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RC2798
Integrated QAM IF Downconverter
Features
* * * * * * * * RF input frequency range 30 to 250MHz On chip VCO with LO frequency range 30 to 250MHz IF amplifier with AGC setting High dynamic range -9dBm IIP3 On chip Video Amplifier Built in ESD protection Supply voltage range 5 to 10 V Space saving 20-Lead TSSOP package
Description
The RC2798 is an integrated solution for the downconversion of QAM IF signals in the front-end design of cable modem and set-top receivers. It is intended for use in 64QAM and 256QAM IF downconversion applications. The RC2798 integrates IF amplifier with AGC , mixer, VCO, and a video amplifier on a single chip. It accepts the QAM IF signals via SAW filter and downconverts it to 5MHz baseband signal. The baseband signal can be digitized using Fairchild Semiconductor's 8 bit A/D (TMC1175 series) or 10bit A/D (TMC1185 series) and decoded further with a QAM demodulator. The IF, Oscillator and Mixer section work at 5V. The video amplifier works at 5V to 10V. The RC2798 is available in a 20 Lead TSSOP package.
Applications
* * * * * Digital Set-top receivers Cable modems Internet surfboards Network Interface Modules Multimedia PCs
Block Diagram
AGC_IN1 AGC_IN2 VAGC VCC MIX OSC_OUT GND B2 C1 C2 1 2 3 4 5 6
LO
RC2798
20 GND 19 MIX_OUT2 18 MIX_OUT1 17 G1A 16 G1B 15 VampIn1 14 VampIn2 13 VccAmp 12 OUT1
VideoAmp
AGC + MIX
7 8 9
B1 10
11 OUT2
Rev 1.0.0
PRELIMINARY INFORMATION describes products that are not in full production at the time of printing. Specifications are based on design goals and limited characterization. They may change without notice. Contact Fairchild Semiconductor for current information.
RC2798
PRODUCT SPECIFICATION
Absolute Maximum Ratings (TA = 25C unless otherwise specified)
Parameter Video Amplifier 5V Operation Vcc_Mix Supply voltage 1 (for AGC amplifier, oscillator, and mixer) AGC amplifier, oscillator, and mixer block Video amplifier block TA = 85C1 -40 -55 AGC amplifier, oscillator, and mixer block Video amplifier block TA = 75C1 -40 -55 6 6 430 +85 +150 6 11 500 +75 +150 V V mW C C V V mW C C Test conditions Min. Typ. Max. Unit
VccVamp Supply voltage 2 ( for video amplifier) PD TA Tstg Vcc_Mix Power dissipation Operation temperature range Storage temperature range Supply voltage 1 (for AGC amplifier, oscillator, and mixer)
Video Amplifier 9V Operation
VccVamp Supply voltage 2 ( for video amplifier) PD TA Tstg Power dissipation Operation temperature range Storage temperature range
Notes: 1. Mounted on 50 X 50 X 1.6mm double epoxy glasss board.
