A.R.F. PRODUCTS, INC. 

ENGINEERS AND MANUFACTURERS OF PRECISION ELECTRONIC EQUIPMENT 

23 OCTOBER 1978 

A COMPARISON BETWEEN 

SSB AND FM MODULATION PROCESSES 

IN A NEAR FIELD MEDIUM FREQUENCY 

WIliELESS COMMUNICATION SYSTEM 

BY 

LARRY G. STOLARCZYK, SC. D. 

DIECTOR OF RESEARCH 

A.R.F. PRODUCTS, INC. 

RATON, NEW MEXICO 

HOME OFFICE AND MANUFACTURING PLANT - RATON, NEW MEXICO 

RESEARCH AND DEVELOPMENT LABORATORY - BOULDER, COLORADO 
23 October 1978 

A COMPARISON BETWEEN 

SSB AND FM MODULATION PROCESSES 

IN A NEAR FIELD MEDIUM FREQUENCY 

WIRELESS COMMUNICATION SYSTEM 

Larry G. Stolarczyk, Sc. D. 

Director of Research 

A. R. F. Products, Inc. 

Raton, New Mexico 

Prepared for: 

United States Department of the Interior 

Bureau of Mines 

4800 Forbes Avenue 

Pittsburgh, Pennsyl vania 1521 3 
1 .0 FRAMEWORK FOR COMPARISON 

This document describes the decision factors that bear on the selection 

of the modulation process for the wireless underground medium frequency 

(MF) comunication system. 

The selected modulation process must satisfy the Bureau of Mines stated 

objective of improving general comnunications within an underground mine. 

The improvement being sought incl udes the devel opment of a wi re1 ess comnunication technology of measurabl e benefit to both mine safety and mine 

productivity. 

The improvement will allow wireless communications paths to exist between 

roving miners. The wireless comnunication paths will also be made compatible with the existing underground telephone system to extend paging and voice 

comnunication throughout the underground mine. 

The new wireless equipment will include man-pack, vehicular, repeater, and 

base station transceivers. Repeaters and base stations will feature coup1 ing networks for interfacing with the existing telephone circuits. 

Frequency modul ation (FM) and sing1 e sideband (SSB) modul ation processes are 

analyzed in view of the special conditions associated with underground mining. Of primary importance is the tremendous el ectrical noise generated 

in an operating mine that could adversely affect comnunications quality. 

High noise coupled with high attenuation rate of near field signals forces 

the communication system to be inherently short range. Fortunately, near 

field signals can propagate over great distances on underground wires and 

cab1 es. 

The framework for comparison is generated by presenting all factors that 
bear on the problem. Then alternative modulation processes are considered 

in terms of the advantages and disadvantages o-,' the factors as they bear on 

the problem. Final ly, the best a1 ternative modul ation process is recommended 

for the wireless mine communication system. 
LIST OF DECISION FACTORS 

Factor 1: The signal level must exceed the noise level by reasonable 

margin. 

For acceptable communications quality, the signal power must always 

exceed the noise power within the transmission bandwidth by a considerable margin within the useful operating range of the system. 

Therefore, a useful communications limit is reached when the noise power 

presented to the input of the receiver becomes comparable with the signal power. 

Factor 2: System must be capable of wireless comnunications along 

haulage ways. 

Communications with miners along haulageways using wireless techniques will be an essential improvement in the present underground 

mine communi cation system. 

Underground haul age incl udes rai 1 and conveyor (be1 t ) systems. In 

recent years, conveyor haulage systems have emerged as a significant a1 ternative to the rail haulage systems. The wireless communication system must be compatible with and useful in both systems. 

Factor 3: Medium frequency signals offer best propagation characteristics in an underground mine. 

The "propagation" characteristics of medium frequency (MF) signal s in 

an underground mine have been experimental ly measured (5) and theoretical ly studied (6). The transmission of MF signal s re1 ies on the 

propagation or coupl ing of magnetic fields between a transmitter and 

a remote tuned loop receiving antenna. 

The magnetic coupl ing or near field mode of signal transmission has 

been known for a long time. The distinguishing characteristic of 
near field transmissions is that the field strength rapidly decreases with distance from the transmitter. Depending on the conductivity of the coal, the field strength attenuation ranges between 10 to 

40 dB for each doubling of range. By way of comparison, the propagation characteristics of the radiated field in free space decays at 

the rate of 6 dB for each doubling of range. 

Near fiel d communication systems have a unique extended range capabil ity since other nearby conductors can act as a low loss transmission medium for magnetic signals. During the war years in Europe, 

communications over "cut" telephone 1 ines were made possible by near 

field transmission schemes. It is the magnetic coupling characteristic that will be exploited in the wireless mine communications 

probl em. 

