7 communications techniques

8/6/2019 7 Communications Techniques

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7.0 Communications Techniques

Not used for entertainment. TX and RX in same package.transceiver.

Image frequency input freq local osc freq = IF freq

In this example the image frequency is 22MHz


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Double Conversion

Use two stages of IF freqs.

In the above the image frequency is 40MHz. 40-30 = 10MHz.

A 40MHz signal will be greatly attenuated by the first RF amp and mixer circuits.


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Determine the image frequency for a receiver with,,input frequency 20 MHz

local oscillator 30 MHz

IF frequency 10 MHz

image frequency = 40 MHz


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Preselector

Tuned circuits prior to mixer.

Image rejection (dB) = 20 log[ (fi/fs fs/fi)*Q ]

Example 7.6

Two tuned circuitsQ of each is 60

IF freq = 455 kHz

Signal = 680 kHz

Image frequency is 680 + 2*455 = 1590 kHz

Image rejection of each tuned circuit

20 log [ ( 1590/680 680/1590 )60 ] = 20 log 114.6 = 41 dB

Image rejection of the two identical circuits = 82dB


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7.3 Special Techniques

AGC purpose is to increase dynamic range.

Special technique to improve AGC performance

Delayed AGC - no gain reduction for until signal exceeds minimum


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Minimum signal must exceed diode reverse bias voltage. Then AGC starts.

D1 is off at low signal levels. AGC level is zero (only sine wave input to C1 and average is zero).

Strong signal, positive peak > AGC set level.

Positive signal peaks will be attenuated.

Average input to C1 is negative.

Feeds back a negative DC level to the input stages (reduces their gain).


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Auxiliary AGC - step reduction in gain when input signal level is very high

For normal signal levels diode is reverse-biased.Strong signal is detected.

DC current flow into the AGC bus increases.

DC voltage a base of Q2 is decreased.

Collector voltage of Q2 rises. Diode turns on.

This shorts it out and loads down the L1C1 mixer.

Results in much lower signal coupled out of L2.


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Variable-gain amplifier chip - 600MHz

Gain is controlled by value of digital input (assumes A/D converter connected to output of the AGCcircuit).

Gain range is -5dB to 40dB in 3dB increments.

Evaluation board

http://www.analog.com/UploadedFiles/Evaluation_Boards/Tools/499510088AD8369EB_0.pdf

Datasheet

http://www.analog.com/UploadedFiles/Data_Sheets/475634305AD8369_0.pdf
http://www.analog.com/UploadedFiles/Evaluation_Boards/Tools/499510088AD8369EB_0.pdfhttp://www.analog.com/UploadedFiles/Data_Sheets/475634305AD8369_0.pdfhttp://www.analog.com/UploadedFiles/Data_Sheets/475634305AD8369_0.pdfhttp://www.analog.com/UploadedFiles/Evaluation_Boards/Tools/499510088AD8369EB_0.pdf


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Variable Selectivity

Uses variable bandwidth tuning. As the local osc freq is changed the bandwidth is changed.


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Squelch

Mute the audio output when the matching (signal-producing) transmitter is turned off. Eliminate offending

audio noise.

Five techniques listed.

1. Fixed RF level threshold.

2. Variable level controlled by HF audio noise.

3. Pilot tone control signal.

4. Didtal code control signal.

5. Microprocessor controlled algorithm (SmartSquelch).


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7.4 Receiver noise, sensitivity, and dynamic range relationships

Noise Floor - thermal noise in a resistance,

Pn = kTB B = bandwidth

for 1 Hz, 290 deg K, Pn = 4x10-21 = -174 dBm

Noise in dBm = -174dBm + 10logB

Now derive sensitivity in terms of noise and desired So/No.

Sensitivity (S) = (noise)dB + Si/Ni

now,

[(Si/Ni)/(So/No)][So/No] = Si/Ni

10log[Si/Ni] = 10log (Si/Ni)/(So/No) + 10log(So/No)

= NFdB + (So/No)dB

S = [-174 dBm + 10logB] + [NF] + [desired So/No]

for 1 MHz, NF = 20dB, desired So/No = 10 dB

S = -174dBm + 10log1000000 + 20 + 10

= -84 dBm

if So/No = 1 signal = noise then S = -94.8 dBm = noise floor


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Dynamic Range

Upper power limit

1-dB compression point

Output is 1 dB down from the ideal linear response

Intermod intermodulation distortion

(IMD)

Third-order distortion products of two

signals. 2f1-f2, 2f2-f1

Response of amplifier to third-order

signals.