Recommended Operating Range
Parameter Vcc_Mix VccVamp Ta1 Ta2 Supply voltage 1 (for AGC amplifier, oscillator, and mixer) Supply voltage 2 ( for video amplifier) Operation temperature range Operation temperature range 11 22 Min. 4.5 4.5 -40 -40 Typ. 5.0 5.0 +25 +25 Max. 5.5 10.0 +85 +75 Unit V V C C
Notes: 1. @ Vcc_Mix = VccVamp = 4.5 to 5.5V 2. @ Vcc_Mix = 4.5 to 5.5V, VccVamp = 4.5 to 10.0V
2
PRODUCT SPECIFICATION
RC2798
Electrical Characteristics (TA = 25C )
Parameter Icc1 fRF fOSC fIF CGMAX CGMIN GCR NF VAGC H VAGC L Icc2 VOUT G1 G2 Supply current 1 RF input frequency range OSC frequency range IF output frequency range Maximum conversion gain Minimum conversion gain AGC dynamic range Noise figure AGC voltage high level AGC voltage low level Supply current 2 Output voltage Differential gain 1 Differential gain 2 VAGC = 4.0V VAGC = 1.0V VAGC = 1.0 to 4.0V SSB, VAGC = 4.0V At maximum gain At maximum gain At minimum gain No input signal RL = 1KW, differential G1A-G1B pins: short, VOUT = 3Vp-p G1A-G1B pins: open, VOUT = 3Vp-p no input signal RL = 1KW, differential G1A-G1B pins: short, RL = 2KW G1A-G1B pins: open, RL = 2KW G1 G2 G1 G2 300 25.0 150 22.0 7.0 12.5 3.0 200 26.0 250 30.0 4.0 1.0 17.0 24 Test conditions no input signal Min. 17.0 30 30 DC 25 -7 32 9 40 Typ. 23.0 Max. 31.0 250 250 150 Unit mA MHz MHz MHz dB dB dB dB V V mA Vp-p V/V V/V AGC Amplifier, Oscillator, and Mixer Blocks (Vcc = 5V)
Video Amplifier Block (Vcc = 5V)
Video Amplifier Block (Vcc = 9V) Icc2 VOUT G1 G2 Supply current 2 Output voltage Differential gain 1 Differential gain 2 18.0 24.0 3.0 385 28.5 470 32.0 32.0 mA Vp-p V/V V/V
Video Amplifier Block (Vcc = 5V or 9V) BWG1 BWG2 Rin1 Rin2 Cin Bandwidth 1 Bandwidth 2 Input resistance 1 Input resistance 2 Input capacitance 50 50 3.5 7.5 1.6 MHz MHz KW KW pF
Standard Characteristics (VCC = 5V, TA = 25C)
Parameter AGC Amplifier Block (Vcc = 5V) AGC IIP3 AGC input intercept point At minimum gain (AGC amplifier + mixer) -9 dBm Test conditions Min. Typ. Max. Unit
Video Amplifier Block (Vcc = 5V or 9V) CMRR PSRR tr tPD Common mode rejection ratio Power supply rejection ratio Rise time Propagation delay time 80 70 2.6 4.4 dB dB nS nS 3
RC2798
PRODUCT SPECIFICATION
Typical Characteristics
30 Conversion gain - dB 25 20 15 10 5 0 -5 -10 0 5.00 E-01 1 1.5 2 2.5 3 3.5 4 4.5 5 conv. gain Series1
AGC control - Volts
Figure 1. AGC Control Characteristics
4.50E-02 4.00E-02 3.50E-02 3.00E-02 Supply current - A 2.50E-02 2.00E-02 1.50E-02 1.00E-02 5.00E-03 0.00E+00 -5.00E-03 0 4.00 8.00 E-01 E-01 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2 5.6 6 Video Amp AGC amp + Mixer + LO Total
Supply voltage - Volts
Figure 2. Supply Current vs. Supply Voltage
4
PRODUCT SPECIFICATION
RC2798
10nF RF input 51 10K VAGC (1.0-4.0V) Vcc_Mix(+5V) + 10F 0.22F FB 0.1F 10F + 10K
AGC_IN1 1 AGC_IN2 2
RC2798
GND 20 MIX_OUT2 10nF 19 MIX OUT 1K MIX_OUT1 18 10nF 1K
10nF VAGC 3
AGC + MIX
0.1F
VCC MIX 4
G1A 17
Oscillator O/P
OSC_OUT 5 10nF GND 6 LO B2 7 10nF C1 8 C2 9
G1B 16 VampIn1 15
VampIn2 14 VccAmp 13 OUT1 12 VideoAmp OUT2 11
Ext. LO I/P 51 10nF
B1 10
