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--- howto:hambasics:sections:practice [2020/10/24 16:12] – [Selectivity] va7fi
+++ howto:hambasics:sections:practice [2020/12/30 09:08] (current) – va7fi
@@ Line 45: / Line 45: @@
 Now back to the receiver.  The selectivity of a receiver is its ability to pass only the signal of interest and reject everything else.
+==== 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 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.  The narrower the bandwidth of the mode we use, the narrower the band pass filter we can use to clean up the audio.  For example:
+    * 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 "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 }}
-{{notch2.png?350 }} {{ notch3.png?350}}
+This picture has three main parts.  From top to bottom:
+  - 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'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.
+<WRAP centeralign>
+{{notch2.png?400}} {{notch3.png?400}}
+</WRAP>
-===== Stability =====
+===== Stability =====
+This is the ability of a receiver to stay on the right frequency and not //drift//.
 ====== Transmitters ======
@@ Line 59: / Line 78: @@
-======= Questions =======
+====== Questions ======
   * B-003-010-001 -> B-003-011-001
-[[stationassembly |{{/back.png }}]] [[..:section|{{  /next.png}}]]
+[[stationassembly |{{/back.png }}]] [[electronics|{{  /next.png}}]]