SDR I/Q Balancing on a Windows 7 System
When using a soundcard based SDR (software defined radio) like the SoftRock or the many variants based on the Tayloe mixer, it’s necessary to balance the soundcard channel’s amplitude and phase to maximize the opposite sideband supression. To date, I’ve done my SDR work on a netbook running Windows XP and never had a problem with this. Therefore, I was a little surprised when I hooked the hardware up to my Windows 7 system and couldn’t get the channels to balance no matter what I tried.
The SDR I was using consists of a homebrew receiver using the PCM2904-based soundcard I’ve previously described and the HDSDR software. Typically I could get deep notches on one or more frequencies as shown in the figure below, but no suppression over the rest of the bandwidth.
For those of you having this same problem, the solution lies in the Windows 7 sound card properties. Right-click on the sound icon in the lower right hand corner of the Windows 7 desktop and select Recording Devices. When the Sound form is displayed, select Recording tab.
Select the recording device you are using, then click Properties to display the device’s Property window.
Select the Advanced tab and note the value in the Default Format drop down. Mine was set to 1 channel, 16 bit, 44100 Hz (CD Quality). Set it to 2 channel with the highest sample rate available (in my case, that was 2 channel, 16 bit, 48000 Hz (DVD Quality)). Click OK and close the rest of the windows.
I think the key here is not the sampling rate but telling Windows you want to use two channels. In any case, as soon as I made that change I was able to balance the sound card channels just like I did on my Windows XP system.