Radio Links

II

Sounding rocket telemetryPoker Flat telemetry dish

Other telemetry design choices

• Frequency – where (in “frequency space”) is information transmitted– Technological constraints: what can be built?– Natural constraints: how do different frequencies

behave in the environment?

• Bandwidth – how much information is transmitted?

Frequency choices

Propagation of radio waves

Line of sight propagation

• About 400 miles at 100,000 feet

Atmospheric transmission

• Transmission “window” in GHz range

Regulations

Bandwidth• Need more than one frequency to carry

information – need a “band” of frequencies

33,000,000 HzDBS Television

10,000,000 HzEthernet (10 Mb)

5,500,000 HzTelevision

22,000 HzCD audio

3,000 HzTelephone

500 HzMorse code

Bandwidth limitations

• Available frequencies are limited – may be forced by regulations to stay in a narrow band

• A higher bandwidth signal requires more power to transmit with the same signal-to-noise ratio

Noise

• Extrinsic – natural sources, interfering transmitters

• Intrinsic – thermal noise caused by random motion of electrons

• Noise power P = kT∆ f, ∆ f = bandwidth• For best SNR, want to make ∆ f as small as

possible

Whistler

White Noise

Modulation

• Continuous radio wave “carrier” has zero bandwidth but carries no information

• Want to change (modulate) the wave over time to convey a message

• Will increase bandwidth: More information -> More bandwidth

Electric Field,Magnetic Field

Moving at the speed of light -->

Distance

Wavelength, λ

Modulation and coding

• Low level: How can the carrier wave be modified to carry information? (modulation)

• Higher level: How should the modulating information be formatted for best communication? (coding)

Spark gap transmitter

• This is a Marconi 1 1/2 kw quenched spark gap transmitter. This piece of equipment was installed on the yacht Elettra and is featured in photographs of Marconi in the radio room on board the ship. It is similar to the transmitter that was installed in the radio room of the liner Olympic, the sister ship to the Titanic. This transmitter was capable of sending messages over a distance of 4,500 miles.

• Type Q.G. No. 356546, with eight-plate quenched spark gap, four flat copper-strip inductance coils with moveable leads, three-position adjuster switch, nickel-plated guard rail and cast nameplate Marconi's Wireless Telegraph Co. Ltd., on mahogany baseboard -76cm (30in.) high.

• Built c. 1920

On-off keying (OOK)

• Simplest/oldest form of modulation• Morse code (1837) – developed for

telegraphyModulation Coding

• AM radio, broadcast TV• Make amplitude of carrier wave proportional to

the signal of interest (modulating signal)

• Vulnerable to distortion from atmospheric attenuation

Amplitude Modulation

Signal Carrier

• FM radio• Make frequency of carrier wave proportional to

signal

• More resistant to atmospheric effects

Frequency Modulation

Signal Carrier

ESS 205 Balloon Telemetry• Two types:

– Live video, using standard television signal

– Readout of sensor values, using audio FM telemetry

Television “raster scanning”

• Electron beam illuminates one spot on the TV screen at a time, covering entire screen 30 times per second

• Broadcast as an AM signal with modulation proportional to brightness – but where?

Standard NTSC television signal

• In addition to brightness information, contains signals which allow the TV to locate the start of each line (horizontal sync) and the beginning of the first line (vertical sync)

Brightness profile (1 line)

Audio FM balloon telemetry

• Use FM radios designed for voice transmission

• Radio link can transmit frequencies 300 Hz – 3 kHz

• All information to be delivered from the payload must be coded into frequencies in this range.– Voltage to frequency conversion

(CricketSat)– Speech synthesis/DTMF (Moses Lake

launch)

• Simple silicon microchip for construction of timers & oscillators

• Generates a square wave audio signal at a frequency determined by two resistors and capacitor

CricketSat signal generation: 555 timer chip

( )1.44

2A B

fR R C

=+

“Ground”

Signal generation with the 555• Use components which are sensitive to their

environment

– Thermistors – resistance changes with temperature

– Photoresistors – resistance changes with light

– Capacitive humidity sensors – capacitance changes with humidity

• Use in a 555 circuit to generate audio frequencies

Voltage to frequency converter

• Generic device for turning a voltage into a frequency

1 V 100 Hz2 V 200 HzEtc., etc.

• Example: Analog Devices AD537

1”

Multiplexing

• How to measure several sensors over one radio link?