While it is true that communications will be required in conductor 

free areas, operating distance will be relatively short with abrupt 

loss of signal in the fringe zones. This is contrasted to radiated 

signals that exhibit a gradual loss of operating system quality as 

the distance from the transmitter is increased through the fringe 

zone. 

Factor 4: Magnetic field sensors are more efficient than electric 

fiel d sensors in practical mine communication systems. 

El ectric field sensors are very insensitive at lower frequencies 

and cannot be used in a practical mine comunication system. 

At an air-earth interface, only the magnetic field is essentially 

undisturbed while the electric field is severely reduced. 

Any current will induce magnetic fields and hence the magnetic fib-d 
will directly be related to current flow. Thus, magnetic field 

sensors such as tuned loop antennas are effective sensors of low 

frequency communication signals. The signals may propagate along 

a conductor in a transmission line mode or be part of the near field in 

the vicinity of an excited conductor. 

Factor 5: Electrical noise generated in the mine power system greatly 

influences the selection of the modulation process and 

the design of the transceiver. 

The electrical noise generated in the mine power system becomes a 

dominant factor in the underground wireless communications problem. 

A review of the measured noise data (6) (7) reveal s the noise fie1 d 

strength varies over an enormous range (by several orders of magnitude - 60 to 130 dB variation). The noise is greatest near AC to 

DC converters (rectifiers), and along the trolley wire in rail haulage systems. Away from haul ageways, the mine noise level is as much 

as 40 to 50 dB below the noise level in the haulageway. The noise 

level s also change over a 1 arge range during a typical mine work shift. 

Be1 ow 100 kHz, changes are of the order of 45 to 50 dB decreasing to 

about 25 to 30 dB for frequencies around 1 MHz. 

The frequency dependence of the envelope of the measured noise spectrum exhibits some noteworthy characteristics. 

a) The mine generated noise level decreases with frequency. 

Between 3 kHz and 100 kHz, the typical envelope of the noise 

decreases by 50 to 70 dB, depending upon the type of electrical 

equipment operating in the vicinity of the noise measurement. 

b) For frequencies below 50 kHz, the envelope of the noise spectrum decreases rapidly. The average rate is approximately 14 dB 

per octave; however, rates as high as 20 dB per octave are 
1 evident over 1 imited frequency ranges. 

c) For frequencies above 100 kHz, the envelope of the noise 

spectrum decreases at the rate of 6 dB per octave. 

The electrical machinery used in a mine creates a wide range of many 

types of intense electromagnetic interference (EMI). Each type of el ectrical apparatus exhibits a noise signature that can be recognized in the 

spectrum analysis of noise signals. 

I To further expand and analyze the mine noise data (6) (7), time domain ( 

measurements were made on common types of equipment. The time domain 

measurements show that mine noise is predominantly generated by recogn izabl e periodic waveforms. These waveforms have Fouri er transformations with specific frequency domain characteristics. 

Recognition of the periodic characteristic of mine noise signals is valuable since the results can be used to design a better wireless communication system. 

The time domain characteristics of trol ley rectifiers and power centers 

will be described by actual measurements. Then the frequency domain 

characteristics will be described in detail. 

The trolley rectifier represents one of the noisier locations in the mine. 

Rectifiers incl ude a three phase step-down transformer. The secondary 

(of the transformer drives a full wave bridge rectifier. Similar AC to 

(DC rectifiers are also found in power centers feeding working areas 

(in the mine. 1 

(TWO different types of time domain measurements were made on the rectifier.( 

11n the first measurement, the voltage appearing between the pilot and 
ground conductor in the high voltage (7100 VAC) cable feeding the rectifier was measured with an oscilloscope. The measured voltage is representative of current flowing in the feeder cab1 e. The result is shown 

in Figure 1.0: 

Figure 1.0 Time domain characteristic of the induced pilot to 

ground signal measured in the U. S. Steel Somerset 

Mine, Peona , Col orado. 

The time domain measurements show the periodic characteristics of the 

signal. 

The DC 1 evel varied with train movement. The 1 evel ranged between -0.5 

and 5 volts and was caused by stray current flow along the railroad bed. 

The strong 12 kHz and 2 kHz sinusoidal signature signals are noteworthy since similar signal components were also observed in the measured 

data of reference 8. These oscillations are probably caused by Ferromagnetic resonances within the transformer feeding the full wave rectifiers. 

The most striking feature of the measured noise is the periodic nature 

of the 360 Hz semi-rectangular pulse train. Each pulse represents the 

surge current flowing through each diode (6 each) in the full wave rectifier. 

The trolley supply voltage characteristic provides further insight into 

the mine noise problem. 
Figure 2.0 Time domain characteristics of the Trolley voltage 

in CF&I A1 len Mine, Weston , Colorado. 

