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RF design guide
Knowledge required for design |
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Emission classes
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If you look at the specification sheet for radio
equipment or a radio module, if the document is in any way related to the
Wireless Telegraphy Act, you will see the symbols F1D, F2D, A3E and so on.
These indicate emission classes. The emission class indicates the type of
modulation of the radio wave of the main carrier, the qualities of the
signal that modulates the main carrier, and the type of transmission
information. The following are examples related to radio modules.
For example, F1D means 'frequency modulation',
'digital signal, single channel equipment, as well as 'equipment that does
not use a sub-carrier for modulation', and 'data transmission, telemetry
and remote control'.
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1.
Main carrier modulation types |
Symbol |
| (1) Non-modulation |
N |
| (2) Amplitude modulation |
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| 1.Double sideband |
A |
| 2.Full carrier single
sideband |
H |
| 3.Reduced carrier
single sideband |
R |
| 4.Suppressed carrier
single sideband |
J |
| 5.Independent sideband |
B |
| 6.Vestigial sideband |
C |
| (3) Angle modulation |
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| 1.Frequency modulation |
F |
| 2.Phase modulation |
G |
| (4) The main carrier is amplitude
and angle modulated either simultaneously or in a certain sequence |
D |
| (5) Pulse |
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| 1.Unmodulated pulse
sequence |
P |
| 2.Pulse sequence |
K |
| a. Amplitude modulation |
L |
| b. Amplitude modulation
or width/duration modulation |
M |
| c. Position modulation
or phase modulation |
Q |
| d. The carrier is angle
modulated during the angle-period of the pulse |
V |
| e. A combination of a.
through d., or that uses another method |
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| (6) Modulation not covered by (1)
through (5) and using a combination of 2 or more of amplitude
modulation, angle modulation or pulse modulation, simultaneously or in
a certain sequence |
W |
| (7) Other |
X |
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2.
Nature of signals that modulate the main carrier |
Symbol |
| (1) No modulating signal |
0 |
| (2) Digital signal, single channel |
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| 1.Without the use of a
modulating sub-carrier |
1 |
| 2.With use of a
modulating sub-carrier |
2 |
| (3) Analog signal, single channel |
3 |
| (4) Digital signal, with two or more
channels |
7 |
| (5) Analog signal, with two or more
channels |
8 |
| (6) Composite system that combines 1
or more channels for digital signals with 1 or more channels for
analog signals |
9 |
| (7) Other |
X |
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3.
Transmission information types |
Symbol |
| (1) Non-information |
N |
| (2) Telegraphy |
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| 1.For aural reception |
A |
| 2.For automatic
reception |
B |
| (3) Facsimile |
C |
| (4) Data transmission, telemetry, or
telecommand |
D |
| (5) Telephony (including sound
broadcasting) |
E |
| (6) Television (video) |
F |
| (7) A combination of the types (1)
through (6) |
W |
| (8) Other |
X |
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Emission units
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If you look at the specification sheet for a radio
module, or if the document is related to the Wireless Telegraphy Act, you
will see a variety of units. Here we will explain about the basic units.
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dB decibels
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In electrical related fields, units using dB appear
frequently, being used to indicate a relative ratio. dB indicates a
relationship with a reference, such as 'larger dB than something' or
'smaller dB than something'. (There is also a unit dB that is used to
measure sound pressure.)
To explain using the amplification factor of an amplifier's voltage
amplification circuit as an example, if a signal of 1 mV is input into a
circuit with a voltage gain of 10,000 times, an output voltage of 10,000
mV (10 V) will result. In this case, an electrical engineer would say that
the circuit had a voltage gain of 80 dB rather than saying that it had a
voltage gain of 10,000 times. There are the following reasons for this.
* From a visual point of view, 10,000 has 3
more zeroes that 80, and with larger values, the numbers can become
unwieldy. By using dB, the value can be expressed with a smaller number of
digits, making dB more convenient. If we assume that 1,234,567 times is
1.23 million times, expressed as dB it becomes 121.8 dB (1,230,268 times),
and there is no particular inconvenience with using it for practical
purposes.