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What is the approximate dynamic range of a receiver if the 3rd order intercept is 25dBm and the sensitivity

equals -95dBm?

2/3 * (25dBm - -95dBm) = 80dB

A receiver has a dynamic range of 76dB. It has 0.8 nW sensitivity. Determine the maximum allowable

input signal in watts.

76dB = 10log x/0.8nW

x = 0.0318W

A receiver has a dynamic range of 76dB. It has 0.8 nW sensitivity. Determine the maximum allowable

input signal in watts.

76dB = 10log(Sin/0.8nW) Sin = 0.0318W

A receiver has the following characteristics,

25 dB noise figure

2 MHz bandwidth

+3 dBm third-order intercept point

12 dB desired S/N

a) What is the receivers sensitivity?

Si = -174 dBm + 25 dB + 63 + 12

= -74


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b) What is the receivers dynamic range?

Dynamic range = 2/3(3 - -74) = 51.33dB


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Intermodulation Distortion (IMD) Intermod

When two frequencies are amplified, f1 & f2

Distortion

vo = k0 + K1vi + K2vi2

+ K3vi3

+ K4vi4

+

vi(t) = A1sin1t + A2sin2t

2

nd

order distortion products out of passband2f1, 2f2, f1+f2, f1-f2

3rd

order distortion products in passband

2f1-f2, 2f2-f1 are in passband usually

K3vi

3

= K3(A1sin 1t + A2sin2t)

3

Intermodulation products that will fall within the passband are,

3/4 K3 A1 A2 sin(2 1 - 2)t and 3/4 K3 A1 A2 sin(2 2 - 1)t

So both rise as power input rises, thus slope of plot of their power is greater than 1st

order term.

Input third-order intercept = 3rd

order of amp minus added gain of a preampShow how gain of preamp boosts the intermod product power plot.

approximation

dynamic range (dB) = 2/3(input intercept - noise floor), higher 3rd

order intercept is better.

MOSFET has lower values for the cube term.


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Noise Floor = Receiver Output Noise = Noise Power + Noise Figure

At 290 degrees Kelvin

Bandwidth - Hz Noise Power - dBm

1 -174

1000 -144

1,000,000 -104

Sensitivity = Noise Floor + Output Signal-to-Noise Ratio (desired) + Noise Figure (Si/Ni)/(So/No) dB

At 290 degree Kelvin and 1 MHzOutput Signal-to-Noise Ratio

(So/No) - dB

Noise Figure Sensitivity - dBm

0 10 -94

10 10 -84

-70 dBm is -70 dB below a milliwatt or -100 dBW-100 = 10log(X/1W) and therefore, X/1W = 10

-10

X = 10-10

W

For a 50 ohm system, (Vrms)2/50 = 10

-10watts

Vrms = [50*10-10

]0.5

= 7.1* 10-5

For 50 ohm system,dBm dBW Input Voltage - V Input Voltage - uV

-70 -100 7.1* 10-5

71

-90 -120 7.1* 10-6

7.1

-110 -140 7.1* 10-7

0.71


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Input third-order intercept = 3rd

order intercept of amp minus added gain of a preamp

dynamic range (dB) = 2/3(input intercept - noise floor), higher 3rd

order intercept is better.

Sensitivity = temp noise (dBm) + bandwidth noise (dBm) + Si/Ni) dB

= noise (dBm) + Si/Ni dB So/No dB amp characteristic + So/No dB desired

= noise (dBm) + (Si/Ni)/(So/No) dB amp characteristic + So/No dB desired

= noise (dBm) + noise figure (dB) + output signal/noise ratio desired (dB)


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Summary

Sensitivity = temp noise + bandwidth noise + noise figure + output signal/noise ratio desired

= noise (dBm) + [si/ni (dB) so/no (dB)]amp characteristic + so/no (dB) desired

example 7-7

20 dB NF, 1 MHz bandwidth, 5 dBm 3rd

order intercept, 0 dB S/N

S = -174 dBm + 10log1000000 + 20 dB + 0 = -94 dBm

dynamic range = 2/3( 5 dBm (-94 dBm)) = 66 dB


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Example 7.8

Preamp 24 dB gain, 5 dB NF

Sensitivity & dynamic range ??