Sequentially, line phase current flows into the trolley bus during each 

conduction period of the bridge rectifiers. On the decreasing part of 

the phase vol tage cycle, the conducting bridge diode becomes reversed 

biased and current flow terminates. In a short period of time, the 

next bridge diode goes abruptly into conduction during the next current 

surge. 

The repetitive pulses of current can be represented by rectangular pulse 

train of duration and period T. The Fourier series expansion of the 

pulse train is given by 

where A is the ampl itude of the current pulse and kj is an integer (harmonic 

order). The ampl itude of the frequency spectrum can be written as 

The frequency spectrum will exhibit a frequency component at each harmonic 

of the current pulse rate (360 Hz). The level of the harmonic will decrease 

with the order of the harmonic (that is as 1 /* ). The ampl itude will follow 
a sin l/x distribution with minimums of the distribution occuring whenever 

the argument of the sin function becomes equal to i\T . 

For the periodic waveform at hand, the harmonics of 360Hz do not occur 

at the minimums of the sin r /x distribution. The periodic waveform was 

applied to a specturm analyzer and the resulting spectrum was plotted in 

two different frequency ranges. 

Figure 3.0 shows the frequency range extending to 3 kHz. The spectrum 

shows the harmonics of 360 Hz. The spectrum exhi bits a sinX/ * ampl itude dependence with minimums occuring every 997 Hz. These minimums are 

not at the zeroes of the sin X/Z functions. 

Figure 4.0 shows the frequency range used by trolley carrier current radio 

communication systems. It is not surprising to find that the commonly 

used 88 kHz frequency occurs at a minimum of the amplitude distribution. 

For completeness, the time domain characteristic of the 88 kHz trolley 

carrigr phone was measured as is shown in Figure 5.0. 

Figure 5.0 Time domain characteristics of the trolley voltage in 

CF&I A1 len Mine, Weston , Colorado. 

The 88 kHz carrier signal exhibits a peak signal voltage of 60V and is 

ampl itude modulated to the extent of 70 percent. 

The spectrum signature of induction motors exhibits two distinguishing 

characteristics. During startupthe feed power cab1 e pi1 ot to ground 

voltage exhibits a triangular waveform with a repetition rate of 60 Hz. 

After startup, the voltage approaches a sinusoidal waveform. 

The frequency spectrum of an induction motor EM1 exhibits an amplitude 

spectrum with harmonics of 60 Hz decaying at the rate of lh. Further, 

two spectral lines are sometimes evident: one at the "slip" frequency, 

the difference between the actual speed and the synchronous speed of the 

motor, the other at the synchronous speed of the induction motor. 

The spectrum signature of arc welders has also been investigated by 

researchers at the Bureau of Standards (8). The EM1 spectrum produces 

no strong spectral component. The amplitude characteristics of the spectrum go below the measurement system noise by 60 kHz. The power system 

a1 so includes infrequent switching transients that can produce short duration noise pulses. 

SUMMARY 

The predominant EM1 within an operating mine will be generated by periodic 

current pul ses. The amp1 itude function of the resul ting frequency spectrum will decay at the rate of l/n where h is the harmonic of the 360 Hz 

pulse rate. Less significant EM1 will be caused by induction motors, 

electrical arcs and switching devices. At low frequencies, below 100 kHz, 

the noise spectrum will decay at the rate of l/h where n is the harmonic of 

the 60 Hz. Above 100 kHz, the noise spectrum decays at 6 dB per octave. 
Factor 6: The repetitive EM1 pulse train will cause the reactive 

networks in the wireless receiver signal path to exhibit 

a transient response behavior (ring). 

The EM1 reaching the receiver front end will be a periodic signal within 

the passband of the receiver. 

Figure 6.0 Time domain characteristic of the EM1 signal in the 

receiver signal path. 

Whenever the EM1 signal exceeds the level of the legitimate signal, 

undesi rabl e operation of the receiver occurs. 

In general, the source of undesirable operation is the transient response behavior of the reactive networks in the signal path. The transient 

response behavior depends upon the transfer function of each reactive 

network. Linear phase transfer functions produce less over-shoot than 

do transfer functions with non-linear phase characteristics. The disturbances will be significant in the narrow-band highly selective circuits of the IF amp1 ifier. Much smal ler disturbances occur in single 

tuned stages of the tuned loop antenna, RF amplifier and mixer stages 

(sing1 e tuned networks exhi bit nearly 1 inear phase response characteristics). 