* If, for example, we connect amplifiers with
voltage gain of 50.11 times (34 dB) and 89.12 times (39 dB), the voltage
gain is 50.11 times×89.12 times = 4,466 times, requiring complicated
multiplication. However, if we express it in decibels, we have 34 dB + 39
dB = 73 dB (4,466 times), making it possible to calculate using addition
or subtraction. If an attenuator or the like is introduced, we would have
to use division, but with dB, it can be done using subtraction.
* 2 times, 3 times, 4 times, and 10 times
each become 6 dB, 9.5 dB, 12 dB, and 20 dB, while 997 times, 999 times,
1003 times and 1005 times can all be expressed by 60 dB, so it may be said
that the decibel expression more closely matches the human perception of
amplification.
Formula
Voltage ratio = 20 log 10 (V2/V1), voltage gain = 20 log 10 (output
voltage/input voltage),
attenuation = 20 log 10 (output voltage/input voltage),
field strength = 20 log 10 (E2/E1), ordinary amplification = 20 log 10
(A/B)
Power gain = 10 log 10 (output power/input power)
* Decibel units using the absolute level for
power or voltage as a reference are dBm, dBV, dBμV and so on.
* When looking at units, it is necessary to
pay attention to what the reference is. In particular, we must be careful
where power gain is concerned.
* Units that express an absolute level like
dBm and the like should not be added to or subtracted from each other. So
20 dBm + 20 dBm = 23 dBm. * Absolute gain and relative gain can be added
to or subtracted from each other. So 20 dBm + 20 dB = 40 dBm. Here we
calculated what we would get when 20 dBm is input into a 20 dB power
amplifier, so we used 10 log for the calculation.
* A mixture of units is used, so we must pay
careful attention. The same unit dBμ is used for noise voltage, field
strength and so on, and while ordinarily dBm or dBk is used for power, it
is also sometimes expressed as dBμ, so caution is required. Recently,
noise voltage, field strength and so on are being expressed with units
such as dBμV and dBμV/m.
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Radio related units
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Here we will explain about some of the units used in
relation to radio, and units expressed in decibels. The following table
contains some values that are somewhat meaningless in terms of practical
applications, but they simply show the position of commonly used units.
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| * O
dBμ |
is used in all examples, so it is misleading.
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Used frequently.
0 dBmW
is easy to understand, but for some reason is not used. |
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dBm |
An expression in decibels for voltage ratio using 1
μV
of voltage as reference, this is 0 dB.
This gives
* 1 mW = 0 dBm, 10 mW = 10 dBm, 100 mW = 20 dBm
* 1 mW = 30 dBm, 1 μW = -30 dBm, 1 nW = -60 dBm
If we reverse this calculation we get
Digression: The 'm' in dBm indicates the prefix 'milli', so is the
correct way of saying dBm "dee bee milli"? Also, why don't we say "dee
bee milliwatt"? You would have thought that it would be easier to
understand if the way we say this unit tallied with the others... |
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dBμV |
An expression in decibels for voltage ratio using 1
μV
of voltage as reference, this is 0 dB.
This gives
* 1 μV = 0 dBμV, 500 μV = 54 dBμV, 1 mV = 60 dBμV, 10 mV = 80 dBμV, 1
V = 120 dBμV
If we reverse this calculation we get
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dBμV/m |
An expression in decibels for voltage ratio using 1
μV
/m of field strength as reference, this is 0 dB.
This gives
Example: With 500 μVm 20log10 (500 μV/m / 1 μV/m) = 54 dBμV/m
* 1 μV/m = 0 dBμV/m, 500 μV/m = 54 dBμV/m, 35 μmV = 31 dBμV/m
If we reverse this calculation we get
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dBi, dBd |
Expresses antenna gain
* When using an isotropic antenna as reference, the gain is called
absolute gain, and the unit used is dBi.
* When using an ideal half wave length (λ/2) dipole antenna as
reference, the gain is called relative gain, and the unit used is dBd.
