The RF Gain – The Other Part of the “Magic”…

On August 2 2015 I wrote about the “IF Shift ‘Magic’” in the ICOM 703 (see: Here is the other part of the 703 “Magic”, one that makes an older receiver, such as the one in the ICOM 703, still able to compete with more modern units and technologies: reducing the RF Gain.

It is really very counter-intuitive that by reducing a gain one could improve on a received signal. Yet this is exactly how it works. The ICOM 703, as do most HF radios, has two gain adjustments: Radio Frequency gain and Audio Frequency gain. Both gains increase or decrease the volume presented to the speaker, but those controls happen at two different moments in the processing of the signal.

The ICOM 703 manual summarizes the optimizing of a received signal in the following manner: “The noise reduction level [of the DSP filter] should only be set as high as is necessary. Use this setting, along with RF gain, NB (noise blanker, if needed), and IF filters as well, to minimize the effects of noise on the target signal.”

With the Noise-Reduction (NR) of the DSP filter set at a level between 3 and 5, the Optional FL-52A filter on, and the IF-shift set to cancel most of the unfiltered noise above the CW signal is how I obtain the best results. Then (with the “Set Mode 20 RF/SQL” set to “Auto”) I use the two separate knobs in the 703, one for the RF gain and the other for the RF gain (in the Elecraft KX3 both these functions are in the same knob, which makes this adjustment a bit trickier), to reduce the RF gain while increasing the AF gain to keep the overall volume at optimal levels and, at the same time, obtain maximum discrimination between the desired signal and the background noise.

Here, to visualize the cumulative effect of these four controls, are pictures of the same CW signal as seen in the FlDigi of waterfall:

Optimized signal

The effect on the audio as perceived by the human ear is, I believe, even more dramatic.

Before you get rid of your old transceiver, give this try… if you have not tried it before, you may be in for a pleasant surprise, one which might even change your mind about disposing of it…


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