KB6NU's Ham Radio Blog

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2016 Extra Class study guide: E8C - Digital signals

Posted: 22 Feb 2016 05:00 PM PST
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E8C Digital signals: digital communications modes; information rate vs.
bandwidth; error correction

Digital modes have become very popular in amateur radio lately, but Morse
Code, the type of modulation that has been around the longest, is the
original digital mode. One advantage of using Morse Code is that it has a
very narrow bandwidth. The bandwidth necessary for a 13-WPM international
Morse code transmission is approximately 52 Hz. (E8C05)

The bandwidth needed for digital transmissions increases as the data rate
increases. The bandwidth necessary for a 170-hertz shift, 300-baud ASCII
transmission is 0.5 kHz. (E8C06) The bandwidth necessary for a 4800-Hz
frequency shift, 9600-baud ASCII FM transmission is 15.36 kHz. (E8C07)

PSK has become a very popular digital mode. One reason for this is that it
occupies a very narrow bandwidth only 31 Hz. One technique used to
minimize the bandwidth requirements of a PSK31 signal is the use of
sinusoidal data pulses. (E8C04) When performing phase shift keying, it is
also advantageous to shift phase precisely at the zero crossing of the RF
carrier because this results in the least possible transmitted bandwidth
for the particular mode. (E8C03)

When digital communication systems were first developed, data was sent one
bit at a time. As the need for faster data transmission grew, engineers
figured out how to send multiple bit simultaneously. Instead of sending
single bits, these systems send and receive “symbols,� which stand for
multiple bits. The definition of symbol rate in a digital transmission is
the rate at which the waveform of a transmitted signal changes to convey
information. (E8C02) The relationship between symbol rate and baud is they
are the same. (E8C11)

Whenever digital data is sent over a radio channel, it is encoded. Gray
codes are often used for this purpose. Gray code is the name of a digital
code where each preceding or following character changes by only one bit.
(E8C09) An advantage of Gray code in digital communications where symbols
are transmitted as multiple bits is that it facilitates error detection.
(E8C10)

There are many things that can cause errors in a data stream. For example,
an interfering signal might cause a receiver to interpret a transmitted
symbol incorrectly. When these errors are not allowable, digital
communications systems implement some form of error detection and
correction.

One way to achieve reliable data communication is to use the Automatic
Repeat ReQuest, or ARQ, protocol. In systems that use ARQ error control, if
errors are detected, a retransmission is requested. (E8C08) Senders will
also re-transmit a data packet if they do not receive an acknowledgement
from the receiver that it has correctly received a packet.

Another way to correct errors is a technique called forward error
correction. Forward Error Correction is implemented by transmitting extra
data that may be used to detect and correct transmission errors. (E8C01)
When a receiver receives erroneous data, it can correct the errors itself.



The post 2016 Extra Class study guide: E8C Digital signals appeared first
on KB6NUs Ham Radio Blog.


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VK3YE shows how to make the Pixie "less appalling"

Posted: 22 Feb 2016 12:00 PM PST
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The Chinese Pixie QRP transceiver kit isÂ*incredibly cheap. Of course, Â*you
get what you pay for. The kit comes with a crystal for 7.023 MHz, which is
in the Extra Class portion of the band, and the receiver (if you can call
it that) is incredibly wide.

In this video, VK3YE, shows how to make it less appalling. He shows how to
add crystal and ceramic resonator VXO circuits to allow you to tune the
transceiver and to allow you to receiveÂ*SSB signals as well.



The post VK3YE shows how to make the Pixie less appalling appeared first on
KB6NUs Ham Radio Blog.