Figure 3. Measurement Circuit 1--AGC + MIX Block
5
RC2798
PRODUCT SPECIFICATION
AGC_IN1 1 AGC_IN2 2
RC2798
GND 20
MIX_OUT2 19 AGC + MIX
VAGC 3
MIX_OUT1 18
VCC MIX 4
G1A 17
OSC_OUT 5 GND 6 LO B2 7 C1 8 C2 9 B1 10 VideoAmp
G1B 16 VampIn1 15
0.01F VampIn1
VampIn2 14 VccAmp 13 OUT1 12 OUT2 11 10F
0.01F VampIn2 FB + 0.1F 10F + 0.1F OUT1 10nF OUT2 10nF
VccVamp (+5 or +9V)
Figure 4. Measurement Circuit 2--Video Amplifier Block
6
IF INPUT OUT+ R5 29 200 1% GND R6 200 1% R7 1K1% 5.6H C14 56pF L2 C19 1 C17 0.01F 0.01F C13 56pF 18 17 16 15 14 13 12 11 C26 15pF C16 82pF GND C18 0.01F C15 82pF 5.6H C11 0.1F C12 0.1F L1 GND OUT3 VAGC MIX_OUT1 G1A G1B VampIn1 VampIn2 VccAmp OUT1 OUT2 C25 0.01F VCC MIX OSC_OUT GND C3 R3 10K 4 5 GND FB1 6 + C6 10F GND C24 0.01F GND C7 0.1F GND FB + C5 10F GND 0.22F 4 MIX_OUT2 5 GND 20 GND
F1 SAWFILTER RC2798
PRODUCT SPECIFICATION
J1
1
1
IN+
2 x6964
R1 51
2
IN-
C1 GND 1 AGC_IN1 10nF C2 2 AGC_IN2 10nF
GND
3
GND
IN_GAIN
J3 VAMPIN2_TP R9 2 51 GND
Applications Discussion
GND
R2
GAIN CONTROL INPUT (1.0 - 4.0V) +5 P1
10K
+5V IN
C4 0.1F
R8 1K1% FB2 FB +
GND
OSC_OUT
GND
+9
P2 +9V IN + C20 0.1F C21 10F GND GND R10 C22 10F GND J4 1 1.0K BB_OUT R12 49.9 2 GND C23 0.1F GND
C8 0.01F C30 D1 MMBV809 R15 20K L3 1.2H C10 100pF C9 J2 1 EXT. LO I/P 2 1000pF GND GND GND GND R4 51 GND C29 1000pF 1000pF
OSCILLATOR OUTPUT
7 B2 C31 15pF 8 C1 C33 10pF 9 C2 C32 15pF 10 B1
VCOC D2 MMBV809
R18
R17
Figure 5. Application Circuit
R16 20K
TP_VCOC
20K
C28 0.1F
20K
GND
GND R11 OUT2_TP 1.0K R13 49.9 GND
Notes: 1. For self oscillation, do not load C9, and C10. 2. Fro external injection (VCO), do not load C28, C29, C30, C31, C32, C33, L3, D1, D2, R15, R16, R17, and R18. 3. For down convesion with video amplifier, do not load C19. 4. For using video amplifier only, do not load C17, C18, C16, C26, and change C15 to 0.1F.
RC2798
7
RC2798
PRODUCT SPECIFICATION
Crystal Oscillator Implementation
AGC_IN1 1 AGC_IN2 2 RC2798 GND 20
MIX_OUT2 19
VAGC 3
MIX_OUT1 18
+5V + 10F Oscillator O/P
FB + .1F 10F .1F
VCC MIX 4
G1A 17
OSC_OUT 5 GND 6
G1B 16 VampIn1 15
220pF
B2 7 C1 8 C2 9 B1 10
VampIn2 14 VccAmp 13 OUT1 12 OUT2 11
XTL 38.886 MHz Fundamental mode Series Resonant
Figure 6. Fundamental Mode--Series Resonant XTL
AGC_IN1 1 AGC_IN2 2
RC2798
GND 20
MIX_OUT2 19
VAGC 3
MIX_OUT1 18
+5V + 10F Oscillator O/P
FB + .1F 10F .1F
VCC MIX 4
G1A 17
OSC_OUT 5 GND 6
G1B 16 VampIn1 15
B2 7 220pF C1 8 C2 9 B1 10 Lo 180nH
VampIn2 14 VccAmp 13 OUT1 12 OUT2 11
XTL 38.886 MHz
10nF Co 93pF
Figure 7. Overtone Mode--Series Resonant XTL
8
PRODUCT SPECIFICATION
RC2798
Overtone Mode--Series Resonant XTL
If it is desired to operate a XTL at non-fundamental or overtone frequency, an AC coupled parallel resonant network should be connected to feedback input pin, B1. The typical impedance looking into B1 with B2 AC grounded is approximately Rin =1KW @ 38MHz. It is recommended to design the value of Qo at approximately 15 to 25. The Lo and Co values can be calculated from the following equations:
Qo = wo Co Rin wo = 2 p fo = (1/LoCo)1/2 The XTL is a series resonant type and it is operated at third overtone frequency.
