Most modern soundcards will produce quite a good display once everything is setup correctly but soundcard mixer settings can be a problem
Soundcard settings. Bruce KF1Z
"In Windows volume control settings (Properties). XP Right click the Speaker Icon - Open Volume Control - Options - Properties - Recording Input.
First, you must use Line-input to the soundcard. Not microphone input.....
Make sure that in Windows volume controls for input, the ONLY thing that is checked is LINE-IN... [Set the slider to maximum]
If there is a "Stereo-MIX" make sure it's muted.
In OUTPUT [Playback] settings (still windows volume controls)
Make sure the only things checked is WAVE .... (and "master volume" if there is one..) [Set the sliders towards maximum]
Make SURE that LINE-IN is MUTED in playback (output) settings. "
Make sure any special effects are switched off.
Any ground loops/PSU/external noise will also create a big mess in the middle. A single sharp spike in the centre may be considered
A quick test with a 9V battery will eliminate PSU noise and ground loop problems. Other noise and ground loops can take longer to
Screenshots using "Rocky" http://www.dxatlas.com/Rocky/ But any other software will show similar displays.
Rocky will work with USB devices using Windows operating systems but has problems with internal cards. Some have had success using VAC to direct the audio to the internal card.
HDSDR is another that you may use. Link and information here http://sites.google.com/site/g4zfqradio/installing-and-using-hdsdr This will show similar spectrums. It does not have automatic image rejection.
For receive SDR#, details here http://sites.google.com/site/g4zfqradio/connecting-soundcard-sdr-to-computer is worth trying.
SAMPLE RATE:- Leave the SDR program set at 48KHz for testing. Some soundcards will operate at higher rates but check that later.
Turn off Windows sound. Make sure Line In is selected.
Here I use Rocky to illustrate, but similar displays will be seen with other SDR software.
Windows mixer settings can be a problem. Soundcard software is usually not intended for technical applications. All special effects must be turned off. Obscure settings are sometimes difficult to find. One such setting is "Stereo Mix" or "What You Hear", maybe called something else.
This may cause a "signal" in the centre of the spectrum. This setting must be found and disabled, in this case "Mute" must be ticked. More screenshots on this subject at the bottom of the page.
Something similar is seen if a microphone input is used.
SHARP SPIKE AT CENTRE
Many soundcards will produce a sharp spike right in the centre of the spectrum. This is not really a fault and can be ignored. It is at the SDR's centre frequency. The reception is +/- half the soundcard's sampling rate each side of this centre frequency. This centre frequency is actually at 0Hz (zero frequency at the soundcard) and nothing can be received here. A technical explanation
Most current SDR software has a feature to hide the effects of this.
The pictures below are using a good card, the M-Audio Delta 44, which does not have this spike. The signals around the centre are pickup of the power line frequency and it's harmonics, 50Hz in Europe 60Hz in USA.
Sometimes low quality cables can pickup interference.
In cases of received interference find out where to look. Remove the antenna, try battery power, even disconnect everything.
In some cases I have seen a few spurii that are at fixed audio frequencies, present when nothing is connected. They do not move when the tuning is altered. These must have been due to pickup of an audio frequency inside the computer. maybe moving the card to a different slot could clear the problem.
Sometimes a bad power supply can cause this. Try a battery to eliminate the possibility.
The basic technique is to remove connections until the problem is found.
Below, click on thumbnail. to see it full-size. The effect when there is no image rejection. The image to which I refer to is the audio image that appears opposite, exactly the same distance from centre as the wanted frequency.
The signal shown here is AM, shows the carrier and both sidebands.
If the image is EXACTLY ( I mean... exactly) the same amplitude as the wanted carrier, then you are not feeding a stereo signal from the (SDR) to the software.
Most likely causes include:- you are using a laptop that does not have a stereo line-input, you have connected the cable to a microphone input on the card. Or a shorted cable, or a cable with a bad connection. Bruce KF1Z
Then look for a fault in the SDR.
Below a reasonably good display, click on thunbnail to see it full-size. There are signs of residual noise right in the centre but I have seen much worse!
IQ Swap. The signals should tune correctly with the high frequencies on the right.
If the IQ signals are swapped over then the high frequencies will be on the left and the opposite (incorrect) sideband will have to be selected for SSB.
Swapping IQ will NOT affect image rejection, it will move the image to the other side of centre..
The recommended way to set IQ connections is to make them correct for PSDR which does not have ability to swap IQ.
Transmit checks. There should be just one main signal, on the same frequency and sideband of the correctly adjusted receive signal. With a receiver it will be possible to find the image on the other side of centre with sidebands reversed. (You will not notice reversed sidebands with Rocky, it has no SSB TX.) The image will need to be reduced until it disappears by using the software's image reduction facility.
Below, some further information about image rejection.
With Rocky you will not normally see the top spectrum of the picture below. The "Correct Balance" box is ticked by default.When receiving Rocky will normally automatically reduce images so they are not seen, as in the lower spectrum.
The top spectrum serves to show how bad the Softrock/soundcard combination actually is. The software is initially set to expect perfect balance of the IQ signals, it does not get this so a spectrum like the top is produced with images at a reduced level but certainly not eliminated.
The lower spectrum shows how things are seen after the software has set itself to accept the signal it gets from the Softrock/soundcard combination.
Some software does not have automatic receive image rejection. These typically require you to adjust sliders manually to remove the images. These WILL show a spectrum like the top one in the picture until the controls are adjusted.
NO SOFTWARE YET CAN ADJUST TRANSMIT IMAGE REJECTION.
When SDR is used to transmit it is essential to check and adjust the image rejection.
The software will initially present perfectly balanced IQ signals to the soundcard. But, like the receiver, balance will only be approximate.
You must use the built-in image rejection tool. Rocky's is in "Tools" "TX IQ Balance" It offers two calibration points for each tuning range, telling you where to look on your receiver.
Note this must be done for every frequency range where you intend to transmit.
If you do not adjust your transmit image you are very likely emitting an interfering signal that may well contravene your licence requirements.
Click picture for full size.
BELOW, TWO MORE SCREENSHOTS SHOWING MIXER SETTINGS
G4ZFQ March 2011
alan4alan at googlemail com
More of my pages related to SDR
All aspects of the Softrock SDR http://homepages.wightcable.net/~g4zfq/Si570.htm