Quadrature output from si5351a

The SI5351a is a key secret to the success of the BITx range of transceivers.  This chip puts out three PLL signals for the two local oscillators and the VFO in the µBITx.   Without this chip (or something similar) a double superhet design such as that used in the µBITx would require considerably more complexity, with either an analogue VFO that was prone to drift (as in the BITX20) or a PLL circuit that added cost and complexity.  The Raduino integrates a 16×2 line display along with an arduino nano and the SI5351a chip … prebuilt for US$25.

So along comes Miguel PY2OHH who announces that he as designed a new
VFO using the SI5351 and Arduino nano, which produces an output  “in quadrature” from 4.76 MHz to 220MHz.  This means is has two outputs on the same frequency, but which are 90 degrees phase shifted from each other.    Note that there is also a small frequency window that can’t be used between 144.66 MHz and 150MHz.

You can read all about the idea, and see a prototype VFO here.

So why is this such a big deal?

What you may not know is that a quadrature mixer, which requires two signals from a local oscillator fed at 90 degrees from each other, is at the core of most SDR receivers.   In an SDR receiver, there is very little “front end” analogue circuitry, with signals rapidly routed to a processor in digital form to apply filtering and band-limiting.    Miguel’s breakthrough paves the way for simple, low-cost SDR transceivers that are of low cost and high performance.   We know that Ashhar Farhan VU2ESE has said in the past that he is experimenting with SDR designs (and with a VHF/UHF version of the BITx).   Let’s hope this innovation spurs him on to produce a stunning new SDR design in kit form!


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