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 for  availability,  exact pricing and shipping info. ).

The following pictures   show the professional quality ISDR  .

Indian SDR reciever from vu2dev
Indian SDR reciever from vu2dev
ISDR opened
Inside view of 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)

ISDR block diagram

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.

ISDR Circuit diagram sheet 1
ISDR Circuit diagram sheet 1
ISDR Circuit diagram sheet 2
ISDR Circuit diagram sheet 2

If  you want to look at the circuit in finer detail,  download this  pdf file.   The info sheet provided by vu2dev is here .  The component values for the circuit can be found here.  T

I have successfully used  Rockykgksdr and winrad on my PC running Windows XP ( 🙁 , I had to install it at last  ).  I am yet to test the SDR on linux and will update as and when I am able to do so.

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.

Update  22-9-2011
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 <>.

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




  1. Yeah, Sunil, its a wonderful contribution from VU2DEV, which has taken me back to SWLing days once again. For CW, I find rocky the best software for isdr.

  2. Hello VU2SWX,

    What kind of antenna you have used for the SDR? Have you used the active antenna along with this SDR?..

    I have seen one photo of the old SXA, Do you have any hints to tune this for receiving the HAM bands. I got one SXA ( not in working condition) , I am looking for one circuit diagram of the SXA to make the Receiver part in working condition. Can you plaese share the circuits of SXA ( if you have)..


  3. I used a 40m dipole with SDR. I could get stations even with a long wire.
    As to SXA , I have the diagram somewhere . Right now I am in IIT Bombay and will be back to Trivandrum next month. Email me around first week of December. I will find out it for you.

  4. Dear All Hams,
    Here is a link to copy the audio file generated from ISDR Designed by OM Prabhu. Please do not forget to tune in to around 7095 KHz and to listen to CW of OM Venkat, 2VNK qso with OB Chandra, VU2JBE. Njoy listening the 40m band.
    73’s, DE Gururaja, Scientist, NAL, Bangalore, VU2GRU.

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