Transient signals generated in the signal path are recovered in the 

receiver detector and appear in the audio output signal. 
In an FM receiver, the limiter and discriminator networks may produce 

a transient response. The recovered audio signal will include both ampl itude and frequency variations of the transient signal. The audio output 

wi 11 exhi bit short duration damped sinusoidal pul ses. It should be 

noted that if the damped sinusoidal frequency band occurs out of the 

audio passband, the resulting disturbance would' not be noticeable. 

In an SSB receiver, the product detector will recover both the amp1 itude 

and frequency variations of the transient signal. Further, the AGC network will respond to the product detector output signal by producing a 

gain control signal with its own transient characteristics. The actual 

audio output disturbance is much greater in an SSB receiver. 

Noise blanker circuits can be designed to reduce noise pul se interference. Noise blanker circuits prevent the noise pulse from reaching the 

highly selective circuits of the IF amplifier. This can be achieved in a 

number of ways. However, the most common method is to open the receiver 

signal path immediately after the first mixer. Other methods incl ude lowering either the RF or mixer gain. These approaches are less satisfactory 

since they introduce new transients upon the restoration of normal gain. 

There are some pitfalls that must be recognized in a noise blanker design. 

First of all, the detection of the noise blanker signal must be made out 

of the frequency range of the legitimate signal. Otherwise, noise blanking will occur with the legitimate signal level. By selectively tuning 

the noise blanker circuits to an operating frequency below the system 

operation frequency range, an enhancement of the noi se bl anker detection 

process occurs. Figure 7.0 shows that the ampl itude spectrum of the noise 

pulse increases with decreasing frequency. 
FSEQUELCY iG \<\LGHE~TZ 

Figure 7.0 Typical amp1 itude spectrum of a noise pul se. 

A noise blanker tuned to a frequency (fN) below the operating frequency 

band will detect noise signals much greater in level than any noise 

signal appearing in the operating range of the system. 

A further refinement in the design of the noise blanker is to include an 

AGC detector in the noise blanker circuit. The AGC should be designed 

to control the gain of the noise blanker such that only noise pulses 

above an average noise dependent threshold will result in blanking. 
IMPULSE NOISE TESTING - 

FM and SSB receivers were evaluated using the test set-up below: 

Figure 8.0 Impulse noise test set-up. 

With the pulse generator turned off, the sensitivity of the receiver 

V 0 LnFTrlp f 

AUDIO 

was determined by using a standard test signal defined by the Electronic 

s\ GN~L 

Industries Association (EIA) for receiver test purposes. In the case of 

the SSB receiver the pulse generator is initially turned off. The signal 

-7 J 

GEKERATO~ 4 i 

Y b RPCr\vER 

OSC 

generator (CW mode) is tuned to 1 kHz above the receiver operating fre3, 

quency. The CW signal level is increased until the receiver output signal 

CdMo - 

PAD ~~orbS8 v L 

PULSE J 

is 10 dB above the output noise when the signal generator is turned off. 

~ENBR~~R 

b 

The pulse generator is then turned on and the controls set to provide a 

ScaPE 

L 

recurring pulse signal at the 360 Hz rate. The pulse width is set to 1 

millisecond. The CW signal generator level is increased until the output 

signal to noise ratio is again 10 dB. The difference between the initial 

and final generator level setting is a figure of merit (not to exceed 

lo dB). 

The behavior of the SSB receiver output signal with a recurring noise 

pulse was studied with the oscilloscope shown in Figure 8.0. The particular SSB receiver incl uded an inband noise blanker. For pul se rise 

times of more than 1 microsecond, the noise blanker effectively reduced 
the pulse interference as the rise time approached .1 microsecond. (The EM1 

signal in the receiver passband increases by 20 dB when pulse rise time 

is reduced to .1 microsecond). The recovered output became noisy at the 

360 Hz pulse rate. When the signal generator level was increased above 

30 microvolts, the effect of noise in the audio output disappeared. The 

output noise appeared to be silenced because the signal level exceeded 

the pulse noise (See Figure 6.0) within the bandwidth of the receiver. 

An FM receiver was evaluated in the same way except the signal generator d 

was frequency modulated with a peak frequency deviation of - + 5 kHz. The 

audio output of the receiver exhibited a damped sinusoidal behavior. 

The SSB receiver response to a noise pulse is determined by the transient 

behavior of the automatic gain control (AGC) network. 

\ST TF AMOLlFlfS 

M\XER 

GAIN PRO DUCT 

C~NTTQQCLE~~ 1. r, DEf CWOR 

AnQLtF\ FR 

I . d 

DC 

co rcTRaC 

SIGN~L 

r---- 

I 

I- - 

Law PASS RccGvERED 

FILTER Cl uD\o 

4,- r u o 

Figure 9.0 SSB receiver automatic gain control network. 

