HYNIX HDM8515P Datasheet

1

HDM8515 Users Manual

DVB/DSS Compliant Receiver

Dec. 2000

Priliminary

2

Direct Broadcast Satellite (DBS) has been one of the most successful new product
introductions in the history of consumer electronics. This product represents the first
application of digital video compression for broadcast television. Originally intended to
provide cable quality television services to remote areas, this product is now offering a
competitive replacement to cable services in many urban areas.

The first operational systems employ closed proprietary signaling structures. The
European Broadcasting Union (EBU) has developed the first open standard (DVB-S) for
DBS services. The broadcasting community has embraced this standard which is now
being adopted for new systems throughout the world. This widely accepted open
standard is essential for DBS to achieve full market potential.

The HDM8515TM is a fully DVB-S&DSS compliant ADC/QPSK demodulator/FEC device
which provides an MPEG-2 stream to be processed by the conditional access and video
decompression circuits. The demodulator clocked with a fixed frequency is true variable
rate over the range of 1 to 55M symbols-per -second. This product achieves the highest
performance and flexibility. It minimizes the cost of external circuits, thus reducing
overall system cost.

3

Hy nix Semiconductor Co., Ltd reserves the right to make changes to its products or
specifications to improve performance, reliability, or manufacturability. Information
furnished by Hynix Semiconductor Co., Ltd is believed to be accurate and reliable.
However, no responsibility is assumed by Hynix Semiconductor Co., Ltd for its use; nor
for any infringement of patents or other rights of third parties which may result from its
use. No license is granted by its implication or otherwise under any patent rights of
Hy nix Semiconductor Co., Ltd.

For more information contact:
Address: Youngdong Bldg. 891, Daechi-dong, Kangnam-gu, Seoul, 135-738, Korea

Tel: [phone]
Fax: [phone]
E-mail: kosb@h ynix.com

4

TABLE OF CONTENTS

1. INTRODUCTION TO THE HDM8515...................................................................................................................7

1.1 FEATURES AND BENEFITS..................................................................................................................................8

2. HARDWARE SPECIFICATION..............................................................................................................................9

3. TECHNICAL OVERVIE W.....................................................................................................................................19

3.1 DUAL CHANNEL ANALOG TO DIGITAL CONVERTER .................................................................................. 19

3.2 V ARIABLE RATE DEMODULATOR.................................................................................................................. 21

3.3 NOISE MEASUREMENT CIRCUIT .....................................................................................................................23

3.4 V ITERBI DECODER.............................................................................................................................................25

3.5 AUTONOMOUS ACQUISITION..........................................................................................................................26

3.6 REED SOLOMON DECODER..............................................................................................................................28

3.7 CLOCK G ENERATION PLL .................................................................................................................................30

3.8 DBS R ECEIVER...................................................................................................................................................35

3.9 DISEQC I NTERFACE ...........................................................................................................................................36

4. MECHANICAL SPECIFICATIONS.....................................................................................................................37

4.1 100 P IN Q UAD FLAT PACK................................................................................................................................37

4.2 64 P IN THIN QUAD FLAT PACK........................................................................................................................39

4.3 RECOMMENDED ANALOG P IN CONNECTION...............................................................................................41

4.4 RECOMMENDED CLOCK G ENERATION CIRCUIT...........................................................................................41

5. SIGNAL DESCRIPTION....................................................................................................................................... 42

5.1 INPUTS..................................................................................................................................................................42

5.2 OUTPUTS.............................................................................................................................................................42

5.3 MONITOR AND CONTROL INTERFACE ...........................................................................................................45

5.4 I2C MODE.............................................................................................................................................................46

6. REGISTER DEFINITIONS.....................................................................................................................................48

6.1 WRITE REGISTERS..............................................................................................................................................48

6.2 READ REGISTERS................................................................................................................................................61

APPENDIX....................................................................................................................................................................66

A1. L OOP FILTER PROGRAMMING APPLICATION NOTE................................................................................67

A2. FALSE LOCK ESCAPE APPLICATION N OTE.................................................................................................70

A3. PERFORMANCE WITH INTERFERENCE. .........................................................................................................71

A4. NYQUIST CRITERIA CONSIDERATIONS.........................................................................................................75