The following relationship obtains between dBi and dBd. dBd
= 2.14 dBi |
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dB/m |
Expresses the attenuation of cables and
the like. 0.033 dB/m means that for 1 m there is attenuation of 0.033
dB, and for 100 m there is attenuation of 3.3 dB. |
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ppm |
ppm is
1/1,000,000 (1×10-6). |
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bps |
This is a unit that expresses bit rate,
indicating the number of bits that can be sent in 1 second. At 4,800
bps, 4,800 bits (600 bytes) of data can be sent in 1 second. |
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Voltage EMF and PD notation
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PD
is short for potential difference, and it indicates the voltage of load
impedance in a terminated state. 50 Ω is generally used as load impedance
for high frequencies.
EMF
is short for electromotive force, and it indicates signal source voltage
in a state with no load (open voltage).
In voltage notation EMF and PD have the relationship EMF = 2 × PD, and in
dB notation, EMF = PD + 6 dB.
Example:
When impedance is 50 Ω power of 0 dBm is 0 dBm = 113 dBμVEMF = 107 dBμVPD
= 223.8 mV.
7 μVEMF = 3.5 μVPD = 16.9 dBμVEMF = 10.88 dBμVPD = -96.1dBm
4.47 μVEMF = 2.235 μVPD = 13.0 dBμVEMF = 6.98 dBμVPD = -100.0 dBm
For the voltage notation in this Design Guide, EMF is always specified,
and if nothing is specified, PD voltage is indicated. Impedance is 50 Ω.
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Impedance
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When handling units, it is
necessary to consider those relating to impedance. For low frequencies there
is 600 Ω impedance, and for high frequencies there is 50 Ω impedance. At the
same impedance of 0 dBm, the terminal voltage differs as follows, so caution
is required.
600 ohm: Terminal voltage a V = √(PR) = √(1 mW × 600) = 0.775 v
50 ohm: Terminal voltage a V = √(PR) = √(1 mW × 50) = 0.224 v
75 ohm: Terminal voltage a V = √(PR) = √(1 mW × 75) = 0.274 v
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50
Ω voltage V to power dBm conversion |
When converting voltage to power P50 dBM when the impedance is 50 ohms, with
P [W], V [V], and R [Ω], we get the following.
 we get
For a voltage value, load power of 50 ohms is sought, and if we perform
dBm conversion (1 mW reference),
* 1 V = 13.01 dBm, 1 mV = -46.98 dBm, 10 μV = -86.98 dBm, 1 μV = -106.98
dBm
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50
Ω
power dBm to voltage V conversion |
To find the terminal voltage corresponding to power expressed in decibels
when impedance is R = 50
Ω, first, find the corresponding power
value P [W] for the converted power P50 dBm, and then find the terminal
voltage.
* 20 dBm = 2.23 V, 10 dBm = 0.707 V , 0 dBm = 224 mV, -20 dBM = 22.4 mV
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50
Ω power W to voltage V conversion |
When converting power to voltage V50 V when the impedance is 50
Ω, with P [W], V [V], and R [Ω], we get
the following.
 so the result can be found
easily.
* 1 W = 7.07 V, 100 mW = 2.23 V, 10 mW = 0.707 V, 1 mW = 22.4 mV
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Reading specifications
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| Here we explain about the main items in a
specification for a radio module. |
General characteristics
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Compatible
specifications |
These show the standard
specifications to which the equipment conforms. |
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Channel
span |
This shows the frequency between
each channel in the frequency band used and that is laid down in the
standard specification. |
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Number of
channels |
This shows the number of channels
used by the equipment in the frequency band used and that is laid down
in the standard specification. |
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Data signal
speed (bit rate) |
This shows the speed at which the
data signal itself is processed in radio transmissions and general
transmissions.