The AGC network is a closed loop feedback network. The IF amplifier gain is 

controlled on the basis of holding the peak value of the recovered audio 

signal constant during the spoken word. The DC control signal is derived 

by detection of the product detector output signal (recovered audio signal ). 
The AGC control loop is designed to have a fast attack and slow release 

time constants. In practice, the attack time constant is selected to be 

10 mil 1 iseconds or less to avoid first syllable "thumps" at the beginning 

of each word. The release time is greater than 10 mil 1 iseconds. 

A major problem in the design of the AGC control loop is that it must not 

respond to signals in the voice frequency band. Therefore, the AGC 

control loop must exhibit a very low cut-off frequency. The low cut-off 

frequency of the AGC loop extends the release time of the network to 

seconds. 

An SSB receiver will respond to a noise pulse by immediately lowering the 

signal path gain for a period of time equal to the release time of the 

AGC circuit. Each recurring noise pulse is evident in the recovered 

audio signal . 

The FM receiver produced recovered audio pulses at the 360 Hz disturbance 

frequency. The damped sinusoidal frequency was we1 1 above the 3000 Hz 

frequency of the receiver audio amp1 ifier. The recovered noise disturbance 

was not as noticeable in the FM receiver as it was in the SSB receiver. 

SUMMARY 

A noise blanker network is required in the design of an SSB receiver to 

avoid receiver desensitization with noise pulses. A noise blanker will also 

extend the operating range of an FM receiver. 

Factor 7: FM systems can offer noise quieting advantages when two conditions are satisfied: 

1 ) The FM carrier to noise ratio must be greater than 13 dB, and 

2) The FM modulation index (AF/fm) must be greater than approximately 0.6. 
The modulation index of 0.6 defines the transition between narrow-band and 

wide-band FM. Narrow-band FM offers no improvement over SSB so far as the 

demodulation process is concerned. The FM noise improvement is due to the 

generation of signal compohents within the transmission bandwidth. The 

signal components act to overcome the noise components, thereby increasing 

the receiver output signal to noise ratio. Spread spectrum modulation 

processes always exhibit a very sharp threshold in the demodulation process. The threshold is the price that must be paid for improvement. 

In an FM receiver, a noise pulse applied to receiver can cause the receiver 

output noise to increase during and after the noise pulse period. An FM 

limiter is not a substitute foranoise blanker. Without a noise blanker, a 

noise pulse can cause the 1 imiter network to ring. 

Factor 8: The action of a limiter in an FM receiver has been investigated by 

Davenport (1). A limiter being a device exhibiting a constant 

output power independent of input power will a1 ter the signal to 

noise ratio in passing through the limiter. 

The change in the signal to noise ratio of a signal passing through a limiter 

ranges betweenr/4 < K (2 the lower band being approached for very low 

input signal to noise ratios, the upper band for very high signal to noise 

ratios. 

Under low signal to noise ratio conditions the presence of a limiter in the 

signal path will degrade the system performance. Under high signal to noise 

ratio conditions, the 1 imiter will enhance the qua1 ity of FM communications. 

Factor 9: FM requires greater IF bandwidth than an SSB communication system. 

The required bandwidth for an SSB communication system is equal to the voice 

baseband frequency range. 
The bandwidth required in an FM system is given by Carson's Rule as: 

BW=2Af + 2fm or 

BW = ~AC- when M > 1 

where Af is the peak frequency deviation of the carrier signal and 

m is the modulation index ( Af/fm). 

When the modulation index is high, the required bandwidth is independent 

of modul ation frequency. 

Sensitivity based upon SINAD measurements will exhibit degradation whenever the peak deviation - + Af exceeds 75% of the IF bandwidth. The modulation acceptance bandwidth is characterized by the frequency deviation 

required to produce 12 dB SINAD for a specific signal that is 6 dB larger than 

a signal with a standard modulation that produces 12 dB SINAD sensitivity. 

The FM bandwidth must be large enough to avoid the degradation effects of 

distortion. The FM receiver bandwidth ranges from 13 to 15 kHz in commercial 

grade FM transcei vers ( Af = +- 5 kHz). 

The second order distortion in a narrow band FM system is given by: 

where @ (w) is the phase response of the receiver selectivity function. 

Distortion increases with peak frequency deviation and modul ation frequency (fm). The distortion is also dependent upon the behavior of the 

phase response within the system transmission path bandwidth. 

Factor 10: FM transceivers are less costly to design and manufacture. 

The reasons for the lower cost are: 

1) Low demand current FM IF amplifiers are available in integrated 

circuit form. 
2) Mono1 ithic quartz crystals are avail able for use in FM receivers. 

These filters are less costly than SSB filters because they require 

lower resonator Q. 

3) The design does not require ALC control circuits in the transmitter. 