5

LIST OF FIGURES

FIGURE 1: T OP LEVEL BLOCK DIAGRAM....................................................................................................................7

FIGURE 2: I NPUT DATA TIMING DIAGRAM.............................................................................................................10

FIGURE 3: I NTEL 80C88A READ TIMING DIAGRAM...............................................................................................11

FIGURE 4: I NTEL 80C88A WRITE TIMING DIAGRAM.............................................................................................12

FIGURE 5: I NTEL 8051 READ TIMING DIAGRAM.....................................................................................................13

FIGURE 6: I NTEL 8051 WRITE TIMING DIAGRAM...................................................................................................14

FIGURE 7: MOTOROLA READ TIMING D IAGRAM....................................................................................................15

FIGURE 8: MOTOROLA WRITE TIMING DIAGRAM.................................................................................................16

FIGURE 9: O UTPUT TIMING DIAGRAM FOR NORMAL PARALLEL....................................................................... 17

FIGURE 10: OUTPUT TIMING DIAGRAM FOR N ORMAL SERIAL...........................................................................17

FIGURE 11: OUTPUT TIMING DIAGRAM FOR REGULATED PARALLEL...............................................................18

FIGURE 12: OUTPUT TIMING DIAGRAM FOR REGULATED SERIAL.....................................................................18

FIGURE 13: ADC B LOCK DIAGRAM............................................................................................................................ 20

FIGURE 14 DEMODULATOR BLOCK DIAGRAM ........................................................................................................21

FIGURE 15: NOISE MEASUREMENT CIRCUIT...........................................................................................................23

FIGURE 16: NOISE ACCUMULATOR AS A FUNCTION OF SNR AND TIME............................................................ 24

FIGURE 17: V ITERBI D ECODER...................................................................................................................................25

FIGURE 18: REED SOLOMON DECODER....................................................................................................................29

FIGURE 19: TYPICAL SET TOP BOX DEMODULATOR............................................................................................ 35

FIGURE 20: MECHANICAL CONFIGURATION...........................................................................................................38

FIGURE 21: MECHANICAL CONFIGURATION...........................................................................................................40

FIGURE 22: ANALOG P IN CONNECTION....................................................................................................................41

FIGURE 23 : CLOCK GENERATION CIRCUIT...........................................................................................................41

FIGURE 24: I2C WRITE TO THE HDM8515..............................................................................................................46

FIGURE 25: I2C R EAD FROM THE HDM8515............................................................................................................47

FIGURE A1: SYMBOL TIMING RECOVERY TRANSIENT RESPONSE....................................................................... 67

FIGURE A2: CARRIER PHASE RECOVERY TRANSIENT RESPONSE ........................................................................68

FIGURE A3: CARRIER PHASE RECOVERY TRANSIENT RESPONSE WITH LOW SNR..........................................69

FIGURE A4: ADJACENT CHANNEL INTERFERENCE OF 10 DB, 1.35 SPACING....................................................72

FIGURE A5: PERFORMANCE WITH INTERFERER AT DIFFERENT CARRIER SPACINGS.....................................73

FIGURE A6: PERFORMANCE WITH +10 DB INTERFERER......................................................................................74

6

LIST OF TABLES

TABLE 1: ABSOLUTE MAXIMUM RATINGS ...............................................................................................................9

TABLE 2: DC C HARACTERISTICS.................................................................................................................................9

TABLE 3: D EMODULATOR SPECIFICATIONS...........................................................................................................10

TABLE 4: AC C HARACTERISTICS...............................................................................................................................10

TABLE 5: I NTEL 80C88A READ CYCLE TIMING PARAMETERS (BUSMODE = 1)................................................11

TABLE 6: I NTEL 80C88A WRITE CYCLE TIMING PARAMETERS (BUSMODE = 1).............................................12

TABLE 7: I NTEL 8051 READ CYCLE TIMING PARAMETERS (BUSMODE = 1)......................................................13

TABLE 8: I NTEL 8051 WRITE CYCLE TIMING PARAMETERS (BUSMODE = 1)...................................................14

TABLE 9: MOTOROLA READ CYCLE TIMING PARAMETERS (BUSMODE =0)....................................................15

TABLE 10 : MOTOROLA WRITE CYCLE TIMING PARAMETERS (BUSMODE =0).................................................16

TABLE 11: OUTPUT TIMING....................................................................................................................................... 17

