I have been playing with the ISDR software defined radio from OM VU2DEV , Ramaprabhu ( from Bangalore )for the past one week. ISDR is a low cost SDR for 40m band. The radio is performing wonderfully. I bought 2 assembled kits from DEV and I recommend the kits to any one who wants to have a taste of SDR technology. The fully assembled radio is available with VU2DEV at Rs 750 ( US $15 approximately) and comes packed in a rugged metalic enclosure and with necessary audio cable. ( Email vu2dev at yahoo.com for availability, exact pricing and shipping info. ).
The following pictures show the professional quality ISDR .
OM VU2DEV has also sent me the schematic diagram and component values and permitted me to post the same on my blog.
The block diagram of the radio is shown below. (Click on the picture for a larger view)
At the heart of ISDR is a 28.224Mhz crystal which is used to create stable oscillations. This frequency is divided by 4 using two 74HC74 dual flip flops to produce two 90 degree out of phase local oscillator signals for the radio.
On the RF side signals from the antenna are filtered for the 40m band and some RF amplification is given. The RF signal is then mixed with local oscillator signal using two diode ring mixers. The resultant signals are sent to a simple low pass filter to get rid of unnecessary mixer products and then amplified. The two quadrature signals are fed to the mic input of a PC sound card. ( You need a sound card with stereo input).
The sound card is used as analog to digital converter. The normal sound card found on your PC will have a sampling rate of 44khz ,with 16 bit accuracy. The sampled signals are processed digitally inside the PC using SDR software and you can hear the radio via PC speakers. Your tuning range will be half of the PC sound card’s sampling rate. So with 44Khz card , you can tune from 7034Khz to 7078Khz. If you have a professional quality sound card, your milage with SDR will be better.
The circuit diagram and component values are given below.
My Intel G35 mother board seems to supports ( I am suspect ) 96khz sampling via the onboard Realtech card. Hence , I am able to tune almost the entire 40m band. Here is a recording from the radio using Rocky.
OM VU2DEV at his morning QTH M/s Micronova Impex Pvt Limited is making a large number of kits including ISDR for the benefit of the community. You may contact them directly. The contact email id is <firstname.lastname@example.org>.
I have used his JOTA transceiver and reviver . Both are valuable additions to your shack. He has also provided me with PCBs of his new 72 watt linear, I am planning to build them during my next vacation