The unit is expressed as bits/second (bps). However fast the data
signal speed in the specification is, time is taken when switching
between sending and receiving and in processing the protocol, so
actual data transmission speed will be slower. |
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Modulating
speed (baud rate) |
This shows the number of modulations
within a unit of time, and the unit is a baud. Ordinarily called baud
rate, it is sometimes confused with bit rate, but strictly speaking
they are different. With multi-level modulation, although the baud
rate is the same, the bit rate is different, and when data is
transmitted in parallel, depending on the degree, the bit rate between
transmitting and receiving is different. |
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Data
transmission rate |
This shows the mean data amount
moving between transmitter and receiver within a unit of time, and the
unit is expressed as bits/second, characters/second, characters/minute
and so on. With actual transmissions, control data such as error
control, equipment identification and the like is attached to the
data, and if data errors occur, retransmission processing and so on is
carried out. For these reasons, the data transmission rate is
naturally slower in comparison with the data signal speed. |
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Emission
class |
This is shown as F1D, F2D, G1D and
so on. They are a combination of symbols that correspond to various
classifications such as the type of modulation of the main carrier,
the qualities of the signal that modulates the main carrier, and the
type of transmission information.
F at the start means frequency modulation and G means phase
modulation, while the next number 1 indicates digital signal, single
channel equipment that does not use a sub-carrier for modulation. D at
the end means 'data transmission, telemetry, and remote control'. |
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Communication method |
There are two communication methods,
two-way and one-way, and two kinds of two-way communication, half
duplex operation and duplex operation. |
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Transmission output |
This is the radio output of a
transmitter. Each country has its own limit, so caution is necessary. |
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Range |
Manufacturers present the results of
tests performed in places where line of sight is possible, but the
range differs considerable according to the environment of use, so
this should only be taken as a rough guide. It differs according to
buildings, people, vehicles and the topology of the vicinity, as well
as in rain and snow. It is also affected by humidity from the ground.
In addition, under similar conditions, if the wave length of the
frequency used is shorter, naturally the range will be shorter too. |
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Transmitting equipment characteristics
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Antenna
power |
The allowable output differs
according to the frequency channel used, however in the 434 MHz band
there are limits of less than 1 mW and less than 10 mW, and in the 868
MHz band, limits are divided into sub-parts of less than 5 mW, 10 mW,
25 mW, and 500 mW. |
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Frequency
error |
The frequency error of the
transmitter is the difference between the measured unmodulated carrier
frequency and the nominal frequency as stated by the manufacturer.
The standard values are different for fixed stations, mobile stations,
and portable stations. |
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Adjacent
channel power |
For devices with specified channel
bandwidth, the adjacent channel power is that part of the total power
output of a transmitter under defined conditions of modulation which
falls within a specified passband centered on the nominal frequency of
either of the adjacent channels.
The regulation value differs with different frequencies and different
channel separation.
With EN300220, the regulation is as follows for 25 kHz steps.
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25 kHz channel
separation |
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Normal test
conditions |
200 nW |
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Extreme test
conditions |
640 nW |
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Power of
spurious emissions |
Spurious emissions are unwanted
emissions that fall outside the radio wave of interest, and this
tolerance is laid down in the standard specification.
With EN300220 they are regulated as follows.
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47 MHz to 74 MHz
87.5 MHz to 118 MHz
174 MHz to 230 MHz
470 MHz to 862 M |
Other frequencies
below 1 000 MHz |
Frequencies above
1,000 MHz |
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Operating |
4 nW |
250 nW |
1 mW |
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Standby |
2 nW |
2 nW |
20 nW |
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Receiving equipment characteristics
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Receive
sensitivity |
This shows the received signal level
when SINAD is 12 dB. This is sometimes expressed as power and
sometimes as voltage. |
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Spurious
emissions |
Spurious radiations from the
receiver are components at any frequency, radiated by the equipment
and antenna.
With EN300220, the regulation is 2 nW (less than 1,000 MHz) and 20 nW
(more than 1,000 MHz). |
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Frequency
error |
The frequency error of the receiver
is the difference between the measured local oscillation frequency and
the nominal frequency as stated by the manufacturer. |
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Modulation and
demodulation
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We will consider modulation and demodulation
using an example in which a person's voice is sent.
