Jason has been working on a portable software defined transceiver design for the past year. Every day this week he’ll discuss a different part of the hardware in a series of guest posts. You can chat with the designer in the forum. Today’s post is about variable-frequency oscillators.
The VFO is the heart of any radio. It determines both the transmit and receive frequency in any modern radio equipment. The performance of the VFO is closely tied to the performance of the radio — especially the receiver. Any phase noise (jitter) or supurious responses will degrade the receiver performance. I have chosen the Silicon Labs Si-570 for this purpose. Think of it as a software controlled crystal oscillator. You can specify the frequency with approximately 0.04 Hz resolution, and the range goes from 10MHz to 160MHz for the CMOS version, or to over 1GHZ for the LVDS version.
Lots more below.
Since I’m interested in only covering the frequency range from 0-30MHz, I need a VFO which covers 4x this range in order to implement the type of mixer and the type of modulation I want to enable. Which means I ideally need 0-120MHz of range. The CMOS version of the Si-570 gives me almost this range at 10-160MHz, which will enable operation from 2.5MHz to 40MHz. Lower frequency operation is possible by simply dividing the output of the Si-570 chip further if desired. The CMOS version of the Si-570 is also easier to interface to, so I have selected it to be used in my design.
The VFO clocks a pair of flip flops, which divide the output frequency by four. The outputs from the flip flops are decoded by four NAND gates to produce four phases of output at 0, 90, 180, and 270 degrees (I, Q, I~, Q~ respectively). These quadrature outputs are needed by the receiver mixer which is a quadrature mixer as well as by the transmitter, to enable BPSK and QPSK modulation.
Here is the schematic. I’ve also attached a copy of a PDF file containing the schematic since it is a little fuzzy at the resolution it is limited to by the web page
Via the forum.