ETM3086:Mobile and Satellite Communications


FACULTY OF ENGINEERING

LAB SHEET

MOBILEAND SATELLITE COMMUNICATIONS

ETM 3086

TRIMESTER II (2010/2011)

MSC1: Testing of Mobile Radio

MSC2 : Propagation Path Loss Measurement

*Note: On-the-spot evaluation may be carried out during or at the end of the experiment. Students are advised to read through this lab sheet before doing experiment. Your performance, teamwork effort, and learning attitude will count towards the marks.

YL Foo / Feb08, Feb09, Dec09, Sep101/8

ETM3086 Mobile and Satellite CommunicationsMSC1

EXPERIMENT MSC1

TESTING OF MOBILE RADIO

1.0OBJECTIVES

  • To understand the importance of the following mobile radio tests: transmit power, transmit frequency, modulation characteristics, audio frequency (AF) bandwidth tests, and harmonic analysis.
  • To acquire the technical skill in measuringthe power of a transmitted radio signal.
  • To acquire the technical skill in measuring the frequency of a transmitted radio signal.
  • To acquire the technical skill in measuring the modulation parametersof a transmitter.
  • To acquire the technical skill in measuring the AF bandwidth of a radio transmitter.
  • To acquire the technical skill in making harmonic analysis at radio frequency (RF) range.

2.0INTRODUCTION

In order to check whether a mobile radio transceiver is working correctly as per recommended specifications, it is necessary to check the performance of both transmitter and receiver.The number and type of tests carried out on a radio transceiver depend on whether development, manufacturing or servicing of the radio is being carried out. The tests set out in this experiment are those normally used to check the performance of a mobile radio that is within the set limits, which are stipulated by appropriate regulatory bodies of the country.

Transmitter testing is carried out by modulating the transmitter with an audio source usually an 1-kHz tone. The signal is then captured for analysis with something that emulates the receiver. The instrument normally used for this function is a modulation meter.

In the receiver testing, the receiver requires some form of signal source, usually a signal generator which can be suitably modulated to emulate a transmitter. Once the receiver has demodulated the RF signal to produce an audio signal, we want to carry out tests on the audio output to determine the performance of the receiver. The instrument normally used for this purpose is an audio power meter, possibly incorporating a switch-able 1 kHz notch-filter, or a distortion factor meter.

Some of the In-Band tests and experiments which can be carried out are:

  • Transmitter Testing:Transmitter Power Output, Transmitter Frequency, Modulation Characteristics, Modulation Frequency Response / AF Bandwidth, Modulation Distortion, Hum and Noise
  • Receiver Testing: SINAD Sensitivity, Quieting Sensitivity, Signal to Noise (S/N), Sensitivity, and Audio Frequency Bandwidth.

Other tests and experiments,especially Out-of-Band tests which can be carried out are:

  • Transmitter Testing:Spurious emission measurement, supply voltage variation test
  • Receiver Testing: Selectivity, inter-modulation and blocking (desensitization), spurious response, image frequency rejection, AGC response, supply voltage variation tests.

In this experiment, we will carry out the following tests:

Transmit Power Test

The maximum allowable transmit power of a mobile device is governed by local regulatory body. Excessive transmit power may cause adjacent channel interference to other devices or health problem.

Transmit Frequency Test

The frequency at which the transmitter operates (i.e. the carrier frequency) is an important parameter for the following reasons:

  • If the transmitter is not operating on the correct frequency, it may well interfere with other transmissions.
  • If the transmitter frequency is slightly wrong, the receiver may receive an apparently distorted signal. (i.e. for proper operation of the radio, the transmitter and receiver must be frequency matched).

Modulation Characteristics Test

This test is preformed to ensure that the transmitter is not over modulating. If a transmitter is allowed to over-modulate it may cause interference to other users. Over-modulation is caused by incorrect gain settings at the output stages of the transmitter, extending the amount of modulation for large amplitudes of the modulating signal to a level for which the system is not designed. When a transmitted signal is over-modulated, the frequency spectrum of the modulated signal will expand beyond the allocated occupancy and could cause interference to other receivers.

Audio Frequency Bandwidth Test

This test checks the audio frequency bandwidth of the transmitter modulator circuits.

If the audio frequency bandwidth is too narrow, the person receiving the transmission will have great difficulty in determining what has been said due to the limited audio frequency range (it may sound very restricted).

If, on the other hand, the audio frequency bandwidth is too wide, there will be few detrimental or beneficial effects to the received transmission, because the receiver will be configured to the normal, narrower audio bandwidth. However, a wide audio bandwidth transmission causes an increase in channel bandwidth, which could affect adjacent channel uses.

RF Harmonic Analysis

The harmonic content of a transmitter output can be analyzed using the harmonic analyzer function.

