Hello Chavdar, > By the way I am using "single tone" method to obtain IP2 figure. I > feed the DC RX input with a strong pure sine wave signal (X-tal > generator & 7th order LP filter to reduce the 2nd harmonic) and > measure the 2nd audio harmonic amplitude with PC spectral analyzer > carefully testing whether the square law for the amplitude is valid. > The input frequency of the test signal is 5 20 KHz above or bellow > LO frequency so that the second audio harmonic spur must be in the > baseband. I do not know where exactly the 2nd order distortion takes > place since in the chain there are mixer, audio pre-amplifier and > ADC. This method measures IP2 of the total system and does not > depends from input filters. OK. Have a look at fig. 6 here: file:///home/bsz/sm5bsz/linuxdsp/rxhfa/imd2_hfa.htm
Two generators at 7.080 and 7.085 present 0 dBm to the input of the test object. The sum frequency 14.165 is 6 dB above the each of the second harmonics at 14.160 and 14.170. (A third generator at 14.164 is used as an amplitude reference) Surely it is enough to use a single tone for IM2 measurements on "normal receivers" but in case you use a Tayloe mixer it could be quite different. You may find that two signals that have a frequency separation that is much larger than one over the time constant of the Tayloe mixer give a significantly worse result just as you may find that the Tayloe mixer is excellent for close range IM3 but much less so for wide range IM3. > One of the articles (in Bulgarian language) at my home page is > named "Measuring the parameters of SDR. Single tone method." At > least the figures and photos can be understood. > I can not say at the moment how single tone method compares to the 2- > tone method since I do not have access to high quality signal > generators. Also different origin of distortions might be assumed > and comparison should be taken with care. To compare correctly the > IP2 figures, the following level correction formula must be applied: > IP2 (2-tone) = IP2 (1-tone) 6 dB > So the before mentioned IP2 figures from my measurements should be > reduced with 6 dB in order to compare them with standard 2-tone test. > The measured IP2(corrected) of my DC RX is between 49-59 dBm. MDS is > around -126 dBm @ 500Hz @14MHz. The mixer I am using is a double > balanced Tayloe QSD mixer with s&h capacitors in the switch output. > 74HC4052 is used and it works fine up to 14 MHz. The audio pre- > amplifier is TS462 (4nV/Hz) and Audigy SE 24bit sound card. I can say > that for a high grade RX this figure is not sufficient , specially > for AM detection problem. Figures above 70 dBm are desirable. > > I tried to use single tone method to measure IP3 but the results were > unreliable. The cubic low requirement was not fulfilled. > Can you give some IP2 figures for your mixer ? The difficult problem is not linearity, but spectral purity of the LO. Therefore my design consists of cascaded converters with low noise crystal oscillators. For this reason the I/Q mixer operates at a fixed frequency so it is easy to make sure that it will never experience any signals anywhere as low as half the frequency. Therefore I can not give any IM2 numbers. The inband second order intermodulation is a little better than -100 dB for a signal that is 0.5 dB below A/D saturation. http://www.sm5bsz.com/linuxdsp/rxiq/rx2500lynx.htm 73 Leif / SM5BSZ