4) The local oscil lator stabil ity versus temperature specification is 

less critical for FM than SSB transceivers. 

Factor 11: The SSB demodulation process does not exhibit a threshold 

effect. SSB demodulation is a linear process and will provide 

a paging capabi 1 i ty even when the comnuni cation system i s operating below threshold. 

The SSB demodulation process is linear, meaning that the signal to noise 

ratio is not changed in the demodulation process at low signal to noise 

ratios. 

A very narrow band post-demodulation tracking detector can be used to detect 

paging and squelch control signals in areas of low carrier to noise operation. The tracking filter can extend the threshold of the comunication system. The roving miner can be paged even when voice conunication qua1 i t y i s 

poor. The miner can then move closer to a repeater or base station to 

establish quality voice communications. 

Factor 12: The narrow band FM articulation index is about the same as SSB. 

A comparison of SSB and FM modulation systems based upon articulation index 

consideration shows that for a peak envelope power to noise power density 

in dB - Hz of less than 40 dB, FM (m = 1 or 2) has no advantage over SSB. 

Factor 13: The sensitivity of an SSB receiver is better than a narrow band 

R4 receiver by approximately 2.0 dB. 

The sensitivity improvement of SSB over FM is given by 
where BW is the required signal transmission bandwidth. 

The SSB system will exhibit better threshold sensitivity. 

Factor 14: SSB transmitter is more efficient. 

SSB will require less battery capacity and hence smaller physical battery 

vol ume. 

Considerations: 

1 ) SSB Class AB or B final amp1 ifier efficiency of 38%; 

FM Class C final amplifier efficiency of 50%. 

2) During voice transmi ssion SSB speech tack requires approximately 

50% less battery power. 

FM speech transmission with a VOX circuitry requires full transmitter 

output for the duration of the speech transmission. 

The transmit period battery consumption is less for SSB transmissions. 

Factor 15: Selective fading less in SSB communication system. 

SSB modulated signals have an advantage over FM and AM if selective fading 

is 1 ikely to occur in the transmission path. Dispersive medium and multiple 

path effects can suppress carrier signals and degrade the demodulation 

processes. 

Since SSB does not require a carrier signal in the demodulation process, 

the SSB transmissions are less susceptible to these effects. 

Factor 16: Spurious receiver desensitization depends upon frequency 

transposition process. 

Spurious response depends upon the receiver transposition scheme. FM 

communication systems require wider IF bandwidths and are therefore more 

suscepti bl e to spurious response desensitization. 

-20- 
Factor 17: A tone controlled squelch will be required in the operating 

wi re1 ess comunication system. 

An audible tone controlled squelch will be required. 

Squelching schemes based upon noise detection cannot be expected to perform satisfactorily in the high noise environment of an underground mine. 

In an FM system, subaudible tones will produce an FM signal with a high 

modulation index. The FM detector threshold will not allow proper operation whenever the carrier to noise level is low. 

In an SSB system, subaudi ble tones recovered in the receiver output will 

include the frequency drift of both the transmitter and the receiver. 

A coded squelch signal can be used for paging purposes. 

Factor 18: FM comunication system will be compatible with FM comunications used in rail haulage appl ications. 

Mining companies have a1 ready invested substantial sums of money in the 

trol ley carrier current phone system. A wireless communication system that 

would be compatible with these communication systems would enhance mine 

safety. 

Nearly all of the carrier current phone systems use FM modulation. 

Factor 19: FM threshold extension not worth the effort. 

Frequency modulation feedback (FMFB) and phase lock loop bandwidth compression can extend the threshold downward to a certain extent (a few dB) at 

the cost of considerable complication in the practical transceiver design. 

See article by J. J. Hupert (9). 
Factor 20: Simple wire antennas can extend induction field range in an 

en t ryway. 

Dedicated wire antennas can be used to transmit near field signals down 

haulageways. Wi reless communications can be easily extended throughout the 

underground mine. Communications will be possi bl e even across broken 

conductors. Dedicated conductors can avo id impul se noi se probl em 1 i kel y to 

occur near power cables. 

Factor 21 : The design must be easily serviceable in the field. 

Service is a major problem in a mine. The design must use plug-in PC 4 

boards wherever practicable. Service and repair manual s must be cl early 

written for mine personnel to use. 

Factor 22: Collins MF Transceiver BW is 12 kHz. 

BW= 2Af + 2 fm 

= 2 (3 kHz) + 2 (3 kHz) 

= 12 kHz 
COURSES OF ACTION 

1.0 Select an FM system with a rnodul ation index of 4 or greater. 

Advantages: 

a) In a high carrier to noise ratio area, FM improvement could be realized. The modulation index wou1.d be 4 or slightly greater. The 

receiver transmission bandwidth (BW) must be wide enough to pass 

the FM spectrum without excessive distortion in audio qua1 ity. The 

required receiver bandwidth is defined as 

where m is the modulation index, 

fm is the highest audio frequency, and 

.75 is a factor insuring that the receiver bandwidth is 

sufficiently wide for 1 ow FM distortion transmissions. 

