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DAA: A Hybrid Design Program for the p-A212AI AT and p-A V22 National Semiconductor Application Note 515 Gary Shapiro » z • UI .... UI The attached listing is for an interactive program (DAA) which performs calculations related to the design of an active line hybrid (2-wire/4-wire converter) with minimized talker echo (reflected transmit energy) for modems utilizing the J.'A212A, J.'A212AT and J.'AV22 single-chip modem IC's. Two design examples are included. The program is written in IBM Basic and will run on the IBM PC and PC-compatibles; a printer is normally required. This version supplants previous versions and includes provision for primary surge resistors and insufficient transmitter drive, and a different set of line termination impedances over which to optimize performance. The program employs a simple "T" model for the line coupling transformer and presumes a near-unity coupling coefficient for the transformer windings; distributed capacitances are not considered. The model and its terminations are shown in Figure 1. Required keyboard inputs are primary and secondary DC resistance (Rpri, Rsec), secondary selfinductance, turns ratio (N), "surge" (external primary) resistance (Rsurge), desired nominal modem input impedance (Rnom) and the frequency at which this impedance is to be initially provided (Fnom). "Primary" refers to the line side and "secondary" to the modem side of the transformer. Turns ratio is from secondary to primary; if not provided, it can be measured as either the open-circuit voltage transfer ratio or the square root of the ratio of the open-circuit primary and secondary winding (self-) inductances. Winding inductances are usually not provided and should be measured on a bridge. Surge resistance includes resistors added to meet requirements for minimum input DC resistance (e.g. 100n for Canada) or the on-resistance of a solid-state relay; the entered value is subsequently lumped with the primary resistance as Rpri'. AC input impedance is unspecified in FCC Part 68, but is specified as 1600nl ± 20% by EIA RS-496, and is specified by other countries. Fnom is normally chosen in the center of the band of interest (Le. 1800 Hz for a 212A), but can be offset, for example, to compensate for the low frequency roll off of a small transformer. DAA displays the secondary termination (RO, CO) providing the specified Rnom at Fnom, a frequency sweep of input impedance, and values for Insertion Loss: Voltage Gain (dB) and Transducers Gain (dB). Voltage Gain is the signal gain or loss in the receive path; Transducer Gain is insertion (power) gain or loss in the transmit path; they are not equivalent. The user is then offered the choice of specifying RO and CO, which can be used to shape the input impedance or to enter a standard value for CO. If this option is selected, the new impedance sweep is accompanied by a relative insertion loss sweep. DAA then calculates the effect of the transformer (and CO in shunt) on a set of 14 values of loop input impedance (Rline + jXline), over which talker echo is to be minimized. These values were selected from scatter plots of data from the AT&T 1977 Loop Surveyl, using the following rationale. Most of the presented data were taken at 1 kHz and 3 kHz, but 3 kHz is out-of-band for 212AIV.22 modems; 3 kHz scatter data were therefore discarded in favor of median data points at 2000 Hz and 2500 Hz, which are in-band for answer mode. Also, originate (calling) mode performance is demonstrably more critical than answer mode for fixed-equalized 212AIV.22 modems; the 1 kHz data are therefore favored by being 10 of the 14 values. The 1 kHz data also favor non-loaded loops, which are both a majority (-80%) and statistically more tightly grouped than loaded loops. Note that all chosen points are capacitive, that 600 + jOn is not included, and that the user can indulge his own rationale by altering program lines 1500-1520. DAA next calculates and displays the relative transmit path gain (Kt) providing minimum mean gain (meanGd8) from transmit output to receive input (talker echo) for the above set of 14 line impedances, as well as the talker echo (Gd8) for each point. The equivalent circuit is shown in Figure 2. The RC compensator in the transmit path has a -3 dB corner (Fc) at 1 MHz; thus, the initial calculation is essentially uncompensated and serves as a reference. One performs subsequent iterations by choosing new values of Fc-usual Iy between 5 kHz and 10 kHz-and noting both meanGdB and the spread of GdB. Note that talker echo is cancelled by subtraction and that these calculations employ simple models for both transformer and line. Therefore, calculated values of GdB are essentially not resolvable below - -20 dB. Varying Fc will yield broad, non-critical minima for meanGdb and GdB. Finally, DM proceeds to the hybrid design, asking for transmit drive level at the TXO pin (Vt) in rmslV, output power to a 600n load (dBm), and net receive path gain (dB). Vt is 0.71 Vrms for the J.'A212AT/J.'AV22 and net receive gain of + 4 dB will provide nominal threshold levels. Corrections to GdB and meanGdB which include receive path gain provide absolute talker echo figures. The specification of output power should consider tolerances: e.g. the Part 68 specification for Permissive mode is not to exceed - 9 dBm. High turns ratio and/or high output power (e.g. 0 dBm) may require a drive level in excess of VI: DAA will state the required level, which can be provided with the second half of a dual op-amp. Figure 3 shows the completed hybrid corresponding to the equivalent circuit of Figure 2. Ra and Rb from RO. R should be large enough not to affect Ra, Rb. R4 and R5 form R': the group R4, R5, C1 may be scaled, if desired. Two design examples are included with the program listing. The first utilizes a Tamura TTC-143 coupling transformer and is similar to the design used in the J.'A212K/TK and J.'AV22K Designer's Kits-demonstration and evaluation modem boards for the J.'A212A1 AT and J.'AV22. The second uses a passive hybrid transformer with N = 1.5 and demonstrates the inclusion of surge resistance and provision for insufficient drive. 1 L.M. Manhire, Physical and Transmission Characteristics of Customer Loop Plant, BSTJ, Vol. 57, No.1, Jan. 1978, pp. 35-59. 5-37
.... ~ ~ U) r---------------------------------------------------------------------------------~ Termination Transformer Line Ideal ~G::3JiIL~'h Rpr!' = Rpr! + Rsurge FIGURE 1. Transformer Model and Terminations Tl/H/9442-1 ...----+RXIN Kt C .D FC=I/2ri·C .--... 'VV~.-....... vt FIGURE 2. Hybrid Functional Equivalent Circuit TL/H/9442-2 ,....--4~-... RXIN 110- I I I FIGURE 3. Complete Une Hybrid TL/H/9442-3 5-38
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~ National ~ Semiconductor 400014 R
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TRADEMARKS Following is the most cu
