Hello Leif, You are absolutely right about in-band 2nd order spurs created from strong single tone in-band signals. 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. 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 ?
Chavdar, LZ1AQ [EMAIL PROTECTED] , www.lz1aq.signacor.com
