howto:hambasics:sections:practice
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howto:hambasics:practice [2020/07/17 21:04] – [Sensitivity] va7fi | howto:hambasics:sections:practice [2020/12/30 09:08] (current) – 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/ | + | * [[howto:hambasics:sections:wavemodulation# |
- | * [[howto/ | + | |
====== Receivers ====== | ====== Receivers ====== | ||
- | There are three main characteristics of a receiver. | + | 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}}\$$ |
</ | </ | ||
Since SNR is a ratio: | Since SNR is a ratio: | ||
- | * If SNR > 1, then the signal is stronger | + | * If SNR > 1, then the signal is stronger |
- | * If SNR = 1, then the signal and the noise have the same strength | + | * If SNR = 1, then the signal and the noise have the same strength. |
- | * If SNR < 1, then the noise is stronger. | + | * If SNR < 1, then the noise is stronger |
- | 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 | + | * If SNR > 0 dB, then the signal is stronger |
- | * 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. |
- | * If SNR < 0 dB, then the noise is stronger. | + | * If SNR < 0 dB, then the noise is stronger |
+ | Now back to the receiver. | ||
+ | For example, here's the specs sheet from the IC-7300: | ||
+ | <WRAP center box 90%> | ||
+ | {{ howto: | ||
+ | </ | ||
+ | |||
+ | For example, a receiver with a sensitivity of -123 dBm can pick out a signal of 0.0000000000005 mW.((-123 dBm = 10< | ||
+ | |||
+ | ===== Selectivity ===== | ||
+ | Recall from the [[howto: | ||
+ | * [[wp> | ||
+ | * [[wp> | ||
+ | * SSB signals usually have a bandwidth between 2 kHz and 3 kHz. A 2.4 kHz filter would work well to isolate the signal and reject neighbouring signals. | ||
+ | * FM, needs about 20 kHz, which is why FM radio stations sound better than AM radio stations. | ||
+ | |||
+ | {{ scope01.png | ||
+ | |||
+ | 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. | ||
+ | * Combining a high pass and low pass filter we get a //band pass// filter, which lets audio between two frequencies. | ||
+ | * An SSB voice signal can use a 2.4 kHz filter where as | ||
+ | * A CW signal can use a much narrower 250 Hz filter. | ||
+ | * The " | ||
+ | |||
+ | {{ 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 (with a 2.4 kHz band pass filter). | ||
+ | |||
+ | You'll notice that there' | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | {{notch2.png? | ||
+ | </ | ||
+ | ===== Stability ===== | ||
+ | This is the ability of a receiver to stay on the right frequency and not //drift//. | ||
====== Transmitters ====== | ====== Transmitters ====== | ||
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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