Previously I described the original project, and how I’m hoping to improve on it.

In this article, I’ll present an overview of the transceiver from a high level.

[See the full list of articles in this series HERE]

My aim is to build a useful QRP CW multiband transceiver, with a simple user interface, that can be built from a well-stocked parts store, or obtained from current component sellers at a reasonable cost. It should be buildable by relatively inexperienced constructors – like the original, it is possibly too hard for a beginner, but I’ll try to provide explanations for everything, from the theory of operation to the construction.

The block diagram of the transceiver is shown below (click diagram for larger version):

The receiver is a direct conversion circuit, as a superheterodyne would be too complex. It’s CW, because CW is awesome 🙂

It also makes for an easy design that doesn’t require complex setup/test/alignment.
Rather than using a capacitor-tuned variable frequency oscillator as in the original single-band transceiver, I’ve opted to use one of the cheap AD9850 direct digital synthesis modules available from eBay at low cost. The module I’m using is shown below:

This needs to be driven by a microcontroller, and I’ve chosen the Arduino Micro, as I’ve used it before, it has just enough I/O/Timer capability, is readily available and it’s small. I want the transceiver to be compact, but not built directly from surface-mount components: it should be constructable by those with not-so-perfect eyesight! The only surface-mount work is in pre-assembled boards (Arduino, DDS, LCD).

To control the transceiver, I’ll display the current frequency on a cheap 16×2 LCD display, and allow tuning and other control with a rotary encoder plus single button. I’ll have more to say on the Arduino / control / user interface in the next article.

I’ll add receiver incremental tuning (RIT) by providing a changeable offset when receiving, and sidetone generation when transmitting as an audio output from the Arduino.

In the original circuit, a bandpass filter (built from hard-to-obtain TOKO coils; now available as Spectrum coils from Spectrum Communications) was used before the mixer to filter out strong out-of-band/broadcast signals, with a separate low-pass filter (built from readily-available T50 toroids) after the transmitter power amplifier to reduce harmonics, restricting transmission to the amateur 80m band. In this version, rather than switching pairs of different bandpass and low-pass filters into the receive and transmit circuits, I’m using the same bandpass filter for transmit and receive, switching the selected filter in just before the aerial.

The bandpass filters (constructed with T50 toroids) are pluggable via SIL headers – there are three slots for filters, and you can swap in new ones. The transceiver needs to know which filter is installed, configuring this via the setup menu so that the appropriate relays can be switched in when the bands are changed. I chose three bands; you could build for fewer – or more if you use the three band-select digital output wires to drive a shift register.

The filtered input signal, plus the buffered DDS oscillator frequency is presented to the receiver’s NE612 mixer, with audio output being filtered by a narrow-band audio filter (the Hi-Per-Mite filter, with changes to select my preferred CW audio frequency), then passed to an audio amplifier. The receiver and audio filter are only powered during receive.

In the original project, switching between transmit and receive was achieved manually with a switch. This revision controls transmit/receive switching, keying, and break-in with the Arduino. In transmit mode (as controlled by the Arduino when the key/paddle is used) the receiver is off, and the Arduino’s sidetone output goes to the audio amplifier. When switching to transmit mode, the RX/TX relay switches the selected bandpass filter and aerial to the transmitter. At the same time as the sidetone audio is being produced, the Arduino is also generating a keying signal that keys the class C power amplifier (PA). This RF – hopefully in excess of 1W – goes into the bandpass filter to reduce harmonics, then into the aerial. The transmitter always produces a driven version of the buffered DDS output; this is amplified on demand by the keyed PA. Break-in is controlled by the Arduino; shortly after the final keying ends, the RX/TX relay returns the bandpass filter and aerial to the receiver and supplies power to the receiver.

All Arduino band-switching, RX/TX switching and keying signals are opto-isolated, to prevent RF getting into the digital side of the system.

I intend to build the transceiver as a series of separate modules, rather than building it all on one board. This should aid testing – and presentation – as each module can be built and tested separately, then integrated. Note that in the schematics and parts lists shown in future articles, each circuit will be stand-alone in terms of component numbering, so there will be an R1, C1, etc. on each circuit module.

I’ll be using a mixture of stripboard / matrixboard / prototype board and copper-clad Manhattan-style construction for the various modules. Interconnects are all 0.1″ 2.54mm single in-line headers, with ribbon cable, shielded audio or RG174 coax RF cable.

Stay tuned for the next article, where I consider the user interface….