The noise floor
What does a receiver noise floor of −174 dBm represent?
The theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature.
−174 dBm per hertz is a physical constant of the universe at 290 K, not a specification. Every receiver’s noise floor is that number plus the bandwidth in decibels plus its own noise figure.
How much does increasing a receiver’s bandwidth from 50 Hz to 1,000 Hz increase the receiver’s noise floor?
13 dB
10 log₁₀(1000/50) = 10 log₁₀(20) = 13 dB. Twenty times the bandwidth, twenty times the noise, 13 decibels.
That single calculation is the quantitative justification for every narrow filter in the material: matching filter bandwidth to the mode is worth real decibels of sensitivity, for free.
What is the noise figure of a receiver?
The ratio in dB of the noise generated by the receiver to the theoretical minimum noise
How much worse than perfect. 0 dB would be a noiseless receiver; 0.5 dB is a good UHF preamplifier, from the components lesson; 10 dB is unremarkable on HF, where atmospheric noise dominates anyway.
What does the MDS of a receiver represent?
The minimum discernible signal.
The weakest signal the receiver can produce a usable output from — noise floor plus a small margin. It combines everything above into one number.
What power level does a receiver minimum discernible signal of −100 dBm represent?
0.1 picowatts.
0 dBm is 1 mW, so −100 dBm is 10⁻¹⁰ of that: 10⁻¹³ W = 0.1 pW. Worth doing once, to feel how small a receivable signal is.
Phase noise and reciprocal mixing
This is the Extra-level insight into why two receivers with identical published sensitivity can behave completely differently on a crowded band.
What is reciprocal mixing?
Local oscillator phase noise mixing with adjacent strong signals to create interference to desired signals.
No oscillator is a perfect single frequency; each has phase noise skirts either side. Those skirts mix with a strong nearby signal exactly as the wanted local oscillator mixes with the wanted signal — so the strong neighbour appears as a raised noise floor right on top of what you are trying to hear.
What is an effect of excessive phase noise in an SDR receiver’s master clock oscillator?
It can combine with strong signals on nearby frequencies to generate interference.
Same mechanism in a direct-sampling receiver, where the clock takes the local oscillator’s role.
The consequence: a receiver with a clean oscillator hears weak signals next to strong ones, and one with a noisy oscillator does not. No amount of filtering after the mixer helps, because the damage was done in the mixing.
Roofing filters
How does a narrow-band roofing filter affect receiver performance?
It improves blocking dynamic range by attenuating strong signals near the receive frequency.
A roofing filter sits early in the IF chain, before the stages that would be overloaded. Narrowing it early means the later stages — and the AGC — never see the strong neighbour at full amplitude.
This is why “roofing filter” appears on high-end receiver spec sheets: it is one of the few things that genuinely improves behaviour in a crowded contest environment.
Front-end filtering and attenuation
Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?
A front-end filter or preselector.
Reject them before the mixer, where they would generate intermodulation.
An SDR receiver is overloaded when input signals exceed what level?
The reference voltage of the analog-to-digital converter.
A hard limit with a hard failure. Below it, an SDR is clean; above it, the converter clips and produces spurious signals across the whole band. There is no gentle degradation, which is why SDR front ends have switched attenuators.
Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?
Atmospheric noise is generally greater than internally generated noise even after attenuation
The key insight. On 160 and 80 metres, what you hear is atmospheric noise, not receiver noise — so throwing away 10 dB costs you nothing detectable while removing 10 dB of overload. Attenuation is free on the low bands and expensive on the high ones, and that asymmetry is worth internalising.
Choosing an IF
Which of the following choices is a good reason for selecting a high IF for a superheterodyne HF or VHF communications receiver?
Easier for front-end circuitry to eliminate image responses
The image is two IFs away from the wanted signal, from the General mixing lesson. A high IF puts the image far away, where a simple front-end filter removes it. That is why up-conversion receivers use IFs above the whole HF band — and why they then need very good crystal filters at that high IF, which is the design trade.
Controls
What is the purpose of the receiver IF Shift control?
To reduce interference from stations transmitting on adjacent frequencies
IF shift moves the passband relative to the carrier, so an interfering signal at one edge can be moved outside it without changing the pitch of the wanted signal. It is the more surgical relative of the reverse-sideband trick from the General track.
What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency?
Capture effect.
The Technician answer, at Extra level.
Check yourself
- You narrow your filter from 2400 Hz to 500 Hz. How much does the noise floor improve?
- Two receivers have identical MDS. One hears a weak signal next to a strong one and the other does not. What differs?
- Why is switching in 12 dB of attenuation on 160 metres nearly free?
Answers
- 10 log₁₀(2400/500) = 10 log₁₀(4.8) = about 6.8 dB.
- Phase noise in the local oscillator, causing reciprocal mixing. The strong neighbour’s energy mixes with the oscillator’s noise skirts and raises the noise floor on the wanted frequency.
- Atmospheric noise exceeds the receiver’s own noise there, so attenuation removes overload without making anything less audible.