> According to the current FLEX website, they are now claiming > a 14 MHz IMD3 at 2 kHz spacing of >100 dB using ARRL measurement > methods. This puts it 6 dB (or more) better than the K3 at 2 kHz > spacing, assuming identical measurement methods. I think it's > arguable whether even a 6 dB difference in IMD3 is meaningful > when both are well above 90 dB, but there it is. Comments on > this thought would be interesting.
I seriously doubt that Flex can achieve IMD3 performance > 100 dB in any except a carefully controlled TWO SIGNAL case. With any receiver using DSP and a broadband front end all signals in the front end filter are applied to the analog to digital converter at the same time. In order to avoid IMD the ADC must be able to handle (and properly digitize) the absolute peak voltage of all signals present. That peak is more than the scalar sum of the average power of each signal - it is the vector sum of the peak power (peak voltage) of all signals including the instantaneous peak voltage of things like static crashes/etc. While the Flex-5000 may be able to handle two strong signals in isolation (and with their architecture 2 KHz or 100 KHz does not make a difference), what happens in the real world on 80 meters or 160 meters when the band is full of S9+20 dB "local" signals plus static crashes during a contest? I suspect the presence of the multiple signals will eat up the dynamic range in a hurry. The ARRL IMD testing methodology is seriously flawed when testing receivers with wideband front ends because it assumes 100% of the voltage that causes IMD comes from two "clean" and well isolated sources. In the real world when the "window" through which the signals that generate IMD can be viewed will accommodate more than two signals, ALL signals will contribute to the IMD. Understand that the two signals in an IMD3 performance test do not need to be the same strength - it is the combined peak voltage that drives the stages being tested (preamplifier, first mixer, post mixer amplifier, IF stages and DSP ADC) into non-linearity. It does not matter if that non-linearity is due to shifting the bias in a mixer or overflowing the ADC - both effects will show up as IMD. With an increasing number of DSP based receivers, we need a test (rating) system that attempt to measure the "total (peak) receive signal" necessary to generate a IMD products which exceed the receiver noise floor. This is different than either two tone IMD3 measurements at 2/5/10/20/100 KHz or a blocking measurement and is the only way to absolutely compare receivers with different design philosophies and different IMD "windows." 73, ... Joe, W4TV receive power" and front end bandwidth. _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
