SDR Transmit and Receive Image Rejection
2020, Most modern SDR receivers do not use soundcards, do not have image problems. But those that transmit may need checking. read the manual.
What are the signals called images in soundcard SDR?
Images are unwanted signals caused by the way a mixer works.
In a receiver the Local Oscillator (LO) mixes with signals from the antenna to create intermediate frequencies (IF) that are processed by the radio. In all mixers two signals are produced, the wanted one that is normally passed through a crystal filter and the unwanted one (image) that does not pass through.
With souncard SDR these IF signals are at, or just above, audio frequencies. Images cannot be filtered.
So soundcard SDR uses two mixers. The RF is split into two signals 180° apart and fed to two mixers that are actually in the same FST3125 chip. These two signals are individually mixed with phase-shifted LO signals to produce the two IQ signals 90° apart. These signals are passed to the stereo soundcard and computer to demodulate them.
For example, if the LO is at 10MHz then it mixes with signals each side of 10MHz. A signal at 9.99MHz AND a signal at 10.01MHz will both produce an IF of 10KHz. The mixing process produces two signals I and Q which are both at 10KHz but phase shifted from each other. The hardware, soundcard and computer is configured to cancel the unwanted 10KHz image and select the required 10KHZ signal. It does this by accurately balancing the phase.
The SDR hardware, the transformer and mixer produce an initial, partial cancelling of the unwanted image. Due to component tolerances and stray capacitance, phase and amplitude variations in the soundcard itself this cancellation will never be perfect. The software is configured to enhance this cancellation and reduce the image to a very low level.
What effect do images have?
On receive they are seen and heard as extra, unwanted signals. But NOTE most current SDR software has automatic image rejection, you may not be aware of these images. Winrad and HDSDR are the only commonly used Windows SDR programs that have manual adjustment.
When transmitting they are unwanted outputs that can interfere with other users. They must always be checked with a receiver. NO SDR program will adjust the transmit IQ balance. It is always an essential manual adjustment.
Even with automatic receive adjustment various faults or incorrect settings may cause images to appear.
Where are these images, what do I see?
Images and the wanted signal are always an equal distance each side of the LO frequency. In an unzoomed spectrum display the LO frequency is in the middle. Frequencies above and below that frequency are seen each side of the centre.
Below, showing the effect of NO image rejection. On receive signals are seen twice at equal levels. SSB images will be on the opposite sideband. When transmitting there will be two equal signals, the wanted and unwanted signals will be an equal distance from the centre and reversed. If it is intended to transmit USB then the image will be LSB.
Rocky has, maybe, the best automatic receive image adjustment. In the top of the image below it is disabled. The basic balance of a Softrock is shown. Corresponding signals and their images are marked. The transmit section of a Softrock may be expected to be similar. But transmit has no automatic rejection.
Below, PSDR with NO image rejection. Note in PSDR the LO is always offset from the tuned frequency by the "IF" setting. It is always in the centre of the unzoomed display. Note signals are identical and opposite each side of centre. Partial, bad rejection is not always immediately obvious.
Reasons for receive image rejection problems.
Normally with automatic receive adjustment you will only see no, or very bad results.
No rejection is usually due to only one of the two IQ signals reaching the soundcard. The soundcard must be stereo.
A very poor image rejection, the image maybe only 10dB down on the real signal may be due to the soundcard. Most software has a sample delay setting. If you see this try setting different delays to see the effect.
This page shows HDSDR's image rejection adjustments. Note all transmit soundcard SDR software have similar manual image adjustments for transmit. This must be checked on a receiver, preferably another SDR.