Philip A Covington’s Blog

Here is a diagram of a SDR that uses ideas from the K2 RF front end and PLL synthesizer: K2LikeSDR.pdf It would use the K2's RF front end. I would substitute the TUF-1 mixer of the K2 with a switching mixer based on a FST3125 bus switch. The QSD would be at a fixed IF of 5 MHz. The LO for the QSD would be derived from a fixed 20 MHz oscillator divided by 4 to generate the quadrature signals needed to driver the QSD switches. The K2's PLL would be modified by eliminating the variable reference oscillator (resulting in the PLL tuning in 5 kHz steps) and substituting one of the LMX National PLLs for the MC145170 PLL used in the K2 design. The K2's VCO design is good, but I would also eliminate the VCO AGC circuit of the K2.

SDR LO Ideas: There has been a lot of discussion of using DDS or PLLs for the LO generation for HPSDR and SoftRock like receivers. In the current SoftRock series the LO is generated by a crystal oscillator. The downsides of the above approaches are: 1. DDS: spurs and cost of a decent low jitter/low noise clock source. Some of the 400 and 500 MHz DDS chips are on the expensive side. 2. PLL: phase noise. A scheme using a microwave VCO/PLL and dividing it down into the HF range has been discussed. The latest feeling is that the phase noise reduction even after dividing down is not enough to provide a high performance system. A dual PLL loop approach has been proposed to improve phase noise performance but it is likely to be expensive. There are also PLL/DDS approaches of varying complexity. 3. Xtal: limited frequency agility, you are limited to tuning within the passband of your computer sound card. There is a forth method that might work for lower cost SDR circuits like the SoftR...

QSD/ISD Switches: Here is a link the PDF data sheets of analog switches that can be investigated for use in QSD/ISD circuits: http://www.philcovington.com/HPSDR/QSD_SWITCH/ Interesting ADC: Here is a link to an interesting ADC by Analog Devices: http://www.analog.com/UploadedFiles/Data_Sheets/AD7760.pdf It is a 24 bit, 2.5 MSPS ADC. Here is the circuit idea: http://www.philcovington.com/HPSDR/TEMP_STUFF/AD7760QSD.pdf It uses 1/2 of a QSD (not quadrature sampled) to mix the frequency band of interest down to zero Hz with a bandwidth of something like 250 kHz. The IF would be offset to something like 125 kHz. The ADC samples the 1/2QSD output and results in a real data stream. A FPGA or CPLD mixes the real data stream with a complex NCO to generate a quadrature data stream which is sent over USB to the PC where the remaining processing is done. The advantage is a 1 X LO and no I/Q balance problems. One disadvantage is that your sampling rate should be 4 times the desired bandwidth...