The block diagram of Figure 4-156 shows a Buf- Buf-fer Amplifier connected to a center tapped

trans-former. This is the input to the receive channel

when the modem is electrically coupled to the

tele-phone line. When acoustic coupling is used, the

out-put from the microphone -is passed through a

rela-tively high gain amplifier. The amplifier provides

approximately the same level at the output of the

Band Pass Filter amplifier as the electrical coupling

input. Selection of either type of coupling is

ac-complished by inserting either the acoustic coupting

r---FiG.'i209- -- -

--1

CIRCLED NUMBERS INDICATE SCOPE POINTS FOUND IN TROUBLESHOOTING CHARTS IN SECTION 5

MODEM BLOCK DIAGRAM

DATA TERMINAL CONTllOL LOGIC "a"

.-- - -FIG~4~20a----1

Section IV HCS-220003 connector plug or the electrical coupling connector

plug at the back of the 1030 or 1035 terminal.

The Band Pass Filter Amplifier has two func-tions. First, it increases the level of the received

FSK signal. Second, it supresses noise and interfer-ing signals which are outside of the received FSK signal band. It is an active filter of the infinite gain single feedback type. Its selectivity and other char-acteristics are of a non-critical nature due to the demodulation technique used.

During Duplex operation, data is being trans-mitted simultaneous with the reception of data.

When the modem is coupled acoustically or elec-trically by a two wire arrangement, transmitted data is coupled into the receive channel. The re-ceive channel must therefore provide circuitry which rejects the transmitted signals. The Band Reject Circuit shown in the block diagram is for this purpose. Attenuation of the second harmonic of the transmit signal is accomplished by a network centered at 2540 Hz.

The output of the Band Reject Circuit feeds the Limiting Amplifier. The Limiting Amplifier simply generates a bipolar rectangular signal for signal levels above a predetermined level. This signal char-acteristic provides a linear transfer charchar-acteristic of the phase comparator.

The heart of the receive portion of the modem is the Phase Lock Loop which is used as a frequency discriminator. The characteristics of this type of phase locking circuit make it well suited for the application of FSK demodulation. This circuit is capable of extracting information from a signal with low signal to noise ratios. Additionally, it performs as a very selective band pass filter, a feature that substantially reduces the predemodulation filtering requirements of the modem. Initial calibration of the circuit is extremely simple, because only one adjustment is required.

Operation of the Phase Lock Loop as a frequency discriminator is as follows: The FSK signal appears at the output of the Limiter Amplifier. The

fre-quency of this signal is compared to the frefre-quency of the Voltage Controlled Oscillator. The result of this comparison is an output voltage from the

Phase Comparator which is proportional to the phase difference of the incoming frequency and the frequency of the Voltage Controlled Oscillator. This voltage is passed through the Low Pass Fitter, which removes the carrier frequency components and

establishes the loop band width. The output voltage of the filter adjusts the frequency of the Voltage Controlled Oscillator in such a direction so as to decrease the phase difference. This forces the fre-quency of the Voltage Controlled Oscillator to be identical to the frequency of the incoming signal.

In other words, the Voltage Controlled Oscillator is phase locked with the incoming signal. As the fre-quency of the incoming signal changes, the output voltage of the Low Pass Filter must also change in order to keep the frequency of the Voltage Con-trolled Oscillator in phase with the input frequency.

It is apparent then, that frequency changes of the input signal are converted to voltage changes at the output of the Low Pass Filter in the loop. This is the basic demodulation process ..

As stated previously, the Phase Lock Loop also acts as a band pass filter. The Low Pass Filter in the loop determines, for the most part, the band width of the loop. Information centered about the carrier frequency is translated by the loop to infor-mation centered about zero frequency. The loop will reject frequencies greater than the loop band width. So in effect, the loop looks like a band pass filter whose center frequency is the same as the frequency of the voltage controlled oscillator. Sev-eral very important features are therefore evident.

a. A very selective band pass filter is obtained as an integral part of the demodulator.

b. The center frequency of the equivalent band pass filter tracks the incoming signal over a limited input signal range.

c. The center frequency can be changed relatively easily by changing the Voltage Controlled Oscillator frequency.

d. Extremely narrow band widths can be obtained with non-critical components.

e. Independent adjustment of band width and center frequency is possible.

f. Predemodulation filtering requirements are greatly reduced since selectivity is an inher-ent part of the phase lock loop FS K demod-utator.

g. Since the information has been translated to zero frequency, additional post demodula-tion low pass filtering can be used to further enhance the selectivity. ·

HCS-220003 Section IV

For demodulating the FSK signals, the Voltage Control led Osei llator is set to

2125

Hz with the out-put of the low pass filter at zero volts. The oscillator will then track the

2-25

Hz and

2225

Hz FSK signal resulting in voltage changes above and below zero at the loop filter output. This demodulated signal is now passed through a post demodulation Low Pass Filter which removes noise outside of its pass band. The output of th is filter feeds a Level Detec-tor with Hysteresis to reject additional noise. The output of the level detector is the received data.