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II) c ~ ~National ~ ~ Semiconductor
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Table of Contents (Continued) Secti
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Alpha-Numeric Index(continUed) DS36
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Section 1 Local Area Networks IEEE
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~National ~ Semiconductor DP8390C/N
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3.0 Functional Description (Continu
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5.0 Pin Descriptions (Continued) BU
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6.0 Direct Memory Access Control (D
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PAL20LB Decode and DMA Interface Lo
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Although Cheapernet is intended for
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The transmitted packet from the SNI
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Truth Table for Various Collision D
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CHEAPERNET APPLICATION WITH THE DP8
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DP8390 Network Interface Controller
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ping between the received address a
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If the current local DMA address ev
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6.2 Dual Port Memory One popular me
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StarLAN With The DP839EB Evaluation
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SUPPORTING DOCUMENTS The following
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Pulse Transformer U3 .----. j " "P>
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ELECTROSTATIC DISCHARGE TEST (ESD)
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Section 2 Contents DP8340INS32440 I
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.. 0 C'I C") en .... z 0 C") GO Go
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Timing Characteristics Oscillator F
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Timing Waveforms (Continued) TA REG
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.. ... ~ ~National ~ ~ Semiconduc
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Detailed Functional Pin Description
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Message Format (Continued) DATA REC
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Timing Characteristics (Notes 2, 6,
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Timing Waveforms (Continued) 4T±(T
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Timing Characteristics (Continued)
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Functional Timing Waveforms (Contin
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MUX~ __________________ The BIPLAN
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An EPROM implementation was selecte
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MASTER MUL TIPLEXING/DE-MUL TIPLEXI
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AN·496
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Decoding Bit Fields with the JRMK I
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Receiver Interrupts/Flags for the D
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****************DA ISR*************
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"Interrupts"-A Powerful Tool of the
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TABLE II. (lCR) Interrupt Mask Bits
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Running the DP8344 with wait states
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WHY EPROM SOFT-LOAD? In a stand-alo
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The roll-off frequency, Fro, should
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00044 F90a :J- 226 Z • 227 MOVE A
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Section 3 ISDN Components
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Introduction To National Semiconduc
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NSC Solutions for Layers 2 and 3 Na
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NSC Solutions: Systems Level Basic
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~National ~ Semiconductor PRELIMINA
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ISDN DEFINITIONS "B" Channel, or DS
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3.0 AC Electrical Characteristics T
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3. The Rx FIFO will hold 16 bytes r
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NS16550A E-J r-o ..... IRQ4 ICU CPU
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TITLE 550APP.ASM - NS16550A INITIAL
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3.0 Board to Board Communications w
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DIAPPS.ASM Flow Chart l> z • ~ CO
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» z I "" CD RDAI: DISABLE IRTS, SE
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.set iCu imsk,lO *aO • set icu -c
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holdloop: nop # br hold loop # # #*
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# msint: save [rO,rl,r2,r3] # movd
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#3/30/B7••••• D2APPS.ASM
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# #*************************** 1655
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A Comparison of the I NS8250, NS 16
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cessed (see Figure 4), The output o
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AC Electrical Characteristics T A =
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~National ~ Semiconductor \ J micro
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1.0 Absolute Maximum Ratings 2.0 Op
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4.0 AC Electrical Characteristics (
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Page 525 and 526: Applications Information (Continued
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Section 7 Physical Dimensions
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~National ~ Semiconductor All dimen
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NS Package N16A N16A(REV E) 0.092 X
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NOTES
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~National ~ Semiconductor Bookshelf
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RELIABILITY HANDBOOK-1986 Reliabili
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NATIONAL SEMICONDUCTOR CORPORATION
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NATIONAL SEMICONDUCTOR CORPORATION
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