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mixer: DBM or dual gate mosfet?
....a long explanation, but a needed one.
One of the most important characteristics of a mixer is its ability to handle large input signals without overloading. if overloaded by an unwanted signal, chances are that it will not be able to handle the weak signal you're interested in. the physical mechanism involved is not important, except for the fact that increasing current through an active mixer (re FET or transistor, or the injection power in a DBM tend to alleviate the problem in some measure. the nois figure of both is more or less equal and adequate in the HF range. the main advantage of the dual gate MOSFET is that it needs much less power from the local oscillator and can save you an amplifying stage. Not really crushing, but sometimes neeedful. If you're talking about a simple first receive I'd go with the MOSFET mixer. there are very good examples in the hand book. iI used them for years with success and still using them in one form or another. the problem is much less severe at the second mixer, because the IF filters tend to defend it from large unwanted signals and the amplituse variations are also smaller due to AGC action. Soooo , KISS (keep it simple, stupid!!!), a very good adage. goood luck with your first. Saandy 4Z5KS |
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