The data is sent to either the

1030

or

1035

by way of the logic shown in Figure 4-197.

The Nand gate does not allow data to pass unless there is an output from the Carrier Detector. The Carrier Detector determines if an FSK signal is present. If there is, data is allowed to pass, if not, the data output is inhibited since it is not valid data. Note that an external Data Set can be used since data from our modern, OR received data from an external data set is passed. This is also true of the Clear to Send and the Data Set Ready outputs which are derived from the Carrier Detector.

As shown in Figure

4-156,

a data lamp driver also receives the data signal. The lamp flashes on and off at the data rate and gives a visual indication that data is being received.

Turning the modern ON or OFF is accomplished by the switch connected to the Carrier Detector Output. Closing the contacts disables the Carrier Detector which inhibits the FSK transmit Oscilla-tor and the Received Data Out. It also inhibits the CTS and DSR signals.

4-58. SIGNAL LEVEL AND TRANSMISSION LINE CHARACTERISTICS

a. Prior to initiating a description of the Datel modulator/demodulator (modem) circuitry, we will examine both the form of the in-coming data and the condition under which the data arrives. Figure 4-157 displays the spectrum of the incoming signal as seen by the receiving section of the modem.

b. Transmission line and signal level charac-teristics as seen by the modem are sum-marized below.

1. Input is from the Acoustic Coupler or . Data Access Arrangement.

(a) The receive signal level range is from

-6

to

-30

dbm and is typi-cally

-25

dbm.

(b) A= typical noise level of

-42

dbm.

~ ~

1"RANSMIT

RELATIVE AMPLITUDE

1070 1270

SIGNALS

2nd HARMONIC OF 1270 XM IT SIGNAL

Figure 4-157.

Section IV HCS-220003 2. The line band width: 300 Hz to 3000

Hz compatible with WE 103 A2 Data Phone.

3. The received frequencies are 2225 Hz for a mark and 2025 Hz for a space.

4. The quiescent condition of the input is the mark frequency of 2225 Hz.

5. The transmitted frequencies are 1270 Hz for a mark and 1070 Hz for a space.

6.

The quiescent condition of the output is the mark frequency of 1270 Hz. the signal appears on the secondary of T1 as depicted in Figure 4-157, and from there to the Band Pass Filter Amplifier circuit (Figure 4-160).

c. If the signals are acoustically coupled they appear at the base of Q 1, and from there to Point A of the Band Pass Filter Amplifier (Figure 4-160).

d. The variable resistor (r2) shown in Figure 4-158 is utilized to adjust the transmit out-put level to the DAA.

e. The circled numbers on Figure 4-158, and on all the rest of the schematics in this manual, refer to the actual termination points on the modem circuit board.

4-59. ACOUSTIC OR DIRECT COUPLING a. When the signal arrives at the modem it is

coupled into the first signal conditioning circuit by one of two methods.

1.

Through the telephone company sup-plied Data Access Arrangement (DAA), and then into the Date! sup-plied coupler circuit as shown in Figure 4-158.

2. Through an acoustic package into the microphone input shown on the Band Pass Filter Amplifier circuit (Figure 4-160).

O n

e,. ~

Figure

4-159.

4-60.

BAND PASS FILTER AMPLIFIER (BPFA) two separate functions: first, to pass and amplify the MARK and SPACE signals arriving on that portion of the band cen-tered around 2125 Hz and second, to sup-press all other frequencies.

b. If the signals are acoustically coupled to the BPTA (as shown in Figure 4-160), they ap-proximately 260 Hz at 3 db down.

HCS-220003

Section IV

LI 390-410 MH ·

+ 12 ^;:.

~Dii ^c::Ji

Cl .012i5%

§

^C::IJ

C2 820 PF

CID c::::ID

HARD. c:::::J

COUPLE R3 36K _R4 56K ^ll

..

RCVD

B~~

© ^{c::::m c::!J}

5 CE

~

MIC. INPUT

~@

~

Dl ^CI!

^{c:J Ci] c::rJ}

TO HIGH ^~

PASS FIL TE

[!] C!J _Os

c::::J118 ...

