Quote:
Originally Posted by G0HZU_JMR
If you plot out the various mixer spurious terms for a typical (upconverting) spectrum analyser you should find that the main problematic mixer terms are:
(0*LO + n*RF)
(1*LO - 2*RF)
Other major terms include (n*LO - n*RF) although the two terms above are usually dominant.
I can remember using the 'spider's web' mixer charts about 30 years ago. I didn't find them very intuitive to use. It can all make sense on the day you draw them, but it requires some effort to process the results when the plots are re-visited a few months later.
Back in those days, the company purchased some mixer analysis SW from Synergy Microwave called SPECT and LOCUS. This was also DOS based but it was quite powerful. SPECT was very similar to the Minicircuits program I showed earlier, but LOCUS could present data across the whole of the RF and LO ranges. I still use LOCUS occasionally although it is quite clunky to use.
Back in about 2005, Agilent added 'WhatIF?' to the Genesys CAD suite. This is meant to assist in the design of frequency converters. 'Spectrasys' also proved fairly powerful system analysis within that suite.
However, for a given frequency plan, it's often more powerful to write your own analysis tools. I've created various analysis tools for dual and triple up/downconverter design work. It's also possible to predict the internal spurious signals or 'birdies' that occur in such systems.
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yes the spur web can be confusing to read, making me feel like a fly trapped in a spider web.... kind of experience like listening to my arty-farty friend reading my tarot cards' prediction.
1*LO-2*2RF is one of the troublesome 3rd order intermodulation products. They grow by the slope of 3:1.(n+m)th is the order of mixing products.
To make matter worst, consider the IF down conversion of two tones RF1 and RF2 (RF > LO), these 3rd order products are the troublemakers with m=1, n = +/-1 or n= 2:
low side:
2RF1-RF2-LO
2RF2-RF1-LO
high side:
2RF1+RF2-LO
2RF2+RF1-LO
then the 2nd order
2RF1-LO
2RF2-LO