3.0EQUIPMENT LIST

  • 1 unit of BNC to BNC RF connector
  • 1 unit of Motorola GP300 walkie-talkie with battery fully charged
  • 1 unit of Communications Service Monitor CSM2945A
  • 1 unit of Motorola Portable Radio Test Set, RTX-4005B
  • 1 unit of 13.8V power supply for GP300 walkie-talkie

4.0PREPARATION

  • Charge the battery (by Lab Technician)
  • Read the lab sheet.Conduct background study about the experiment.
  • Bring the followings: a log book for recording observations and results, a scientific calculator, and graph papers.

5.0REFERENCES

  • Marconi Instrument, “Communications Service Monitor 2945A Operating Manual”.
  • Motorola Test Equipment: Portable Radio Test Set RTX-4005B Manual

6.0PROCEDURES

6.1 Transmitter Test Setup

In principle, the Communications Service Monitor (CSM2945A) generates an AF signal that goes into a mobile radio (Motorola GP300 walkie-talkie), which performs frequency modulation (FM) and then transmits the signal in the RF range. The RF signal is then fed into the CSM2945A unit for analysis.

  1. Connect the RF connector to the GP300 walkie-talkie. Connect (with RF test cable) the CSM2945A ‘RF I/O’ port to RF connector on the walkie-talkie. The I/O port must be configured as an Input port (by pressing the SELECT button).
  2. Connect the service cable from GP300 microphone / speaker jack to "Service Cable" receptor on the Portable Radio Test Set RTX-4005B. Make sure the jack is inserted fully.
  3. Connect (with BNC to crocodile clip cable) ‘AF Gen Out’ port of CSM2945A to ‘Audio In’ port of Portable Radio Test Set RTX-4005B.
  4. Place the meter selector of the Interface Unit RTX-4005B to MIC position.
  5. Use a fully charged battery to power the GP300 walkie-talkie, or alternately connect the 13.8Vpower supply towalkie-talkie.
  6. Press the[Tx TEST] button on the CSM2945A. It is now in ‘Auto Tune’ mode now by default. [Tx Freq] [Auto Tune]
  7. Apply 550 mV, 1 kHz tone from ‘AF Gen Out’ port of the CSM2945A.[Audio Gen] > [FREQ] 1 [kHz] > [LEVEL] 550 [mV]
  8. [Tx TEST] > [Audio Gen] > [Gen 1/Gen 2] allows you to choose between AF1 and AF2. Use only AF1. Select AF2, turn it off by pressing the [ON/OFF] button.

Transmitter Test Setup

______

CSM 2945A

______

| |

| AF GEN |

| OUT AF INPUT RF IN/OUT |

| oo o |

|______|

MIC/SPK JACK RF CONN



______

| o o o o O || 13.8V |

| AC/DC AUDIO Service Cable | GP300| Power Supply |

| Mtr IN Receptacle | walkie-talkie|______|

| |

|______|

RTX-4005B

______

6.2 Experimental Procedures

6.2.1Transmit Power Measurement

a)Switch on the walkie-talkieand select a channel (say, Channel 1).

b)At AF signal level as high as 550 mV, the walkie-talkie transmits immediately once you turn it on (as indicated by its red LED being ON). At this point, if its LED remains OFF, it shows that the battery has depleted. Change to use the 13.8V power supply instead of the battery.

c)Observe that the CSM unit quickly tunes to the transmitted signal and it records a stable value of transmission power.

d)Press the [DISPLAY HOLD] button to freeze the display.

e)Turn off the walkie-talkie. Precaution: The walkie-talkie can overheat if it keeps transmitting. Turn it on only when measurement needs to be taken. Turn it off after the measurement is taken.

f)Record the Transmit (Tx) Power of the mobile radio (i.e. walkie-talkie).

Tx Power: ______dBm

Remember: Throughout the experiment, if the Tx Power drops to a very low level (say, < 15 dBm), that means the battery runs out of power. Change battery or use the 13.8V power supply to power up the walkie-talkie.

6.2.2 Transmit Frequency Measurement

a)Record the Transmit(Tx) Frequency.

Tx Frequency: ______MHz

b)Press the [DISPLAY HOLD] button again to release the display.

c)To measure the Tx Frequency Offset from the initial recorded value, enter the reference frequency of the walkie-talkie by selecting the[Tx Freq] function. (If you do not key in any value, the Tx FREQ value at the moment you select the [Tx Freq] function is taken as the initial recorded value.)

d)Press the [DISPLAY HOLD] button to freeze the display.

e)Record the Tx Frequency Offset.Assess the significance of the recorded value.

Tx Frequency Offset: ______Hz

f)Select the [Print] function to print your result. Make sure the printer is ON LINE before you start printing.

g)Press the [DISPLAY HOLD] button again to release the display.

h)Select the [Auto Tune] function again.