• Share the link by switching, or “multiplexing” between them

• Simple technique: Each sensor takes turns modulating the transmitter

• Radiosondes use this technique• Used in ESS205 (2004)

Speech Synthesis

• RC System’s V-stamp text-to-speech synthesizer “reads” English text

• Pro: Simple to use, no special receiver required

• Con: Not machine readable

1”

DTMF (Dual Tone Multi Frequency)

• “Touch Tones” encode digits 0-9, A-D, *, # as sounds containing two different audio tones

• Low frequency indicates row, high indicates column

• Machine readable - devices for encoding and decoding (tones back into numbers) are readily available

DSchmidt Technologies’ DTMF Decoder II

ESS205 audio telemetry (2005)

• Interleave speech output (for casual reception requiring no special equipment) with DTMF (for machine readability)

SpeechGPS

SpeechExpt12-15

DTMFExpt0-15

SpeechExpt8-11

DTMFExpt0-15

SpeechExpt4-7

DTMFExpt0-15

SpeechExpt0-3

DTMFGPS

SpeechGPS

. . . . .. . . . .

Digital telemetry for scientific ballooning: The Sprite project (2002-2005+)

• Capable of high bit rate (3 MBps)

• Inexpensive• Legal to use in USA &

Brazil• Versatile• Transparent• Moderate design

complexity

• Analog: Modulation is interpreted as a continuously varying parameter

• Digital: Electrical signal is interpreted to be one of “N” (usually, two) possible values

Analog vs. Digital Modulation

Modulating Signal Received +

Interpreted

Threshold

1

0=

Received +Interpreted

Modulating Signal

Noise behavior of digital systems

• Digital systems are immune to small quantities of noise

• Larger amounts of noise cause complete system failure

Threshold

1

0=

1

0=

Types of digital radio telemetry• Modulate carrier discretely to form 1’s and 0’s• Examples: OOK, FSK (frequency shift keying)

– FSK: switches between two frequencies (“0” and “1”) at a certainbit rate (baud = bits per second)

– Bell 103 (original 300 baud modem protocol):

• Others: ASK, PSK, QPSK, OQPSK, MSK, QAM…

“Originate” 0 = 1070 Hz, 1 = 1270 Hz

“Answer”0 = 2025 Hz,1 = 2225 Hz

0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001

Digital data transmission: Modulation is not enough

… now what?

Binary numbers• Decimal digits have values 0-9, binary digits (“bits”) only 0-1• Combine multiple digits to form larger values

– 8 bits (one byte) = 0…255– 16 bits (one word) = 0…65536

• Example: 79 decimal = 01001111 binary

97000000

100101102103104105106107

11110010

20 (=1)

21 (=2)

22 (=4)

23 (=8)

24 (16)

25 (32)

26 (64)

27 (128)

Decimal

Binary

Synchronization

0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001

One 8 bit “sync byte” 01001111 inserted every 120 bits (15 bytes)creates a repeating “frame” pattern

• Add structure to transmitted data to allow interpretation

Syncbyte

0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001

Telemetry FramesTemperature Pressure Etc.

• Define frame contents according to telemetry requirements

Syncbyte

• Add redundant content to frames to allow detection & correctionof bit errors

0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001

Error Correction

Error correction byte

• Some methods: Hamming, Reed-Solomon, Golay, Turbo

Sprite telemetry design approach

• Use newest technology – higher integration, higher performance, more features

• Use consumer & amateur radio technology wherever possible

• Frequency: 902-928 MHz– Amateur radio band in US & Brazil– Amplifiers/antennas readily available– Also used for non-licensed devices: cordless phones,

wireless networks, etc.

Sprite transmitter

• Cell phone transmitter evaluation board generates & modulates low-level radio signal

• Modified for 902-928 MHz operation

• Amateur radio power amp boosts power to 3-5 W

• Commercial dipole antenna

Sprite receiver

• Antenna modified from TV satellite dish

• Commercial pre-amp• Commercial wide-band

receiver• Custom intermediate-

frequency (IF) amplifier• Digital demodulator –

commercial evaluations boards• Custom digital interface• Ethernet single board computer• Laptop Laptop

Ethernet controllerµ

Digital demodulator

Receiver PreampIF amplifier