AGC_IN1 1 AGC_IN2 2
RC2798
GND 20
MIX_OUT2 19
VAGC 3
MIX_OUT1 18
+5V + 10F Oscillator O/P
FB .1F 10F + .1F
VCC MIX 4
G1A 17
OSC_OUT 5 GND 6
G1B 16 VampIn1 15
B2 7 220pF C1 8 C2 9 B1 10
VampIn2 14 VccAmp 13 OUT1 12 OUT2 11
Cc 15pF for Fundamental 2pF for Overtone XTL 38.886 MHz Parallel Resonant
Figure 8. Fundamental or Overtone Mode--Parallel Resonant XTL
Fundamental or Overtone Mode- Parallel Resonant XTL
Figure 8 shows the implemenation of parallel resonant XTL at fundamental or overtone frequency. The XTL is a parallel resonant type and can be operated at either fundamental or third overtone frequency depending upon the feedback capacitor, Cc. When used with Cal Crystal Lab's XTL, P/N#CCL-6-38.8860G153, for Cc = 15pF it operates at fundamental mode and for Cc = 2pF, it operates at third overtone mode (38.886MHz).
For symmetrical reasons, the following design is recommended for better duty cycle (50 to 50%) output from VCO (see Figure 9).
9
RC2798
PRODUCT SPECIFICATION
AGC_IN1 1 AGC_IN2 2
RC2798
GND 20
MIX_OUT2 19
VAGC 3
MIX_OUT1 18
+5V + 10F Oscillator O/P
FB .1F 10F + .1F
VCC MIX 4
G1A 17
OSC_OUT 5 GND 6
G1B 16 VampIn1 15
B2 7 Cc XTL 38.886 MHz Parallel Resonant Cc Cc: 15pF for Fundamental 2pF for Overtone C1 8 C2 9 B1 10
VampIn2 14 VccAmp 13 OUT1 12 OUT2 11
Figure 9. Fundamental or Overtone Mode with Improved Duty Cycle--Parallel Resonant XTL
10
PRODUCT SPECIFICATION
RC2798
Package Dimensions
20-pin TSSOP package
Inches Min. A A1 A2 B C D E E1 e L N a ccc -- .002 .031 .007 .004 .250 .240 .168 Max. .047 .006 .041 .012 .008 .257 .264 .176 Millimeters Min. -- 0.05 0.80 0.19 0.09 6.40 6.10 4.30 Max. 1.20 0.15 1.05 0.30 0.20 6.60 6.70 4.50 5 5 2, 4 Notes: Notes 1. Dimensioning and tolerancing per ANSI Y14.5M-1982. 2. "D" and "E1" do not include mold flash. Mold flash or protrusions shall not exceed .010 inch (0.25mm). 3. "L" is the length of terminal for soldering to a substrate. 4. Terminal numbers are shown for reference only. 5. "B" & "C" dimensions include solder finish thickness. 6. Symbol "N" is the maximum number of terminals.
Symbol
.026 BSC .018 .029 20 0 -- 10 .004
0.65 BSC 0.45 0.75 20 0 -- 10 0.10
3 6
D
E1 E
A2 A1 SEATING PLANE -C- LEAD COPLANARITY ccc C a L C
A B e
11
RC2798
PRODUCT SPECIFICATION
Ordering Information
Product Number RC2798G Package 20 pin TSSOP
LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user.
www.fairchildsemi.com 5/20/98 0.0m 001 Stock#DS30002798 O 1998 Fairchild Semiconductor Corporation
2. A critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.

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