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…
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.