howto:hambasics:sections:practice
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howto:hambasics:practice [2020/07/26 21:57] – [Selectivity] va7fi | howto:hambasics:sections:practice [2020/10/24 16:54] – [Selectivity] va7fi | ||
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- | | [[home |Ham Basics]] | ||
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<WRAP round todo center box 60%> | <WRAP round todo center box 60%> | ||
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====== Recall ====== | ====== Recall ====== | ||
- | * [[howto/hambasics/wavemodulation# | + | * [[howto:hambasics:sections:wavemodulation# |
====== Receivers ====== | ====== Receivers ====== | ||
- | There are three main characteristics of a receiver: sensitivity, | + | There are three main characteristics of a receiver: |
===== Sensitivity ===== | ===== Sensitivity ===== | ||
A signal is always accompanied by some sort of noise, and very roughly speaking, if the signal is stronger than the noise, then it can be heard. | A signal is always accompanied by some sort of noise, and very roughly speaking, if the signal is stronger than the noise, then it can be heard. | ||
<WRAP centeralign> | <WRAP centeralign> | ||
- | $$\text{SNR} = \frac{\text{Signal}}{\text{Noise}}$$ | + | \$$\text{SNR} = \frac{\text{Signal}}{\text{Noise}}\$$ |
</ | </ | ||
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* If SNR < 1, then the noise is stronger than the signal. | * If SNR < 1, then the noise is stronger than the signal. | ||
- | Like other ratios, we often express SNR in [[howto/hambasics/mathbasics# | + | Like other ratios, we often express SNR in [[howto:hambasics:sections:mathbasics# |
* If SNR > 0 dB, then the signal is stronger than the noise. | * If SNR > 0 dB, then the signal is stronger than the noise. | ||
* If SNR = 0 dB, then the signal and the noise have the same strength. | * If SNR = 0 dB, then the signal and the noise have the same strength. | ||
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<WRAP center box 90%> | <WRAP center box 90%> | ||
- | {{ :howto: | + | {{ howto: |
</ | </ | ||
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===== Selectivity ===== | ===== Selectivity ===== | ||
- | Recall from the [[howto/hambasics/intro# | + | Recall from the [[howto:hambasics:sections:intro# |
- | * [[wp> | + | * [[wp> |
- | * [[wp> | + | * [[wp> |
- | * SSB, which needs about 2.7 kHz of space to encode human voice. | + | * SSB signals usually have a bandwidth between |
* FM, needs about 20 kHz, which is why FM radio stations sound better than AM radio stations. | * FM, needs about 20 kHz, which is why FM radio stations sound better than AM radio stations. | ||
+ | |||
+ | {{ scope01.png | ||
Now back to the receiver. | Now back to the receiver. | ||
+ | ==== Filters ==== | ||
+ | Sometimes a specific noise or interference is mixed in the signal and we'd like to reduce or eliminate it. One way is to use filters. | ||
+ | * A filter that lets high frequencies through but blocks low frequencies is called a high pass filter. | ||
+ | * A filter that lets low frequencies through but blocks high frequencies is called a low pass filter. | ||
+ | * A filter that blocks a narrow band of frequencies in the middle of the audio spectrum is called a notch filter. | ||
+ | |||
+ | {{ notch1a.png }} | ||
+ | |||
+ | This picture has three main parts. | ||
+ | - The frequency is 7.100 MHz on LSB. | ||
+ | - The scope shows this frequency relative to the entire band (7.000 Mhz to 7.300 Mhz) | ||
+ | - At the bottom is the audio spectrum of what we hear (300 Hz to 2.5 kHz) | ||
+ | |||
+ | You'll notice that there' | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | {{notch2.png? | ||
+ | </ | ||
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======= Questions ======= | ======= Questions ======= | ||
- | * B-003-009-001 -> B-003-008-006 | + | * B-003-010-001 -> B-003-011-001 |
- | [[stationassembly |{{/ | + | [[stationassembly |{{/ |
howto/hambasics/sections/practice.txt · Last modified: 2020/12/30 09:08 by va7fi