Most legacy receivers are designed for 10 meters. Within this point,
this generally says the receiver gain on lower frequencies is somewhat
excessive. Plus typical of most environments, the band noise increases
as frequency decreases. Additionally, the ear has the ability to easily
discern two signals, one being noise, with a 10 dB differential. With
hearing, if two signals are of equal level, the signal of the higher
frequency component will sound louder. Usually this is the noise
component. Thus we desire to make a difference for ease of hearing.
As to the physics of the receiver, anytime gain is added there is also
some noise component added.
I have had in-depth discussion with the likes of Rob Sherwood on the
topic. Part of this has to do with "is your antenna adequate to allow
weak signal copy". {Topic for different discussion.} Also having been
involved with another company that made ham radio products for 40+
years, I've had in-depth discussions with their engineering staff
regarding optimization of receiver performance. The general approach is
to add attenuation as needed and or reduce RF gain and the signal will
pop out of the noise. From these sources, the 10 dB to 15 dB number
seems to always come to the forefront of the discussion.
Here is a quote from my files:
<snip>
If receiver noise floor is 10 dB below band noise, the receiver is
contributing less than 0.5 dB of the total noise.
Band noise varies by band over 30 dB, 160-10 meters. It also varies by
direction and time of day, plus what the sun is doing. In an noisy urban
environment it is anybody's guess as to your band noise level.
A simple test is to see how much the noise coming out your speaker
increases when you switch between a dummy load and your antenna, when
tuned to a dead spot on the band.
Example on 10 meters at my rural QTH, IC-756 Pro III: preamp OFF, noise
goes up 3 dB. That means the receiver is contributing half the noise.
Preamp 1 ON, band noise goes up 9.5 dB.
Almost all legacy receivers are designed for 10 meters, and attenuation
is desirable on the low HF bands. Most SDR receivers have a preamp in
the circuit all the time to buffer the antenna connection to the ADC.
The Flex 6000 radios can have the preamp out of the circuit, and will
need the preamp enabled on some bands at certain times of day.
If you can barely hear your antenna connect, you are not going to hear
weak signals.
<snip>
73
Bob, K4TAX
On 8/26/2017 3:01 PM, Mike Markowski wrote:
Bob K4TAX,
Why 10 to 15 dB above - is your idea to have some margin for band
noise floor changes? It seems you'd maximize dynamic range by making
the two equal.
Tnx es 73,
Mike ab3ap
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