-12

Figure 4-160. . Figure 4-161.

d. Starting at point Bon the BPFA, the signal 4-62. BAND REJECT FILTER conditioning process is started by:

1. Attenuating all signals outside the a. Function:

2000

Hz to

2250

Hz band.

1.

Suppress the

2540

Hz second

har-2.

Amplifying the signals centered manic of the

1270

Hz transmit (Mark)

around

2125

Hz. carrier frequency.

4-61. HIGH PASS FILTER (HPF)

2.

Amplify the

2225

Hz (Mark) and

2025

Hz (Space) frequencies.

a. The main purpose of the High Pass Filter, illustrated in Figure

4-163,

is to pass the

2025

and

2125

Hz signals and suppress the

1070

Hz and

1270

Hz signals. The transmit b. The signals from ~High Pass Filter are signals are coupled via transformer T1 sec- coupled through

C24,

Figure

4-164,

to the ondary, into the receiver section of the input to filter network

L2, C5,

and R12 modem during transmission. (point A). Inductor

L2

is utilized to tune

the circuit to

2540

Hz, therefore, signals of b. The impedance characteristics of the

RC

this frequency are rejected by the high

im-network, plus the impedance of

02

as an pedance of the tuned network.

emitter follower, combined with the

feed-back through

RB,

results in the 12db/

c.

Amplification of the signals passed by the octave cutoff point at

2000. L2-C5

network is carried out by

Z2.

4-87

Section IV

n u ⁿ

^{i ~ I}

,.,

"""

0

FROM HIGH PASS FILTER

I 1~2000 H1

Figure 4-162.

Rl2 470K

Rll 2.4 K

Figure 4-164.

CMD D

...

HCS-220003

I Ocoo I

c:::J i ^I

CH I

n

~

ol

c:.• I

FROM BAND PASS

FILTER AMP.

C4

.00~

RS 3.3K

R9 16K

-

-+12

C24~

~ (I]

&...:::..J TO BAND REJECT CKT

-12

Figure 4-163.

470 K 510

IT]

Rl4 50

TO BAND PASS AND LIMITER AMPLIFIER

0

Zl'J

.,.,

w q]J

^T•

CMO Q ^Oc10

c:::J

Cll

c ..

0

Figure 4-165.

HCS-220003 Section IV 4-63. BAND PASS FILTER AND LIMITER

AMPLIFIER (BPF) (LAmp)

a. The output of the BPF is coupled to the Limiter Amplifier (LAmp) via the Band Pass Filter (BPF) made up of R 15, L3 and C6. This filter:

b.

1. Has minimum impedance at 2125 Hz.

2. Therefore rejects currents of all other frequencies.

Zener diodes wired back to back, so that at point B we get a bi-polar limiting action.

This limiting action is a result of the large signal at the Z3 output causing the diodes to conduct in the reverse direction.This effectively shorts the feedback loop, RAD-ICALLY DECREASING the gain of the circuit.

The incoming signal, either a MARK signal of 2225 Hz or a SPACE signal of 2025 Hz, has now been amplified and conditioned to the extent that it now represents informa-tion that can be correctly translated by the rest of the modem circuitry as data.

0

4-64. POWER SUPPLY VOLTAGE

CONTROLLED OSCILLATOR (PSVCO) a. The VCO is a voltage sensitive circuit.

There-fore, the supply voltages to this circuit must be closely regulated.

b. Close inspection of the VCO PS (Figure 4-170) indicates that it is not a supply in the true sense of the word. It is a regulating system for the voltages supplied to the VCO from an external supply.

c. The regulating characteristics of the circuit are established by Zener Diodes V R5, R33, R35, R37 and R40. Zener Diode VR5 establishes a fixed reference of +6.2 VDC at point F on Figure 4-168.

d.

As the +VDC supply attempts to go more positive, point B tries to go more positive.

Since this positive going change is fed back to the inverting input of Z6, the output then goes negative at point D .

e.

06

is turned on harder causing a larger IR drop across R41, and has therefore caused

a

negative voltage change at point A which offsets the attempted rise in voltage.

4-65. VOLTAGE CONTROLLED OSCILLATOR (VCO)

a. Although this circuit is part of the Phase Lock Loop, the following description will be developed as if the VCO were a "stand alone circuit". The discussion defining the circuit's dynamic operating characteristics,

Section IV

HCS-220003

+12

R32 390 VR5 IN5234 R33

75 K

!1% R35 IOOK'!'l°lo

TO R43 vt

--820

Figure 4-168.

with respect to the other parts of the Phase

Im Dokument CORPORATION HARRIS (Seite 100-108)