NR = log-1(NF/10)

NR1 = log-1(5dB/10) = 3.16

NR2 = log-1(20dB/10) = 100

NR = NR1 + (NR2 - 1)/PG1 from 1-16

PG1 = log-1(24dB/10) = 251

NR = 3.16 + (100-1)/ 251 = 3.55

NF = 10log3.55 = 5.5dB

S = -174dBm + 60dB (due to bandwidth) + 5.5dB= -108.5 dBm

input third-order intercept +5dBm 24dB = -19dBm

dynamic range = 2/3[ -19dBm (-108.5dBm)] = 59.7 dB


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Example 7-9

10 dB gain preamp in place of 24

Sensitivity and dynamic range?

NR = 3.16 + (100 1)/10 = 13.1

NF = 10log1.31 = 11.2 dB

sensitivity = -174 dBm + 60 dB + 11.2 dB = -102.8 dBm

dynamic range = 2/3[( -5dBm (-102.8dB)] = 65.2 dB

Receiver Only Receiver + 10dB preamp Receiver + 24dB preamp

Preamp NF 5dB 5dB NFdB 20 11.2 5.5

Sensitivity (dBm) -94 -102.8 -108.5

Third-order intercept

point (dBm)

5 -5 -19

Dynamic range (dB) 66 65.2 59.7


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Ex 7-8 Ex 7-9 Ex 7-7

Noise at 1Hz and 290deg K -174 -174 -174 -174

Bandwidth 1,000,000 1,000,000 1,000,000 1,000,000

Noise - dBm -114 -114 -114 -114

Preamp Noise Figure - dB 5 5 0 5

Preamp Gain - dB 24 10 0 24

Preamp Power Gain 251 10 1 251

Receiver Noise Figure - dB 20 20 20 20

Noise Ratio 1 3.16 3.16 1.00 3.16

Noise Ratio 2 100 100 100 100

Noise Ratio 3.56 13.06 100.00

3.56Effective Noise Figure - dB 5.51 11.16 20.00 5.51

Noise Floor - dBm -108.5 -102.8 -94.0 -108.5

Desired So/No Ratio - dB 0 0 0 10

Sensitivity - dBm -108.5 -102.8 -94.0 -98.5

3rd Order Intercept - dBm 5 5 5 5

Input 3rd Order Intercept - dBm -19 -5 5 -19

Dynamic Range - dB 59.7 65.2 66.0 53.0


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Intermod Testing


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Two frequencies applied to Class AB linear power amplifier


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Cross Modulation

Two-tone test again, but allow one tone to be amplitude modulated.

Third-order products of the two-tone signal include modulation of one tone causing amplitude modulation

of the other tone,

A1[1 + m1(t)]sinw2t

3/2 K3 A12 A2 [1 + m1(t)]2sinw2t]

Passive elements may have nonlinear characteristics and produce distortion products.

Metal rool with rusted joints.

Toilet.


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Frequency Synthesis

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1 2 3 4

Fill in the blanks in the table below. Show your work; i.e., the formulas/algebra you used for each step.

Noise at 1Hz and 290deg K -174 -174 -174 -174

Bandwidth 1,000,000 1,000,000 1,000,000 1,000,000

Noise - dBm

Preamp Noise Figure - dB 0 0 10 10

Preamp Gain - dB 0 20 20 10

Preamp Power Gain

Receiver Noise Figure - dB 20 0 0 20

Noise Ratio 1Noise Ratio 2

Noise Ratio

Effective Noise Figure - dB

Noise Floor - dBm

Desired So/No Ratio - dB 0 0 10 20

Sensitivity - dBm

3rd Order Intercept - dBm 5 5 5 5

Input 3rd Order Intercept - dBm

Dynamic Range - dB

The formula for Noise Ratio is NR = NR1 + (NR2 1)/PG1

7 communications techniques

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