The requi red recei ver bandwidth becomes 

- 

b) The transceiver local osci 11 ator frequency stabi 1 ity versus temperature specification is r e a l i z a b l e with a commercial grade quartz 

crystal oscill ator. 

c) The noise impulse response would be less than in an SSB system. 

Disadvantages : 

a) In low carrier to noise ratio areas, FM threshold would seriously 

degrade audio quality. 

b) System could not be used for paging purposes in noisy areas of 

the mine. 
C) A 1 imiter will degrade the output signal to noise ratio in noisy 1 

areas of the mine. 

d) The receiver will have increased suscepti bil i ty to spurious response 

owing to the wide IF bandwidth. 

e) Wide signal spectrum requires wide band transmitter final circuits. At low (and medium) frequencies this leads to low-Q cir- 1 

cuits. These cannot, however, be efficiently loaded by the radia- I 

tion resistance, since the system does not operate by means of the 1 

radiated field, but rather by an induction field. Wider band nec- 1 

essitates, then, higher antenna circuit loss, reduces circulating 

current and thus the transmit moment. This is a powerful factor 

militating against even a relatively narrow-band FM system. 

2.0 Select a narrow band FM system. 

Advantages : 

1 a) Although FM improvement would not be possible in the system, the 1 

1 effect of threshold could be avoided in the communication system. 1 

1 The modulation index would be less than unity to avoid threshold 1 

effects. The required receiver bandwidth for acceptabl e audio 

qua1 i ty becomes 

= 3520 Hz 

The voice band would extend to 2200 Hz and is adequate for good 

legibility. 

b) The transceiver local oscillator frequency stability versus 
temperature requirement is r e a l i z a b l e with a commerci a1 grade quartz 

crystal osci 11 ator. 

Di sadvantases : 

a) No demodulation improvement over SSB. 

b) In areas of low signal to noise ratio, receiver 1 imiter will degrade 

the output SIN ratio by 1 dB. 

c) Since the final power amp1 ifier can be operated in Class C, the 

transmitter harmonic output will be greater than in a Class AB or B 

SSB design. A more complex harmonic filter will be required. However, the cost of the additional filter element will be less than 

$1.00. 

d) In a receiver with a properly designed noise blanker, a noise pulse 

will cause the receiver output noise to increase for the duration 

of the bl anking period. 

e) In high carrier to noise ratio areas, the receiver limiter will 

improve the output signal to noise ratio by 3 dB. 

f) If the receiver can be tuned to frequencies used by the existing 

carrier current trolley telephones, the transceiver can be simply 

added to some existing mine conunication system. 

g) The noise impulse response would be better than in an SSB system. 

A noise blanker can extend the operating range of the FM receiver. 

h) A very narrow band tracking filter following the discriminator 

can be used to detect paging and squelch control signals. The 

tracking f il ter can extend the thresh01 d of communication system 

for paging and squelch control purposes. 

-25- 
3.0 Select an SSB system. 

Advantages: 

a) The demodulation system is 1 inear and threshold effects are avoided. 

b) The receiver bandwidth only needs to be as wide as the highest 

voice band frequency (BW = 2200 Hz). The receiver will have a 

sensitivity advantage over narrow band FM. The improvement may be 

less than 2 dB. 

c) More communication channel s can be used in a given bandwidth. 

d) In a receiver with a properly designed noise blanker, a noise 

pulse will cause the receiver output audio signal to become quiet 

instead of a full noise output as is the case in FM. 

e) The antenna Q can be the highest possible to reduce circuit loss, 

allow maximum circulating current and thus a high transmit magnetic moment. 

f) Since the system need not transmit during pauses in the voice 

signal, the highest overall efficiency is achieved. 

g) A below threshold paging capability can be realized by using a 

narrow band tracking filter and decoder following the receiver 

product detector. 

Disadvantages : 

a) To avoid spurious receiver response problems, a high IF frequency 

must be used in the design. This forces the first mixer injection 

oscill ator to have a frequency stabil ity versus temperature of 

- + 1 PPM. The required crystal oscillator will cost considerably mre 
than a commercial grade oscillator for an FM system. The cost 

may exceed $1 00 in production quantities. 

b) The IF circuits must include a means of sharing the IF crystal 

filter in both the transmit and receiving modes of operation. 