6.2.3 Modulation Characteristics

a)Keep the equipment setup as before.

b)In frequency modulation (FM), the frequency deviation varies with the modulating signal level and frequency. To investigatethe modulation characteristic of the radio transmitter, in this step we fix the modulating signal frequencybut vary the modulating signal level, and observe the changes in frequency deviation.

c)Explain the type of signal that serves as the input to the walkie-talkie and modulates the carrier. Identify its range of frequency.

d)Choose a modulating signal frequency that is representative of the above range of frequency.

e)Now vary the modulating signal level and observe the changes in frequency deviation as recorded in the CSM2945A unit. Explain your observation.

To adjust the modulating signal level, you may follow these steps: [Tx TEST] > [Audio Gen] > [Gen 1/Gen 2] > [LEVEL].

Note: When the AF Gen level reduces to lower than approximately 220 mV, the walkie-talkiemay cease transmission (the LED on the walkie-talkie turns off). In this case, press the push-to-talk button to initiate the transmission (the LED will turn on).

To observe the level of frequency deviation, you may follow these steps:[Mod Meter] > [More] > [FM Dev Pk/rms], choose ‘Pk’.

You also need to choose the appropriate cutoff frequencies for the AF filter and the intermediate frequency (IF) filter used in the CSM2945A, by following these steps:

[Mod Meter] > [AF Filter] > Choose an appropriate value

[Mod Meter] > [IF Filter] > Choose a value larger than the cutoff frequency you set for the [AF Filter]

Explain your choices.

f)Prove/Show that the CSM2945A unit successfully demodulates the radio signal transmitted from the walkie-talkie.

Hint: You can view the demodulated signal on the scope by selecting [Scope/Bar] function, which toggles between three different views: a power meter (bar chart display), a scope (standard display), and a larger scope (expanded display). Print out the result to prove your case.

g)Based on your observation above, specify the modulating signal level beyond which distortion might be experienced. Show your proof.

h)Identify the maximum frequency deviation.

6.2.4 Audio Frequency Bandwidth

a)Keep the equipment setup as before.

b)Now we want to examine the range of audio frequencies that is acceptable to the walkie-talkie. This is done by keeping the modulating signal level fixed, we vary the modulating signal frequency and observe the frequency deviation as measured by the CSM2945A unit.

c)Choose an appropriate modulating signal level.

d)Choose an appropriate range of modulating signal frequency to be examined. Explain your choice.

Note: You may need to reselect the cutoff frequencies for the AF and IF filters used in the CSM2945A unit.

e)Now vary the modulating signal frequency within the selected range, and observe the changes in the recorded frequency deviation. Plot the frequency deviation versus modulating signal frequency on a graph to illustrate their relationship. Explain your observation.

f)Based on your observation, concludeon the audio frequency range that is acceptable to the walkie-talkie. Justifyyour conclusion.

Alternative: There is more than one method to conclude on the acceptable bandwidth of the modulating signal. The following is another one:

  • Choose an appropriate modulating signal frequency. The resulting frequency deviation will serve as a reference by setting the following: [Mod Meter] >[More] > [Mod dBr]. The measurement unit of the frequency deviation now becomes ‘dBr’.
  • Now decrease the modulating signal frequency and observe the changes in the frequency deviation (in the unit of ‘dBr’).
  • Stop decreasing when the frequency deviation reaches -3 dBr. Record the modulating signal frequency value. This can be treated as the lower cutoff point of the acceptable bandwidth.
  • Now increase the modulating signal frequency and observe the changes in the frequency deviation (in the unit of ‘dBr’). Explain your observation.
  • Stop increasing when the frequency deviation reaches -3 dBr. Record the modulating signal frequency value. This can be treated as the upper cutoff point of the acceptable bandwidth.
  • Calculate the modulating signal bandwidth that is acceptable to the walkie-talkie.

6.2.5 RF Harmonic Analysis

a)Choose [Tx Power] > [Harmonic Analysis]. The harmonic content of the transmitted signal is shown on the bar charts. The unit is dB relative to the carrier level (dBc).

b)Explain the meaning of harmonic frequencies and the method of calculation.

c)Identify the number of harmonics you observe. Explain why the number is not larger.

d)Print screen and show it as a proof.

Marking Scheme

Assessment Components / Marks
1 / Acquisition of the technical skills in using radio test equipment to measure radio signal power and frequency / 10
2 / Reasonable choices in modulating signal level and its range of frequencies in the investigation of modulation characteristic / 10
3 / Correctness in relating and explaining changes in frequency deviation with respect to modulating signal level and frequency / 10
4 / Reasonable settings of AF and IF filter frequency ranges, followed by justification / 10
5 / Proof of demodulated signal / 10
6 / Correct identification of the modulating signal level that would cause distortion / 10
7 / Reasonable choices in modulating signal frequency and the range of signal levels in the investigation of the bandwidth acceptable to the mobile radio / 10
8 / Reasonable estimation of the bandwidth acceptable to the mobile radio / 10
9 / Correct understanding of harmonic frequencies / 10
10 / Effectiveness in working as a team to ensure smooth running of the lab session / 10
Total / 100

YL Foo / Feb08, Feb09, Dec09, Sep101/8