HI all

What is the "RIGHT" gain of the RF front end amp stage in a radio receiver ? Or maybe it is not about gain but abou isolating the local osc from antenna ?



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This is not so much about right gain (i.e. front end noise figure) rather than front end selectivity vs. overload.

At high-HF and low-VHF a 1-2 dB noise figure with 30 dBm IP3 should be adequate.

Any good RF amplifier and a double balanced mixer should be good enough.

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Well, you might think about it this way. If your front end had a gain of -40 dB (i.e. it was an attenuator) then by improving that to -20 dB, you'd gain 20 dB of sensitivity, unless the received background noise was something fierce.

If it had a gain of +40 dB, it wouldn't be useful in the presence of strong interfering signals, because it would distort like mad. Dropping the gain to +20 dB would get you 20 dB of headroom.

So the right amount of gain is somewhere in the middle, and the optimum gain depends a lot on what frequency you're operating at and what the adjacent channel interference is like.

Most strong mixers (i.e. ones that resist distortion due to stronger adjacent signals) have noise figures of around 6 dB or so, last time I checked. Up at VHF and above, the atmosphere is reasonably quiet, so you can improve the signal-to-noise ratio by using a decent RF amp (

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Phil Hobbs

There is no "right" answer to that question. Setting the gain distribution of a receiver is a balancing act between sensitivity, dynamic range, bandwidth, noise figure, linearity, gain, cost, power consumption, and available board space. You can't change one without affecting the others.

Local oscillator radiation is a problem that will need to be dealt with, but the gain of the RF amp has very little to do with the problem. The selection of IF frequencies, which affects the choice of LO tuning range, is much more important for reducing LO radiation.

You may want to play with AppCAD: which includes a calculator for receiver gain distribution. Plug in some real numbers, extracted from a real working radio, and watch what happens when you change the gains.

The basic idea is to set the gain of each stage so that all the stages overload at exactly the same input signal level. That's not easy.

Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

That's a pretty cognizant explanation. Even up to 14MHz or so the atmospheric noise tends to dominate over thermal, just not quite so much as it does at lower frequencies.

As you get up into the VHF and UHF bands then the received noise can be less than thermal, because often much of what your antenna "sees" isn't terrestrial at terrestrial temperatures -- it's cold black space. That's where really super-low-noise amplifiers become valuable.

And that's all without taking cost or power consumption into consideration -- very often the best bang for the buck is had with a system that's not "optimal" in engineering terms, but puts more onus on the transmitter to produce power for the benefit of lots of cheap receivers (think FM broadcasting or cellular phones).

Wes Hayward, "Introduction to RF Design" or "Hayward and Cambell" "Experimental Methods in RF Design".

Tim Wescott 
Control system and signal processing consulting 
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Tim Wescott

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