Yes, that would make it an I.F. gain control, and that is what it is most of
the time. I don't know why manufacturers still use the misnomer "RF Gain"
for this control.
Concerning the ARRL, when they talk about having only enough gain to hear
the normal background noise they are usually talking about RF amplification
ahead of the 1st mixer. It is important to pay careful attention to the gain
distribution of the receiver, since any gain ahead of the 1st mixer degrades
the dynamic range by that same amount.
As an example, suppose a receiver has an IP3 of +30dBm. Now, if a
preamplifier with 10dB of gain is placed ahead of the 1st mixer the IP3 will
be degraded to +20dBm.
Typically, excess noise contributed by the environment is in the 15 to 20dB
range. Many shortwave receivers have a noise figure in the 10 to 12dB range.
The excess environmental noise can be between 3 to 5dB above any noise
contribution that the receiver has itself. For this reason, most modern
receivers don't use an RF amplifier ahead of the 1st mixer.
Some receivers, such as the AOR-7030 have a post-mixer amplifier. Any
amplification after the 1st mixer won't degrade out of band IMD performance
unless signals fall within the range of the roofing filter that follows the
1st mixer.
Now, another point comes into consideration..............close-in IP3
performance. Three major mechanisms affect this parameter. First of all, the
performance of the 2nd mixer itself. Most receivers use a "weaker" 2nd mixer
since the roofing filter protects this device from out of band signals. If a
stronger 2nd mixer with better IMD performance is used, the close-in IMD
will improve.
The 2nd parameter is the sideband noise of the synthesizer itself. If phase
noise can be improved, close-in IMD performance will improve.
Of course, the parameters of the roofing filter itself will directly affect
the close-in IMD performance. As an example, both the Racal 6790 and the JRC
NRD-515 are using at least an 8-pole crystal filter as the roofing filter at
the 1st I.F.
The 6790 has some amplification that follows the roofing filter, and another
roofing filter is added after that amplification.
The result is exceptional skirt selectivity at the 1st I.F. This affords
quite a bit of out of band protection for the 2nd mixer.
Another example is the Collins 651S-1. This is a triple conversion receiver.
At the 1st I.F. of 110.35MHz, an 8-pole crystal filter is used. At the 2nd
I.F. of 10.35MHz?, another 8-pole crystal filter is used. At the 3rd I.F. of
either 450 or 455kHz, depending on when the receiver was manufactured,
mechanical filters are used. These things are what set the "premium grade"
receivers apart from most of the consumer equipment.
Do we really need this kind of front end protection? Probably not, unless
the receiver is going to be used on an ocean going vessel that has
multi-kilowatt transmitters. Even in this scene, narrow band RF preselectors
are used ahead of the receivers to add additional IMD protection. I hope
this helps.

Pete

"Geoffrey S. Mendelson" wrote in message
...
Pete KE9OA wrote:

When you turn up the RF gain control, you don't desense the radio, but
you
do give the AGC loop more gain. Maybe this is what you meant.

Wouldn't that make it an IF GAIN control?

A while back I read a review by the ARRL of a transciver and they brought
up
an interesting point. Modern receivers have enough gain to "hear" the
normal background noise, so beyond that it really does not matter.

In my environment the noise is so high that almost anything can hear it,
my R-5000 often hears S9 level background noise on 40m and nearby SW
bands. (5-10mHz).

Geoff.

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