TABLE 12: E XAMPLE OF ACQUISITION TIMING.....................................................................................................27

TABLE 13: I2C S LAVE ADDRESS..................................................................................................................................47

7

6

6

I

Q

Variabl

e

8Data ClockData

QPS

K

Loc

k

Nod

e

Sync44Symbol ClockViterbi Bit Clock

Frame

Sync8

Viterbi

Dat

a

I2C

DiSEqC

Interface

Interface

MCU

Interface

BER

Monitoring

PLL

C/N

Estimator

T

u

ner

Byte Sync

QPSK Lock

AIN_I

AIN_Q

WB_AGC

DISEQC

SCL_I2C

SDA_I2C

HI_ADDR[5:0]

HI_DATA[7:0]

XTAL1_IN

DATA_CLK

DATA[7:0]

QPSK_LOCK

FRAME_SYNC

Reference clk

AGC

AGC_Detector

1. Introduction to the HDM8515

The HDM8515 digital demodulator for direct broadcast satellite receivers is a single chip solution fully
compliant with the European Telecommunications Standards Institute (ETSI) specification ETS 300

421. This chip integrates an A/D converter, a variable rate matched filter, a variable rate QPSK
demodulator with a Viterbi decoder, a deinterleaver and a Reed Solomon decoder.

The HDM8515, which is implemented in a 0.25 micron CMOS, Four Layer Metal Process, provides
variable rate capability while operating with a fixed frequency sampling clock. Digital samples of
baseband I and Q data are generated by an internal A/D converter, then provided to the demodulator at
a fixed sample rate. The root raised cosine filter is implemented internally with fully digital techniques.
Similarly, the symbol timing recovery and carrier phase tracking functions are performed entirely in the
digital domain. This approach provides minimum constraints on external circuits, thus reducing overall
system costs.

The HDM8515 may be configured by an external processor for a specific symbol rate, and carrier
frequency along with loop gain parameters. The HDM8515 provides an external AGC signal which is
used to control the gain of the analog signal which is applied to the down-converters. And it also
provides a digital AGC internally which controls the gain of the signal out of the matched filters. In
addition, the HDM8515 provides fully programmable sweep circuitry to aid in initial acquisition when
large frequency offsets may be present.

The digital frequency translation capability of the HDM8515 permits this part to be used in frequency
multiplexing applications. In this application, an entire transponder bandwidth con taining many signals
is sampled at a fixed rate. The digital oscillator within the HDM8515 is programmed to the specific
desired carrier frequency within that band to permit the selected signal to be passed through the
baseband filter and processed by the demodulator circuits.

A/D

Converter

Rate

QPSK

Demodulator

Viterbi

Decoder

Synchronization

and

Deinterleaving

Reed

Solomon

Decoder

FIGURE 1: T OP LEVEL BLOCK DIAGRAM

8

1.1 Features and Benefits

* Fully DVB&DSS compliant
* Dual 6bit A/D converters
* Continuously variable symbol rate from 1Msps to 66Msps (90MHz clock)
* Internal digital root raised cosine filter
* Less than 0.5 dB implementation loss
* Frequency multiplexing capability
* Automated frequency search
* Internal bias cancellation
* Both wideband and narrowband AGC
* Noise calibration for antenna steering
* Output data rate as high as 82Mbps
* Fixed frequency sampling clock
* Simple interface with tuner and analog processing
* Microcontroller interface
* Eight bit parallel or I2C monitor and control interface
* I2C by -pass mode
* DiSeqC 1.2 interface support

* Dual Carrier Loop Filter

Part code Package

HDM8515P 100PQFP

9

2. Hardware Specification

Table 1: Absolute Maximum Ratings

Rating Value Unit
Ambient Temperature under Bias -10 to 70 c
Storage Temperature -65 to 150 c
Ambient Humidity under Bias 85( 85 c,500hrs) %
Thermal Resistance(Ja) 45 c/W
Junction Temperature 120 c
Voltage on Any Pin Vss - 0.3V to VDD + 0.5V V
VDD, IOVDD 4.5 V
Package Material - Compound : CEL -4630SX

