Hi Ronald
Eliminating/attenuating the Image (aka Mirror) signal in an IQ demodulation scheme depends on precise amplitude match between the I & Q signals, and a precise 90 degree (pi/2 radians) phase difference between the I & Q signals.
In a conventional superhet, the image suppression is implemented by
a) raising tne IF to as high as possible, eg 45MHz for an HF receiver, and
b) implementing hi-Q band/low pass front-end preselector filters ahead of the first mixer stage
which adds very considerable cost compared to IQ SDR h/w where the "extra" cost required to achieve sufficient rejection is just CPU cycles which these days are very, very, very cheap - try doing this type of computaion back in the 1960/70s and the cost of CPU cycles was back then much. much greater than coils, capacitors & crystals.
Because the SDR H/W front-end, in this case the Elektor SDR Reloaded, is constructed using real/physical components, even with very high specification/tolerance components, there will always be a slight mismatch in the gain of the I & Q channels, and very small deviation from the precise pi/2 radians phase difference required. Even though these errors are very small, their effect on the Image Signal is still quite large, eg uncorrected, the Image Signal will be perhaps -30dB down, whereas properly corrected the Image Signal can be made to be -60dB down.
The Amplitude & Phase controls in Config enable the operator to make those fine adjustment to achieve the greatest Image Signal attenuation, ie at least -60dB.
The Amplitude control adjusts the s/w gain of the Q channel so that the Amplitude of the I & Q channels are as closely matched as possible. In my case the gain correction is -0.0098, ie if the I Channel has a gain of 1, then the Q Channel will have a gain of 1 - 0.0098 = 0.9902.
The Phase controls adjusts how much of the I channel signal is added to the Q channel signal to adjust the phase of the Q Channel wrt the I Channel to achieve as close as possible pi/2 radians phase difference - only a very small amount is required, in my setup -0.88 degree or -0.0153589 radians, however the controls permit much larger adjustments to provide the greatest flexibility for other IQ SDR h/w.
Using the cursor(arrow keys) provides the greatest control over adjustments - the usual Ctrl+key, Shift+key applies to slow down/speed up the adjustment rates.
As you can see the adjustments are very, very small, but the difference between -1,000 (30dB) and -1,000,000 (60dB) image rejection in the quality of the received signal is very large.
The mathematics of IQ signal processing are explained well by Tektronix at
Also Steven W Smith's book "The Scientist and Engineer's Guide to Digital Signal Processing" is available for free at covers all aspects of DSP in a user and simple mathematically friendly manner - Steven Smith's book was my bible when I wrote the G8JCFSDR - I bought the actual physical book as a mark of my respect and gratitude for Dr Smith's altruism.
In answer to your question about these controls affecting the AF output, lack of good Image Rejection shows up as interference between stations spaced apart at the 2 x IF Fequency, in the case of the Elektor SDR Reloaded at 48KHz sampling, the IF is at 12 KHz, then if another station is transmitting +- 24 KHz away,? that station's signal will interfere with the station being received at the IF frequency, eg in the UK, BBC Radio Scotland transmits on 810 KHz, if there was another station at 834 KHz, ie 810 + 24 KHz, or at 786 KHz, ie 810 - 24 KHz, then that station(s) would be simultaneously received as the wanted signal at 810 Khz. With good Image Signal attenuation, eg -60dB, since the station(s) at 824 (786) KHz will be attenuated by 1,000,000 times compared to the wanted signal at 810 KHz, the interference will be negligible and unheard. Even without another station at the Image Frequencies, the natural noise will be mixed into the wanted signal reducing the SNR of the wanted signal.
Hopefully, my explanation of why The Gain and Phase correction controls are so important is clear, and makes sense.
Great Question :-)
73
Peter - GM8JCF