If you talk to a person who is several kilometers away, the content
(information) will not reach the other party however loud you speak. In
this case we might consider transmission media such as radio waves, wires,
light and the like, but voice itself vibrates the air with sound waves so
sending that alone will not work. In this case, it is possible to change
the sound to an electric signal using a microphone and send that signal.
The electrical signal is a continuous (analog) representation of the
proportional strength of the sound. The following two methods are
available for transmitting the signal using radio waves or wire.
1 Sending the analog quantity as it is
2 Numerically converting the signal (digitizing it) and sending it as a
digital quantity.
FM and AM radio use the first method, while mobile phones and BS digital
broadcasting use the second method. At the receiving end, if the signal
received is digital, it is converted into an analog quantity or voltage,
and uses a speaker to produce sounds based on the strength of the
voltage.
Let's consider sending radio wave data
Analog or digital information signals (called base band data) cannot
simply be sent as they are through space. It is necessary to combine the
base band data with a carrier frequency that is sufficiently high to pass
through space as a radio wave. Converting the electric signal including
the original information into a signal that is appropriate to the
transmission path (in this case radio waves) in this way is called
modulation. So modulation systems comprise analog modulation and digital
modulation.
1 Analog modulation systems
Analog modulation systems include AM, FM, PM and so on. They modulate the
carrier using an analog method.
2 Digital modulation systems
How is data that is originally a numerical value transmitted? There is
apparently no other method of sending it than as a digital quantity
(signal). Digital modulation directly shifts the high frequency parameters
of the radio equipment using the base band data (digital quantity). There
are may kinds of digital modulation systems such as FSK, MSK, CPFSK, GMSK,
GFSK, ASK, PSK, DBPSK, DQPSK, QPSK, BPSK, multi-value QAM, OFDM, CCK and
so on.
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Frequency shift modulation
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Frequency shift modulation includes the
following systems, and all of these types are related.
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FSK |
Frequency Shift Keying |
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CPFSK |
Continuous Phase Frequency Shift
Keying |
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MSK |
Minimum Shift Keying |
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GMSK |
Gaussian filtered MSK |
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The FSK system is a modulation system in which the logic of the digital
code making up the base band data is shifted proportionally in the
frequency of the carrier. The carrier is switched between different
frequencies when the logic is 1 and when the logic is 0. There are systems
in which the phase of the modulated wave is continuous, and others in
which it is not. Those systems with continuous phases are called CPFSK,
which is the most often used of the FSK systems. In addition, in order to
increase bandwidth utilization efficiency, the modulation index m is set
to 0.5, and with a narrower frequency band, the system is called MSK
(Minimum Shift Keying). GMSK (Gaussian filtered MSK) has an even narrower
band than the MSK system.
In order to generate a frequency in accordance with the logical value of
the base band data, the CPFSK system uses a VCO (Voltage Controlled
Oscillator). The VCO changes the oscillating frequency according to the
level of the digital data of the base band that is applied to its circuit,
so that the phase is continuous.
Among FSK systems, those that use two oscillators and do not use
continuous phase are not used much as they result in a wider frequency
band.
The circuit systems of FSK systems (CPFSK) themselves are simple, but as
the frequency bandwidth used (occupied frequency bandwidth) is wide
compared with PSK systems and the like, in order to keep bandwidth as
small as possible without losing the characteristics of the CPFSK system,
the MSK system and GMSK system with their narrower frequency band are also
used. The occupied frequency bandwidth (the extent of the spread of the
spectrum) of FSK systems is decided by the modulation index expressed by
the frequency spectrum of the base band data and the depth of the
modulation.
If modulation index is m, frequency shift is Δf (one side), and the time
length of 1 bit of data if T,
m = 2 × Δf × T = 2 × Δf/bit rate resulting
in Δf = m × bit rate/2
* bit rate = 1/T Unit: bits/second (bps)
It can be seen that with a similar bit rate, the larger the modulation
index, the greater the occupied bandwidth will be. With FSK, the higher
the modulation index, the higher the SN (signal to noise ratio) in the
demodulation at the receiver, but the occupied bandwidth increases and
bandwidth utilization efficiency falls. The MSK system is the same as the
CPFSK system but with a modulation index of 0.5.