This adds complexity to the design. 

c) Receiver AGC must have a short attack and release time to prevent 

receiver desensitization after a noise pul se. There are probl ems 

in achieving this objective because the fast release SSB AGC networks are difficult to stabilize with voice modulated SSB signals. 
RECOMMENDATIONS 

Voice qual i ty communications will require that the signal path carrier 

to noise power ratio exceed 10 dB throughout the required operating 

range. The system must include a network of dedicated conductors, 

repeaters and antennas to maintain the signal path signal to noise 

ratio. 

Rail haulage communications can be improved by use of a tuned filter 

between the trolley rectifier and the trolley bus. The filter should 

be tuned to the operating frequency of the MF communication system. 

A dedicated trolley communication line would improve the communications 

quality. Power line filters could be used elsewhere in the mine to 

reduce EMI. 

In general there are no miracl es in establ i shing voice qual ity communications in an operating mine. The present trolley mine phones which operate in the noisiest part of the mine simply overpower the noise. The 

general characteristics of trolley telephones are shown below: 

Manufacture Transmitter 

Output Power 

Manufacture 

Pyott Boone 

Femco 

15 Watts 

16 Watts 

MSA 20 Watts 

Type of Modulation 

Pyott Boone (X214) 

Femco (731 901 ) 

MSA (1601 ) 

Modul ation Receiver -- - 

BW Sensitivity 

5 kHz 420 uV 

FM Some AM 5 kHz 480 uV 

FM Some AM 13 kHz 820 uV 
The data shows that the combination of high transmitter power and low 

receiver sensitivity solves the noise problem in the trolley communication system. 

Narrow band FM does not offer any significant improvement over SSB 

wi re1 ess comuni cations. A1 though the SSB receiver sensitivity can be 

made 3* dB better than a narrow band FM receiver, the system operating 

range improvement is small because of the high attenuation of near field 

signals. The pivotal point in the selection of the modulation process 

is the relatively high cost of stable local oscillators required i,n an 

SSB communication system (required to avoid spurious receiver responses). 

Further, accidental shock (drop) to the transceiver could seriously 

degrade system voice quality whenever the oscillator frequency drifted 

more than 20 Hz from the required channel frequency. 

An SSB transceiver is more complex to design and manufacture, since the 

IF fil ter (quartz crystal filter) must be shared between TX and RX mode 

of operation. A transmitter automatic level control and a receiver automatic 

gain control are required in the design. 

A final point bearing heavily on the selection is that a noise blanker 

is necessary in the SSB receiver to avoid receiver desensitization under 

repetitive noise pulse EMI. However, this argument against SSB may be 

weak, since a noise blanker applied to an FM receiver will improve the 

system performance in the vicinity of high impul se noise (around rectifiers). 

* 2 dB from smal ler noise bandwidth and 1 dB from the FM 1 imiter operating 

at low SIN. 
Narrow band FM wi re1 ess communication system is recommended. 

The receiver to be free of spurious response (IF greater than 5 MHz) and 

the transmitter spurious emission to be down better than 70 dB. The recei ver design to incl ude a no i se bl an ker tuned to a frequency be1 ow 

88 kHz. The frequency range of the system to incl ude 88 kHz trolley mine 

phone frequency. 
LIST OF REFERENCES 

1. Panter, P. F. 

2. Viterbi, A. J. 

3. Hancock, J. C. 

Modulation, Noise and Spectral Analysis, 

McGraw-Hill Book Company, New York; 1965. 

Principles of Coherent Communications, McGrawHill Book Company; 1966. 

The Principles of Communication Theory, 

McGraw-Hi 1 1 Book Company; 1961 . 

4. Davenport, W. B. and An introduction to the Theory of Random 

Root, W. L. Signal s and Noise, McGraw-Hi 11 Book Company, 

1958. 

5. Cory, T. S. 

6. Lagace, R. L. 

7. Scott, W. W. ,et a1 

8. Kanda, M. , et a1 

9. Hupert, J. J. 

Propagation of FM signals in Underground Mines, 

United States Dept. of the Interior, Bureau of 

Mi nes , Con tract No. H0366028; Fi nal Report; 

September, 1977. 

Transmit Antennas for Portabl e VLF to MF 

Wi re1 ess Mine Communications, USBM Contract 

Final Report (H0346045), Task C Task Order 

No. 1; May, 1977. 

Electromagnetic Noise in Lucky Friday Mine, a 

Technical Report of El ectromagnetics Division, 

Institute for Basic Standards, National Bureau 

of Standards, Boulder, Colo. 80302; June, 1974. 

Electromagnetic Noise in McElroy Mine, a Technical Report of Electromagnetics Division, 

Institute for Basic Standards, National Bureau 

of Standards, Boulder, Colo. 80302; June, 1974. 

"Frequency Modul ation Techniques", El ectroTechnology, February, 1965.