Table 2: DC Characteristics

Symbol Parameter Min. Max. Units Test Conditions

IDD Dynamic Current - 390 mA VDD=2.7, Freq=90Mhz

IOVDD Interface Power Supply

Voltage

VDD Core Power Supply

Voltage

V ADC Powe r Supply

Voltage

VIL Input Low Voltage 0 0.3VDD V

VIH Input High Voltage 0.7VDD VDD+

VOL Output Low Voltage - 0.4 V IOL = 4 mA

VOH Output High Voltage 2.4 - V I

IIH Input High Current - 10 10 uA VIN =3.6, VDD =3.6

IIL Input Low Current - 10 10 uA VDD = 3.6, VIN =0

CIN Input Capacitance - 10 pF Typical 5.75pF

COUT Output Capacitance - 10 pF Typical 5.97pF

- Lead Frame : Copper

3 3.6 V Normal Operation

2.3 2.7 V Normal Operation

2.3 2.7 V Normal Operation

0.5

V

OH

= 4 mA

10

CLOCK

I_IN [5:0]

or Q_IN [5:0]

Table 3: Demodulator Specifications

Parameter Min. Max.

Sampling Clock Frequency 1MHz 90MHz
Analog Input Full Scale Range 0.9 Vpp 1.1 Vpp
Symbol Rate 1Msps 66Msps
Viterbi Data Rate - 90Mbps
Reed Solomon Data Rate - 82Mbps
Implementation Loss - 0.5 dB
Symbol Rate Resolution Clock/(220) ­Carrier Frequency Resolution Clo ck/(220) ­Acquisition Sweep Range - + or - Clock/2

Table 4: AC Characteristics

Symbol Parameter Min. Max. Unit

t

Input Data Setup before Clock 6 - ns

su1
th1 Input Data Hold after Clock 2 - ns

t

Low Pulse Width of Clock 8.7 - ns

pw1

t

High Pulse Width of Clock 8.1 - ns

pw2

tpw1

t

pw2

t t

su1 h1

F IGURE 2: INPUT DATA TIMING DIAGRAM

11

Valid

HI_ADDR [4:0]

/CE

/REDTACKHI_D

A

TA[7:0]

t

t

t

t

t

t

Table 5: Intel 80C88A Read Cycle Timing Parameters (Busmode = 1)

Symbol Parameter Min. Max. Unit

t

Input Address and /CE Setup before /RE Inactive 35 - ns

su1

th1 Input Address and /CE Hold after /RE Inactive 5 - ns

t

/RE Low Duration 200 - ns

pw1
td1 Delay from /CE to DTACK Active - 35 ns

t

Delay from /RE Inactive to DTACK in Tristate Mode - 10 ns

doz1

t

Delay from /RE Inactive to HI_DATA [7:0] Tristate Mode 10 - ns

doz2

pw1

d1

Z

su1

FIGURE 3: I NTEL 80C88A READ TIMING DIAGRAM

Note: HI_ADDR[4:0] is derived from the processor(80C88A) A15-A8 bus and HI_DATA[7:0] is
connected to the AD7 - AD0 bus.

#This page is only for HDM8515P.

h1

Z

doz1

doz2

12

Valid

HI_ADDR [4:0]

/CE

/WE

DTACKHI_DATA[7:0]

th1t

t

t

t

Table 6: Intel 80C88A Write Cycle Timing Parameters (Busmode = 1)

Symbol Parameter Min. Max. Unit

t

Input Data Setup before /WE Inactive 20 - ns

su1
th1 Inp ut Address, Data and /CE Hold after /WE Inactive 8 - ns

t

/WE Low Duration 200 - ns

pw1

td1 Delay from /CE to DTACK Active - 35 ns

t

Delay from /WE Inactive to DTACK in Tristate Mode - 15 ns

doz1

pw1

d1

doz1

su1

FIGURE 4: INTEL 80C88A WRITE TIMING DIAGRAM

Note: HI_ADDR[4:0] is derived from the processor(80C88A) A15-A8 bus and HI_DATA[7:0] is
connected to the AD7 - AD0 bus.

#This page is only for HDM8515P.

