The QRP2001 is being developed as a high performance direct-conversion transceiver for HF bands. However, some work on the transmit circuitry is still in progress, so it is presented here as a complete receiver design, with details of the transmit circuitry to follow in due course.
The circuit is based on a 'Tayloe' product detector (see Credits) together with a polyphase filter to give single-signal receive capability with good audio quality. The excellent strong-signal performance of the Tayloe product detector and carefully selected gain distribution results in a pretty decent performance by any standards, and a quite remarkable performance for a simple design with no pre-selection filters.
Although other designs based on the Tayloe/Polyphase combination have appeared since, we think that the QRP2001 was the first to use this configuration. If you know differently, please don't shatter our illusions!
(The Tayloe Product Detector is a patented design, and therefore any commercial use of the design is subject to agreement. The patent application can be downloaded from the US Patent Office web site under patent number 6 230 000 titled "Product Detector and Method Therefor". Thanks to 9A2HL for this information.)
The output from the polyphase filter passes through a series of audio amplifiers and further filters to define the audio bandwidth. There is plenty of room for experimentation here, and indeed we tried numerous combinations before arriving at the circuit presented in Main Board.
Earlier versions of the prototype receiver used a DSP board to replace either the audio bandpass filters or both the polyphase and bandpass filters, but we haven't included details here because it increased the cost considerably for little performance gain. If anyone feels inclined to produce a design for low-cost DSP processing of the Tayloe product detector output, then please contact us!
Given the outline circuit description above, the block diagram will come as no surprise. The only major feature which has not been mentioned previously is the SCAF (switched-capacitor audio filter). This is a very useful little circuit which provides a low pass filter with variable cut-off frequency, and a steep (8-pole) slope. It offers a cheap and convenient method of changing bandwidth between SSB and CW modes, and would be a useful addition to many other simple receivers.
All of the receiver circuitry apart from the VFO is accomodated on a single board. Both the main board and VFO are described in detail in subsequent sections.
We have included photographs of our prototype hardware throughout these pages. Only fans of VUCM (Very Ugly Construction Method) are likely to find these images attractive. However, after a long period of measuring, tweaking, re-measuring and so on, their performance more than makes up for their appearance. If you like your radios to be neat and tidy or you have an indifferent connection to the internet it probably isn't worth following the links to the pictures - they are provided for interest only.
So, bearing that in mind, here is a picture (41 KB) of one of our prototypes complete with low-noise VFO, nearing completion.
