Differences

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--- howto:hambasics:sections:practice [2020/10/24 16:54] – [Selectivity] va7fi
+++ howto:hambasics:sections:practice [2020/10/24 16:59] – va7fi
@@ Line 49: / Line 49: @@
 ==== 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.  You can think of filters a little bit like the equalizer on a Hi-Fi stereo.
-  * A filter that lets high frequencies through but blocks low frequencies is called a high pass filter.
+  * 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 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.  Here's an example:
+  * Combining a high pass and low pass filter we get a //band pass// filter, which lets audio between two frequencies.
+  * The "inverse" of a band pass filter is called a //notch// filter.  It blocks a narrow band of frequencies in the middle of the audio spectrum.  Here's an example:
 {{ notch1a.png }}
@@ Line 58: / Line 59: @@
   - 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)
+  - At the bottom is the audio spectrum of what we hear (with a 2.4 kHz band pass filter).
 You'll notice that there's a very intense line around 1.5 kHz on the audio spectrum.  This line, coming from an interfering carrier signal, sounds like a loud note (a little above \$ \text{F}_{6}^{\text{#}} \$  for the musicians out there) mixed with the voice.  To remove this tone, we can "notch" it.  We now see a dark spot around 1.5 kHz.  If we need to, we can make this spot more or less wide.