SDR RADIO

Inspired by several websites recently "surfed", this is my build of a SDR or software defined radio.
For display I use a dowloaded "beta" version (freeware) of the software kindly provided at SDR Radio by Alberto I2PHD
This design is basically a direct conversion type receiver, which frequency translates a band of "off-air" radio signals directly down to audio; so as this spectrum-spread (50 KHz approx) can be fed into the PC sound card - for processing.
The SDR program takes care of the display of all parameters.
Hardware is mostly constructed from re-cycled componentry.
The circuit is based on the well known Soft rock QSD circuit (and a few others).
VFO tuning range is 14 MHz to 15.4 MHz for 300 degrees of knob rotation.
This means 7 to 7.7MHz in the 40m band and 3.5 to 3.85MHz in the 80M band.
Dial calibration is by a handmade scale using the swish/plop method of listening to the VFO in a good receiver.
However as an adjunct a simple external frequency counter was constructed, and attaches by 3 wires; +5v, gnd and signal in (TTL level).
VFO tuning is by a pair of back to back varicap diodes.
Diodes measured 25pF approx on my LC meter (nil applied DC), I don't know what type they are; (salvaged from VHF TV tuners!)
VFO is housed in cut-down mustard tin (again!) to provide shielding
Well proven JFET hartley oscillator followed by two cascaded inverters (first biassed as a linear amplifier by 100K resistor) to give TTL (5V) level signal.
The VFO output is divided by 2 and by 2 again by a cascaded pair of 'D' type flip-flops type 74HC574.
A better choice would be the 74HC74 here.
I just happened to use what I had on hand, to get things working.
The 74HC02 and the 2 inverters is configured as a 2 line to 4 line decoder.
We can only employ the HC (high speed CMOS) variant of the available "logic" I.C.'s because of their upper switching speed spec' of approx 50MHz
Antenna tuning (preselector) is with a simple parallel LC circuit.
DC power for the electronics is derived from USB lead from PC (spec' indicates a max of 100 mA can be drawn from a USB port.)
A quadrature switching detector provides an I & Q output pair (in phase and "quadrature" 90 deg lag - signals)
A common QSD is the Tayloe detector, named after designer Dan Tayloe.
The Tayloe detector can be thought of as a four-position rotary switch.


The switch revolves at the same rate as the carrier frequency. Each of the four switch positions is connected to a sampling capacitor, and the 50 ohm antenna impedance is connected to the rotor.
Each capacitor will track the carrier's amplitude for exactly one-quarter of the cycle since the switch rotor is turning at exactly the RF carrier frequency, causing the switch to sample the signal at 0, 90, 180 and 270.
If the switch is off, each capacitor will hold its value for the remainder of the cycle.
The 180 and 270 outputs carry redundant information with the 0 and 90 outputs respectively.
Therefore the 0 and 180 outputs can be summed differentially to produce the in-phase signal (I). Similarly, the 90 and 270 can be summed to form the quadrature signal (Q).

For a well written explanation of the I & Q demodulation technique, I recommend reading :
Watch your I's & Q's by VK6VZ & VK6APH (UK Radcom mag' Jan 2007) - below.

Watch your I's & Q's Pg 1
Watch your I's & Q's Pg 2
Watch your I's & Q's Pg 3

Also:
Fists of Fourier: Software Defined Radios Explained - The Garage Shoppe

















Ref : Youtube video of an I2PHD decoder in action.

At the present time (Sept 2010) this is still a work in progress {WIP} and some exhaustive "off-air" testing still needs to be done !