13

Valid

HI_ADDR [4:0]

/CE

/REHI_DATA[7:0]

t

t

t

t

t

Table 7: Intel 8051 Read Cycle Timing Parameters (Busmode = 1)

Symbol Parameter Min. Max. Unit

t

Input Address Setup before /CE Active 5 - ns

su1
th1 Input Address and /CE Hold after /RE Inactive 5 - ns

t

/RE Active Duration 400 - ns

pw1

t

Delay from /RE Active to HI_DATA [7:0] Valid - 40 ns

pd1

t

Delay from /RE Inactive to HI_DATA[7:0] Tristate Mode 10 - ns

doz1

FIGURE 5: I NTEL 8051 READ TIMING DIAGRAM

#This page is only for HDM8515P.

su1

pd1

h1

pw1

doz1

14

Valid

HI_ADDR [4:0]

/CE

/WE

HI_DATA[7:0

]

t

t

t

Valid

t

t

Table 8: Intel 8051 Write Cycle Timing Parameters (Busmode = 1)

Symbol Parameter Min. Max. Unit

t

Input Address and Data Setup before /WE Active 5 - ns

su1

th1 Input Address and Data Hold after /WE Inactive 5 - ns

t

/WE Active Duration 400 - ns

pw1

t

/CE Setup to /WE Active 5 - ns

su2

th2 /CE Hold after /WE Inactive 5 - ns

FIGURE 6: I NTEL 8051 WRITE TIMING DIAGRAM

#This page is only for HDM8515P.

su2

su1

h1

h2

pw1

15

Table 9: Motorola Read Cycle Timing Parameters (Busmode =0)

Symbol Parameter Min. Max. Unit

t

Setup Time of R/W with respect to /CE Active 5 - ns

su1

t

Address Setup with respect to /DS Active 5 - ns

su2

td1 Delay from DTACK Active to Data Valid - 30 ns
th1 R/W Hold with respect to /DS Inactive 5 - ns
th2 Address Hold with respect to /DS Inactive 5 - ns
th3 Data Hold with respect to /DS Inactive 10 - ns

HI_ADDR[4:0]

t

su2

/CE

/DS

t

su1

R/W

DTACK

HI_DATA[7:0]

FIGURE 7: MOTOROLA READ TIMING D IAGRAM

Note: External pull-up resistor is required on DTACK.

#This page is only for HDM8515P.

Valid

t

d1

t

h2

t

h1

t

h3

16

/CS

/DS

HI_DATA[7:0]

t

ttt

t

Table 10: Motorola Write Cycle Timing Parameters (Busmode =0)

Symbol Parameter Min. Max. Unit

t

Data Setup to /DS Active 5 - ns

su1

t

R/W Setup to /CS and Address 3 - ns

su2

td1 /DS Delay from R/W 5 - ns
td2 DTACK Delay from /DS Active - 40 ns
td3 DTACK Delay from /DS Inactive - 10 ns

t

/DS Active Duration 5 - ns

pw1
th1 Address, /CS and R/W Hold from /DS Inactive 5 - ns

th2 Data Hold from /DS Inactive 5 - ns

HI_ADDR[4:0]

su2

d1

R/W

DTACK

t

su1

FIGURE 8: MOTOROLA WRITE TIMING D IAGRAM

Note: External pull up resistor is required on DTACK.

#This page is only for HDM8515P.

Valid

d2

t

pw1

Valid

h1

d3

t

h2

17

1234n

n-1

n-2

n-3xxxxxxxxxxxxxxxx

xx

12348n-5

8n-6

8n-7

8n-8xxxx

xxxx8n-4

8n-3

8n-2

8n-1

8n

Table 11: Output Timing

Symbol Parameter Min. Max. Unit

tsu Output Data Setup before DATA_CLK and DATA_STB 5 - ns
thd Output Data Hold after DATA_CLK and DATA_STB 10 - ns

t

hd

DATA_CLK

DATA_STB

FRAME_SYNC

DATA_VALID

t

su

DATA

FIGURE 9: O UTPUT TIMING DIAGRAM FOR NORMAL PARALLEL

DATA_CLK

DATA_STB

FRAME_SYNC

DATA_VALID

DATA[0]

