Mixers
4c.1 Recall that when two frequencies are mixed together, the mixing process generates new frequencies. Recall that these new frequencies are equal to the sum of and the difference between the original frequencies. Understanding mixing is essential to understand how radios work. When two different frequencies are fed into a mixer, the output consists of several frequencies One is the sum of the two frequencies
One is the difference between the two frequencies
The output also contains the two frequencies pu into the mixer.
You will come across mixers throughout the Intermediate and Advanced
Exam syllabuses because they are an essential to the working and
understanding of receivers and transmitters.
4d Modulation and sidebands 4d.1
Recall that, when audio frequencies are mixed with a radio frequency,
the new frequencies that are generated are called sidebands. Recall
that this process is called modulation
AM Modulation by tone
This graph shows the results of mixing a 3.7MHz signal from a VFO with a 1kHz tone
3700 + 1 = 3701kHz
3700 - 1 =3699kHz
So, the outputs will be
the original carrier at 3700kHz. We should have mentioned that mixers will also output the original frequencies. The 1kHz signal will be there but is so far away from 3700kHz that it is not visible.
The sum of the two frequencies i.e. 3701kHz
The difference of the two frequencies i.e. 3699kHz
4d.2 Recall that amplitude modulated signals contain two sidebands and the carrier. AM modulation by voice
This graph shows the results of mixing a 3.7MHz signal from a VFO with the output from a microphone amplifier with a 0 to
3kHz bandwidth.The shape will depend on the range of frequencies in the
voice of the operator.
In conclusion an amplitude modulated signal has a carrier and two sidebands.
4d.3
Understand that single sideband (SSB) is a form of amplitude modulation
where one sideband and the carrier have been removed from the
transmitted signal.
Understand that SSB is more efficient because power is not used to
transmit the carrier and one sideband. Understand that a second
advantage is that the transmitted signal takes up only half the
bandwidth, e.g. 3 kHz not 6 kHz.
SSB and other modes
If you look at the AM graph
above the carrier frequency does not carry any information and the two
sidebands are carrying the same information. So we could filter out the
carrier and one of the sidebands and still have a signal that is
intelligible, but more efficient in that energy is only used to amplify
the carrier. In addition this SSB (Single Sideband) signal uses up half the
bandwidth (3kHz) and so more stations can fit into the band.
The drawing to the right shows a
double sideband spectrum with no carrier. There are many QRP
transmitters using double sideband. especially on top band
Below is an upper sideband
signal. The carrier and lower sideband have been filtered out leaving
only the upper sideband. Upper sideband is used by amateurs on
frequencies above 10MHz
At the bottom is a lower sideband signal. The carrier and upper sideband have
been filtered out leaving only the lower sideband. Lower sideband is
used by amateurs on frequencies below 10MHz
Recall that CW occupies the least bandwidth and that FM occupies the most bandwidth. 4d.4 Recall that data transmissions commonly use two or more audio tones to modulate the carrier.
Other forms of transmission
CW (Morse code) uses the least bandwidth
FM uses the greatest bandwidth.
Data modes use two or more audio tones to modulate the carrier.
4d.5 Recall that a variable capacitance diode can be used in an oscillator to produce frequency modulation (FM). FM Modulation
Here is an oscillator modified for FM by placing a varicap diode across
the tuned circuit. Voltage from the microphone amplifier is applied via
the resistor. This produces a varying voltage which causes the varicap
diode to
change the frequency slightly
The carrier moves from side to side as the carrier is modulated. The carrier does not change its amplitude, only its frequency. Narrow band FM does not take up as much bandwidth as wideband FM, and so more channels
fit in the same space. This is why modern FM transceivers have a
"narrow" setting and some repeaters will not rebroadcast an
over deviated FM signal.