Furthermore, with the GMSK system a Gaussian filter is applied to the base
band data input in the VCO, making the occupied frequency bandwidth
narrower.
Binary FSK
In the specification for radio equipment and radio modules, you may
sometimes see the term binary FSK. This refers to normal FSK (MSK, CPFSK
and so on) in which two frequencies are assigned to the data logic, and 1
bit is sent with 1 modulation. Quadruple conversion FSK sends 2 bits with
1 modulation, so that the bit rate is doubled.
Direct FSK
This expression is used when making a distinction from analog modulation
systems to stress that it is digital modulation. |
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FSK
*bit0
~ bit3
expresses inputted digital data
< Click to move to the calculation
window >
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CPFSK
*bit0
~
bit3
expresses inputted digital data
< Click to move to the calculation
window >
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MSK
*bit0
~
bit3
expresses inputted digital data
< Click to move to the calculation
window >
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Phase shift keying
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Phase shift modulation includes the following
systems.
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PSK |
Phase Shift Keying |
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BPSK |
Binary Phase Shift Keying |
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QPSK |
Quadurature Phase Shift Keying |
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DBPSK |
Differential Binary Phase Shift
Keying |
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DQPSK |
Differential Quadurature Phase Sift
Keying |
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The PSK modulation system is a modulation system that changes the phase
of the carrier by proportionally shifting the code of the base band data.
Power and frequency efficiency is very good compared with ASK and FSK, and
PSK is characterized by having a low rate of data errors. In addition,
multi-level modulation is easily performed, while compared with FSK, the
occupied frequency bandwidth is beneficially narrow so that it is actually
used in many applications. However, as the processing circuits can be
complex and phase characteristics are not linear, then data errors will
occur requiring an engineering solution. This applies to all members of
the PSK system family.
The PSK modulation system includes differential phase modulation (DBPSK,
DQPSK) and absolute phase modulation (BPSK, QPSK). Differential phase
encoding is normally used for its reliable demodulation. Of the
differential phase encoding systems, with DBPSK if the data is 1, it
causes the outgoing carrier to undergo an 180-degree phase shift, and if
it is 0, it does nothing.
The DQPSK system shifts the phase of the carrier in 90 degree increments;
phases are allocated to blocks of 2 bits in the received data stream, and
phase shifting is applied to the phases of the outgoing carrier. The DQPSK
system occupies the same bandwidth as the DBPSK system, but it can send
twice the information (it has double the bit rate) so it is used
frequently in applications.
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Amplitude shift keying
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ASK
(amplitude shift keying) |
A modulation system in which the
amplitude of the carrier is shifted proportionally to the base band
data.
ASK is susceptible to noise and interference, so it is not used much
for data transmissions over long distances. However it is a simple and
compact system and is cheap to implement, so it is used by micro-power
radio operators and the like for short range communications. The
oscillation circuit does not stop when the data bit is either 1 or 0,
which differentiates ASK from OOK. |
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OOK (On -
Off keying) |
Similarly to ASK, OOK turns a
carrier of constant frequency and constant amplitude on and off, but
when it is off, the oscillation circuit stops completely. For this
reason modules using OOK can achieve low power consumption. |
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Waves
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Addition and multiplication of waves
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Although
'mixer' is a commonly used word, the output results of mixers used for
audio circuits and those used for radio circuits are very different. When
mixing two signals with an audio circuit mixer, the levels of the input
signals are combined (added), whereas with a radio circuit mixer, the
frequencies are added and subtracted (multiplied). With an audio circuit
mixer, the frequencies of the input signals are not changed but with a
radio circuit mixer, the frequencies are converted.
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Addition of waves |
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The red line is the waveform when two sine waves are added. In this
addition, the levels are added together. The output of audio mixers
corresponds to this result.
