Jane, QSD response to a half of the LO frequency is a second order IMD problem (IMD2) as mentioned by Leif and Grant. Due to 2nd order non- linear response you get at the QSD output either second harmonics of the input signal mixed with LO (when 1 signal is applied) or the sum or difference of two applied signals if the spur is in the band pass range of the QSD.
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. 73 Chavdar, LZ1AQ [EMAIL PROTECTED] , www.lz1aq.signacor.com