FIGURE 10: O UTPUT TIMING DIAGRAM FOR NORMAL SERIAL

NOTE : In case of DVB, n is 188
In case of DSS, n is 144

t

t

su

hd

18

123

4

n-1

n-2

n-3

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

xx

n

12348n-5

8n-6

8n-7

8n-4xxxxxxxxxxxxxxxx

xx

8n-8xxxx

8n-3

8n-2

8n-1

8n

DATA_CLK

DATA_STB

FRAME_SYNC

DATA_VALID

DATA

DATA_CLK

DATA_STB

t

su

FIGURE 11 : OUTPUT TIMING DIAGRAM FOR REGULATED PARALLEL

t

su

t

hd

t

hd

FRAME_SYNC

DATA_VALID

DATA[0]

F IGURE 12: OUTPUT TIMING DIAGRAM FOR REGULATED SERIAL

NOTE : In case of DVB, n is 188
In case of DSS, n is 144

19

3. Technical Overview

3.1 Dual Channel Analog to Digital Converter

The block diagram shown below illustrates internal configuration of the Dual Channel ADC.
Baseband signals, in -phase(I) and quadrature phase(Q), which are generated by down converters,
are applied to the dual channel ADC and quantized to 6-bit digital codes respectively. The ADC is
optimized to allow AC coupled inputs with full scale input range of 1V + or - 10%. An LSB weight is
approximately 15.6 mV.

The full scale input analog conversion range (Vpp) is determined by the voltag es of VTOP and
VBOT and simply equal to (VTOP - VBOT). The full scale range is defined as the voltage range that
accommodates 63 codes of equally spaced LSBs. Also the ADC supplies its own reference
voltages for A/D conversions. The voltages can be monitored by external reference pins. The
VTOP, VBOT represent top and bottom reference voltages respectively. REF_I, REF_Q represent
middle reference voltages for each channel. All these 4 reference voltage pins should be by -passed
to GND via 0.1uF capacitors. The values of internally generated voltage of VTOP and VBOT are

2.0V and 1.0V respectively. Vpp can be adjusted by externally applying voltages to both VTOP
and VBOT pins respectively when different conversion ranges are necessary. VTOP can be
adjusted as high as 2.3V and VBOT can be as low as 0.5V. A larger input range can be
established by taking VTOP higher and VBOT lower than on -chip generated voltages.

To supply necessary bias voltages for AC coupled applications, REF_I and REF_Q, which are
middle reference voltages for I and Q channel, are connected to the analog input pins (AIN_I and
AIN_Q ) respectively through 40 kohm resistors, as shown in the block diagram. For DC coupled
applications, these voltages can be used to feed back offset compensation signals.

To insure optimum performance, a low impedance analog ground plane is recommended and
should be separated from other digital ground planes. The analog power supplies should be by­passed at device to analog ground through 0.1uF ceramic capacitors.

20

AIN_Q

VBOT

CLOCK

REF_I

REF_Q

VTOP

AIN_I

6-bit ADC

Ref.

Voltage

Gen.

6-bit ADC

FIGURE 13: ADC B LOCK DIAGRAM

6

6

DI

DQ

21

3.2 Variable Rate Demodulator

The block diagram illustrates the overall configuration of the variable rate QPSK demodulator.
Baseband in-phase (I) and quadrature (Q) inputs are applied to the demodulator at a fixed sampling
rate. These digital samples are produced by A/D converters which employ AC coupling to minimize
DC offset.

QPSK Lock

Signal

Strength

Symbol

Tracking

Lock Detect

I_in
Q_in

First

Frequency

Trans.

Frequency

Sweeper

Dual FIR NB AGC

Second

Frequency

Trans.

Carrier

Tracking

I_out

Q_out

FIGURE 14 DEMODULATOR BLOCK DIAGRAM

The only significant change to this configuration over the HDM8513A is the addition of the Second
Frequency Translator. The carrier tracking block produces two outputs, one is the frequency
correction which is provided to the First Frequency Translator. This insures that the input to the
Dual FIR is always centered at zero frequency error, although there may be a phase error at this
point. The second output of the Carrier Tracking function provides the phase correction to the
Second Frequency Translator.