< Click to move to the calculation
window >
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Multiplication of waves |
The red line in the middle is the result of multiplication of sine waves
f1 and f2 by a mixer. As a result of this multiplication, f1 + f2 and f1 -
f2 signals occur, so if it is necessary for the circuit, extract the
signal with a bandpass filter. The radio equipment extracts the sum
component with up-conversion and the difference component with
down-conversion. From the point of view of hardware, the multiplication of
a mixer uses the higher harmonics that are generated by the non-linear
part that powers a semiconductor.
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< Click to move to the calculation
window >
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Addition of waves that have phase differences |
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The red line is the calculation result. If the radio waves have been
reflected off a body, a phase difference occurs between the direct waves
so that the level of the composite signal fluctuates between strong and
weak. The fading, in accordance with the multipaths taken through space
and the reflection phenomenon that arises inside the antenna, corresponds
to this phenomenon.
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< Click to move to the calculation
window >
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Multiplication of waves that have phase differences |
The red line is the mid component of the result of multiplication by a
mixer of similar frequencies that have phase differences. A frequency
multiplier is used.
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< Click to move to the calculation
window >
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Frequency conversion of CPFSK
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The next applet shows up-conversion of a CPFSK waveform. f1 is a carrier,
and f2 is a CPFSK signal generated by inputting a base band signal in a
VCO. The red line in the middle is the result of multiplication of f1 and
f2, but this waveform also includes f1 + f2 and f1 - f2 and other
harmonics. (For example, the frequency of the modulated waves of Circuit
Design's MU-1 have a carrier frequency of 1/20 (400 MHz band) ~ 1/60
(1,200 MHz band).) As a result of this multiplication, f1 + f2 and f1 - f2
signals occur, so the f1 + f2 signal is extracted with a bandpass filter
by the transmitter. Similarly at the receiver, down-conversion is
performed by a mixer to extract the difference signal.
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Resolving the waves
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You may have
heard that any waveform is made up of several sine waves with different
frequencies and levels. With this applet, 5 sine waves with different
phases and levels can be added as a means of practical demonstration.
Click
the Square wave, Triangular wave, and Sawtooth wave buttons to set the
phase and level of the higher harmonics, and the respective waveforms are
shown. When the start phase is combined with an odd-order higher harmonic,
the result is a square wave like the red line.
You
can see the relationship between fundamental harmonics and higher
harmonics.
If the signal is passed through a filter circuit with poor performance, a
phase lag in the harmonics that form the signal may occur and the output
signal will be changed. The tops of the square wave and sawtooth wave are
undulating, but when combined with further components such as 11th order
or 13th order waves, it comes closer to an actual waveform.
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Is it OK to use any
frequency?
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Each country and region decides which
frequencies can be used and with which applications.
The 2.4 GHz frequency band is used in Europe, America and Japan as the ISM
band, but 434 MHz and 868 MHz are only used in Europe. Furthermore, 426
MHz, 429 MHz, and 1,200 MHz are assigned in Japan to telemetry,
telecontrol and data transmission, but they cannot be used in other
countries.
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What is the range?
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Users will be concerned to know the operating
range of the equipment, and this is an important issue for equipment
manufacturers as well.
You may want to suggest a long operating range in the specification, but
it is no good if the user comes to view this specification with suspicion
when actually using the equipment. Therefore it is necessary to include a
disclaimer such as "Communication range varies according to the
environment of use."
The range shown in the specification of the radio module itself is only a
guide, and when choosing a radio module it is necessary to test it using
an actual evaluation board.
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Receive
sensitivity |
The difference in communication range within
the limits of the stipulated output indicates the good and bad points of
the performance of the module. Communication range varies according to a
number of conditions so sometimes it may not be included in the
specification, however receive sensitivity is always included so we will
compare the two.
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Difference due to
frequency |
Communication range differs according to the
frequency used. The higher the frequency of the radio wave, the higher its
straightness and the less far it travels. 400 MHz radio waves are subject
to the diffraction phenomenon and travel further than straighter 2.4 GHz
radio waves.
