
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.
OVERVIEW:
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.
The meter is a Calectro part. I got a whole sackful of these meters in various movements, cheap, at a hamfest. They look nice, and mount easy in a single large hole with a lock nut from behind. The meter used here is 0-1 ma movement. Microamp meters could be used with a suitable meter shunt.
The small locking connectors are the XH 2.54 mm type. They are a poor match for, and not very convenient to use, on these prototype boards. They are cheap, however, and do make it very convenient to remove the boards for modification without having to unsolder a bunch of connections. I feel they are worthwhile.
I used store bought mixers, but they can also be homebrewed. I used a store bought low pass filter, but that, too, can be homebrewed.. I just found it convenient to use store bought parts to save time. I have serious doubts that I could make two identical mixers, but Mini Circuits can make millions of them and they will all perform the same. Designs for homemade Si5351 frequency synthesizers are also common but I like the memories and advanced features of the QRP Labs VFO.
SMD / SMT components are used whenever possible here. There seems to be a prevailing attitude among home builders that SMD components are more difficult to use than leaded through hole components, and I also held that perception. When I was working on a simple SSB transceiver, I was encouraged to make the switch to SMD by more experienced builders, and, I have to say, it was really good advice and I have not looked back from that decision to switch. The parts are much cheaper, the circuits work better, and assembly is faster and easier. I have come to prefer 0805 size parts. 1206 parts seem a little big to me. When necessary I use 0603, and sometimes 0402, although the latter is difficult to use on these prototype boards. Two tools are essential: a hot air gun makes removing smd components a snap. A good temperature controlled soldering iron is a necessity. I just use a regular tip with SMD components. Some people say to use a fine point tip. I don’t think it is necessary. I have found that I like solder with a little bit of silver in it. I despise lead free solder.
Test equipment is really an unfortunate necessity when fooling with RF. I have a lot of VOM’s, all cheapies. A set of tweezer type test probes are helpful with SMD parts. For everything else I use a Red Pitaya StemLab 125-14, the 14 being the bit depth of the device. This is a great device - it can do two oscilloscope channels to 60 MHz, two signal generators to 50 MHz, two spectrum analyzer channels to 60 MHz, with frequency sweep and all sorts of other stuff in a package about the size of a pack of cigarettes. It’s just a terrific device - I can do as much with the Red Pitaya on the kitchen table at my second home as I can in my ham shack.
This receiver is literally a kitchen table receiver - it was built on and has never left the kitchen table at my second house.