The carrier frequency error associated with these samples is removed digitally during tracking
operations by a complex multiplier and a digitally controlled oscillator, sometimes called a
numerically controlled oscillator (NCO). During initial acquisition, coarse frequency error is
removed by a combination of the digital AGC within the HDM8515 and external analog tuning
circuits.

A Dual filter performs the root raised cosine filtering of the frequency corrected baseband samples.
This filter, which implements the function of equat ion (1), is always configured to have an impulse
response duration of 8 symbols regardless of the programmed symbol rate. For low symbol rates,
a large number of samples are used, while for high symbol rates a relatively low number of samples
are processed for each filter output. The outputs of the daul filters are applied to a digital

22

narrowband AGC which insures that the signal is optimally scaled to the Viterbi decoder to an
accuracy of + or - 0.5 dB to insure optimum FEC performance.

y[k] = Σ h[n] x[k- n] (1)
In addition to optimizing performance of the Viterbi decoder, the digital narrowband AGC also

insures that the performance of the symbol timing and carrier tracking loops is independent of
signal level variations. An analog wideband AGC is also employed to insure that the analog signal
applied to the A/D converters is properly scaled.

Both the symbol timing and carrier tracking loops are implemented digitally, which eliminates the
need for external connections to analog tuning components during steady state operation. This
causes the requirements on the analog presampling filter to be relaxed, permitting a lower cost
analog front end. For systems which require a narrow band presampling filter, and have the
potential for significant frequency error in the LNB (several MHz) the HDM8515 provides a high
resolution measure of carrier frequency to permit periodic readjustment of the front end tuner
frequency to compensate for drift. The host processor periodically reads the frequency register,
then computes appropriate correction to the tuner frequency.

The nominal symbol rate and the nominal carrier frequency are programmed into the demodulator
to an accuracy provided by 20 bits of resolution, and the system accuracy is equivalent to that of
the fixed frequency sampling clock.

During initial acquisition, the HDM851 5 provides an automated sweep program to facilitate carrier
acquisition. The host processor loads a 20 bit register which determines the initial carrier
frequency. A 16 bit regist er is programmed with the number of symbol times the receiver will dwell
at each frequency. If the receiver remains at the initial frequency for the programmed number of
symbol times without achieving lock, the carrier frequency is incremented by the step frequency
value programmed into another 16 bit register. If no lock is achieved, the receiver will continue to
increment the frequency until the maximum number of search frequencies, as determined by the
value in an 8 register, is achieved. When the maximum number of search frequencies is reached,
the carrier frequency returns to the initial value and the entire process is repeated. Once the host
processor determines that lock is achieved by observing the lock flag, it then inhibits the sweep
function and programs loop bandwidth parameters which are optimized for steady state
performance.

23

AbsoluteValue

255256816

8

AbsoluteValue

2552568168In PhaseComponent

Average MagnitudeInstantaneousDeviation

Deviation

8168In PhaseComponent

-

3.3 Noise Measurement Circuit

When the DBS system is being installed in any place, the most difficult part of the installation is
accurate pointing of the antenna toward the satellite. Inaccurate pointing results in loss of margin
and greater potential for outages in adverse weather conditions. Existing systems use information
from the demodulator forward error correction circuits to provide a measure of anten na pointing.
Unfortunately, this method is useful over a range of only several dB above system threshold.

The HDM8515 employs a unique circuit for accurate measure of signal strength over a 20 dB range
of signal to noise ratio. This method, illustrated in the block diagram, makes use of the fact that
the demodulator provides 8 bits of resolution for each of the quadrature output components. This
high resolution provides a means of measuring the noise component with great accuracy.

The eight bit in-phase demodulator filter output is detected by an absolute value circuit, then
passed through an IIR to provide a measure of average signal amplitude. Each sample is then
subtracted from this average amplitude to provide an instantaneous noise sample. The absolute
value of these noise samples are then averaged by a second IIR to provide a measure of the noise
which is roughly proportional to the noise power and inversely proportional to signal to noise ratio.

Finally, the Figure 16 illustrates the results of simulations under different noise conditions. This
figure illustrates that for signal- to-noise ratio as high as 19 dB, the noise measurement circuit
provides a meaningful measure of signal power with worst case resolution of 1 dB.

R

FIGURE 15: NOISE MEASUREMENT CIRCUIT

Average

R