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Difference due to
modulation system |
ASK is more affected by background noise than
FSK or PSK, and even if the radio wave reaches the receiver, data errors
may occur.
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Increasing the
operating range |
Using a gain antenna
With receive only equipment, you can choose any kind of antenna. You can
expect to increase the range if you use a gain antenna with good
performance. Please conduct field testing with a number of antennas before
selecting one.
The integral and dedicated antennas used in CE approved radio modules are
calibrated for transmission output in conformity with the specification,
and should not be replaced for trivial reasons.
Careful design
When using a radio module, it is important that your design does not in
any way degrade the receive sensitivity that is shown in the
specification. It is also important to make every effort to avoid errors.
Please pay attention to the following points.
* Ensure that the radio module is not subject to noise from the CPU or
other devices in the system in which it is incorporated.
* Reduce noise coming from the system's control switch and the like.
* Make the position of the receiver a place where there is a minimum of
noise.
* Take care in attaching the antenna.
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Incorporating radio
modules in other equipment
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Measures against internal noise
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The equipment in which radio modules are
incorporated will often be equipped with high speed CPUs and logic
circuits. These components emit high frequency noise in the form of
harmonics with the rising edge and falling edge of the control signal, and
these have adverse effects on the frequency band used. It is necessary to
take measures to ensure that this noise is not allowed to impinge on the
antenna of the receiver or transmitter. Next we will consider a list of
points requiring caution when incorporating radio modules in other
equipment. Please note that the following does not necessarily apply to
all modules.
* The case in which the radio module is
incorporated should be of a material that allows radio waves to pass
through it, such as ABS plastic. The module cannot emit or capture radio
waves inside a metal case. If a metal case is used, only the main unit of
the radio module should be built in, and the antenna should be outside.
The body of the module should make electrical contact with the metal case
and there should be no electric potential difference. The same applies to
cases with electric conductive coating
* The radio module and antenna should be kept
separate from noise sources as far as possible.
* If the antenna of the transmitter or
receiver does not match the plane of polarization of the radio wave,
efficient communication cannot be achieved, communication range will be
poor, and errors will occur.
* If the antenna of the transmitter is
vertical, make the antenna of the receiver vertical too. Take the
conditions of use into consideration when attaching the antenna.
* Do not allow the whip antenna of the module
to be attached so that it is bent. Try to keep it as vertical as possible.
* If the module is built into equipment that
includes a source of noise emissions, be sure to connect the antenna using
coaxial cable. Antenna circuits that do not have impedance matching cause
problems with reflection of the radio waves, and besides reducing
efficiency, they cause adverse effects to the equipment.
* The CPU that controls the radio module and
the signal lines from other logic are sources of noise and should
therefore be kept as short as possible.
* Use a separate regulator for the power
source of the radio module and keep it separate from the power sources of
other noise emitting digital circuits. If this is not possible, take the
power directly after that for the main unit power source, and apply RF
decoupling. RF decoupling uses CR filters, LC filters, EMI filters and so
on to cut the noise frequencies as necessary.
* If these power sources are included within
the same case, ground them away from other circuits and separate them with
shield plates and so on.
* When installing a radio module on the
substrate of control circuit, try to make the ground of the control
circuit as extensive as possible. Use the power line directly after that
for the power circuit, and apply RF decoupling. In addition, in order to
locate it as far as possible from noise sources, mount the module on the
reverse side of the substrate from that with digital circuits mounted on
it.
* Do not change the antenna if the module
already has one. If you are designing a new antenna, use one with a length
of λ/4.
* Check whether the clock frequency used by
the CPU is an integer of 1 or thereabouts of the radio frequency used. The
harmonics from this clock frequency will have an adverse effect on the
radio circuit.
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ISM band equipment
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It is often said that there is a frequency
band that is common worldwide as an ISM band, but in fact there are subtle
differences to the standard in each country and careful investigation of
this is required.
Take care not to sell modules or equipment that is in breach of the law.
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