17 - Building a Lowcost HF transceiver from Scratch

Building a Lowcost  HF transceiver from Scratch.

Transceivers have only 4 different types of circuit within them, so making a complete transceiver is not overly difficult, particularly given the availability of very low cost test equipment and modern designs that are simple and convenient to make.  Radio Amateurs who do this are called homebrewers and it is an active and growing part of our hobby. I will document my journey to making a particular design known as the uBitx.

The four types of circuit are;

Filters, two types are often needed; Lowpass made with good quality capacitors and hand wound coils, usually on a simple powdered iron toroid core. Also needed are bandpass filters often made with a number of quartz filters to provide good selectivity or purchased as a ready made crystal filter.

Amplifiers; The uBitx receive chain uses a number of simple three transistor circuit, 4 identical modules are needed. The uBitx also has two simple one transistor circuits as a microphone amplifier and a audio pre-amplifier. The audio pre-amplifier feeds a low component cost audio amplifier but I will replace this with a more powerful audio circuit to drive bigger speakers as I have poor hearing. The other amplifiers in the BitX are in the transmitter portion of the circuit and there are four stages to take the sub-milliwatt RF signals up to 10 Watts. Each stage has only half a dozen to ten components and is easily tested in isolation. In fact building and testing each simple module in isolation is the best way to build.

Mixers; There are three of these, two are identical  each with four diodes and two simple broadband transformers handmade with 8 turns of wire around small ferrite toroid. The third has only one transformer and two diodes. These circuits also have 2 or 3 resistors and the odd capacitor but they are not complicated.

Oscillators; In days of yore these would be the main headache, now they are extremely simple. A single chip can provide the 3 outputs we need. This 8 pin chip is driven by a simple arduino (a nano) so that the frequencies can be changed. This allows a nice modern user interface with lots of flexibility. The software has been written for you so you do not need to be a programmer to successfully construct or buy this module.

And that is it! you can see the circuit of the uBitX at https://www.hfsignals.com/index.php/ubitx-v6/ 

Of course if interested in building a transceiver the uBitX is not the only option, HFsignals.com also describes a simpler design that covers one band only; the Bitx. This design was originally for 40m but can be put onto any other band. 

There is also a most excellent book from the RSGB online shop.

Building a Transceiver by Eamon Skelton EI9GQ and Elaine Richards, G4LFM

This is a very complete and very well explained book. It covers a more complex design but it is a valuable read. The uBitx also has good community support from the groups.io/bitX forum - it covers both Bitx and uBitx. In the near future it will no doubt cover the sBitx SDR transceiver and possibly the unnamed 2m Bitx. 

A block diagram of a conventional SSB/CW transceiver is listed below, it has the four types of object mentioned above, note the RF receiver amplifiers (between the mixers) are paired up but only one of the pair is given 12 volts of power at a time - on the 'R' or 'T' lines. this allows the mixers and filters to be shared by the transmitter and the receiver without needing a lot of relays to switch from one to the other. These amplifiers are designed so that when they are powered down they do not affect the powered up circuitry (much). 

The secret to managing complexity is to divide and conqueror. Once you can build a triangle (an amplifier) a rectangle (a filter) and a mixer (a circle) you have most of the hard work done! I have covered the bandpass filter, the output low pass filter and the crystal filters in previous CONTACT articles, at least partially.

 

 

 

I intend to make my uBitx in a modular way - each module will be on a small piece of singlesided PCB material, each PCB will be placed on a baseplate of a bigger piece of PCB - 6.5 inches by 6 inches is the target. Soldering bits of braid will connect each module to the base and connect up all areas of blank copper to zero volts. Before making the first module I estimated what size each module might be and made a floorplan. Module size for the filters and mixers was dictated by my use of T50 toroids for the filters and FT37 for the mixer transformers. That's what was in my junkbox! T50's have an outside diameter of 0.5 inch and the FT37's are 0.37 inch.

The main construction technique will be "manhattan" this is where islands are glued or created on blank copper clad PCB board. I also etch a traditional printed circuit board for the amplfiers as I have made many boards this way - using glossy photo ink jet paper in a laser printer to create black plastic toner which can be melted onto a copper board using a smoothing iron. The melted toner protects tracks and pads prior to dipping into ferric chloride solution where the  unprotected copper is eaten away in a few tens of minutes. 

However, just for a change I though I might try something new (to me). As well as supergluing small squares of copper I will grind circles using a dremel and a core cutter that i bought from the GQRP club many years ago - it makes a 3mm circle and a 1mm trough. (The GQRP club currently sell a slightly bigger version). Manhattan layout also allows mid-air connections.