Hi Chavdar, > What I want to add is that in my opinion the 2nd order IMD is much > more important in direct conversion (DC) RX than in superhet RX. Not > only around half LO frequency, but any two input strong signals with > close frequencies will give a spur f1-f2 into the low frequency > baseband of the DC RX. This same IMD2 is responsible for direct AM > detection of strong broadcasting stations which sometimes is > difficult to be filtered out with input filters. For example I have > very often AM detection from a strong broadcasting station working at > 13.600 MHz when my DC RX is on 14 MHz band - very near to be filtered > effectively. > > The 2nd order spurs should be removed theoretically when a balance > mixer is used but practically not more than 30 -40 dB additional > attenuation can be achieved. The balance depends very much from the > quality of the broadband transformers and parasitic capacitive > balance of the practical hardware. My own measurements show IP2 in > order of 55-65 dBm (@MDS: -125 dBm) for balanced QSD mixers with > 74HC4052 switch. > If I have to rate the main problems in SDR with DC RX hardware they > are: > 1. Insufficient broadband image rejection > 2. IMD2 giving AM broadcast detection. > Very rarely in normal use I have any IMD3 problems. So it will be > nice if we see more often detailed data for 2nd order response of the > SDR equipment.
There is another second order problem with these mixers. When the LO is set to e.g. 7.050 and the desired signal is at 7.060 (IF=10kHz) a signal at 7.055 or at 7.045 will produce an IF of 5 kHz and its second order harmonic would be on top of the desired signal. I ran into this problem when designing the WSE RX2500 unit. The solution was to add feedback to make the current through the switch zero. http://www.sm5bsz.com/linuxdsp/rxiq/mixer.htm It is a dramatic improvement for the "audio overtone" case. I do not see any of f1-f2 coming out on the IF (audio) port, but I have no idea whether it would help against f1+f2 (half the frequency) because the the RX2500 has a good RF filter that prevents signals far away from 2.5 MHz to reach the mixer. 73 Leif / SM5BSZ
