APPLICATION NOTE #113 Transceiver Input Impedance APPLICATIO

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APPLICATION NOTE #113 Transceiver Input Impedance APPLICATIO

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CIRCUIT TECHNOLOGY
APPLICATION NOTE #113
Transceiver Input Impedance
APPLICATION NOTE #113
Released 11/99
INTRODUCTION
This application note will outline procedure find transceiver input impedance when power removed when power transmit inhibited state. looks transceiver manufacturer's data sheets, this most confusing specifications user. Some manufacturers transceivers call this specification "Transmitter Output Resistance", which not, while other manufacturers give "Receiver Input Resistance" "Transmitter Output Resistance" with condition that transmitter transmitting (inhibited) expect user figure that means power what frequency measured. Very manufacturers state measurement condition even refer MIL-STD-1553B specification. This most under-specified parameters data sheet, and, vague best relates MIL-STD-1553B specification.
MIL-STD-1553 Specification
specification refers transceiver input impedance rather terminal input impedance which transceiver only component. other component "Isolation Transformer" that connects transceiver bus, either directly through transformer coupled stub. applicable paragraphs from MIL-STD-1553B 4.5.2.1.2.3 "Input Impedance" terminals with transformer coupled stubs 4.5.2.2.2.3 "Input Impedance" terminals with direct coupled stubs. Both specifications combined read, "the magnitude terminal input impedance, when transmitting, power removed, shall minimum 1000 ohms (transformer coupled stub) 2000 ohms (direct coupled stubs) within frequency range 75.0kHz 1.0MHz. This impedance that measured line-to-line point Figure (transformer coupled stub) Figure (direct coupled stubs). Figure from MIL-STD-1553B, reproduced here Figures convenience. order determine input impedance transceiver combined with isolation transformer will make terminal input impedance specification, stated above, must know transceiver's input impedance well transformer's self impedance specified frequencies. transceivers input impedance made components; transmitter's input impedance when power removed when transmitting (standby). receiver's input impedance when power removed standby. Since MIL-STD-1553B specification terminal input impedance, transceiver's transmitter receiver's input impedances represent parallel impedance mutually exclusive, some manufacturer's data sheets would suggest. input impedance specification Isolation Transformer, taken from transformer manufacturer's data sheet. almost cases this impedance specification 4000 ohms minimum, frequencies between 75KHz 1MHz. Therefore method measuring transceiver's input impedance needs determined.
Transceiver Input Impedance
method, that will yield accurate results, impedance analyzer. 4192A Impedance Analyzer will used determine transceiver's input impedance over frequency band interest. feature this analyzer, ability "zero" reference null circuit point, will used make this measurement. purposes transceiver input impedance measurement, either Figure Figure used.
APPLICATION NOTE #113
Released 11/99
Databus Interface Using Transformer Coupling Figure MIL-STD-1553B
Terminal TRANSCEIVER UNDER TEST DATA DATA Isolation Transformer Coupling Transformer 1.414 Isolation Resistors 0.75 Data Wire Pair
DATA DATA
Terminal Input Impedance MIL-STD-1553B para 4.5.2.1.2.3 minimum. 75KHz 1MHz Transceiver Input Impedance
FIGURE
Databus Interface Using Direct Coupling Figure MIL-STD-1553B
Terminal TRANSCEIVER UNDER TEST DATA DATA Isolation Transformer
DATA DATA
Data Wire Pair
Terminal Input Impedance MIL-STD-1553B para 4.5.2.2.2.3 minimum. 75KHz 1MHz Transceiver Input Impedance
FIGURE
APPLICATION NOTE #113
Released 11/99
Transceiver Input Impedance (con't)
results will same either circuit configuration; simply which method more convenient user. will circuit Figure this measurement analyzer measure magnitude impedance Display angle theta Display (set radian readout), frequency 1MHz. This frequency chosen, even thought specification frequency between 75KHz 1MHz, because represents worst case impedance. 4192A analyzer leads placed across circuit points which would Terminal's input/output. With transceiver removed from circuit, analyzer's inputs "zeroed" open circuit condition. Then short circuit placed across circuit points analyzer again "zeroed" short circuit condition. transceiver under test inserted analyzer measure impedance transceiver directly. Isolation transformer' self-impedance been zeroed measurement only transformer parameter that remains measurement transformer turns ratio. Once transceiver impedance 1MHz known, then equivalent Resistor/Capacitor parallel circuit found that represents transceiver's input impedance (parallel combination transmitter receiver), frequency. Figure represents Transceiver equivalent circuit. Terminal input impedance found power power on-not transmitting inhibit) cases.
Transceiver Input Impedance, Resistor Capacitor Equivalent Circuits
Terminal TRANSCEIVER UNDER TEST Isolation Transformer
Data Wire Pair
Terminal Input Impedance MIL-STD-1553B para 4.5.2.2.2.3 minimum. 75KHz 1MHz Transceiver Input Impedance
FIGURE
APPLICATION NOTE #113
Released 11/99
Measurement Calculation
following formulas apply circuit Figure with following definitions: ZSEC Magnitude Impedance seen looking into points ZPRI Magnitude Impedance seen looking into points Transformer turns ratio (Secondary turns Primary turns) Equivalent Input Capacitance Equivalent input resistance Radian frequency interest omega Angle ZSEC radians theta
Zpri Zsec Equation
Equation
-tan
Zpri
Equation
magnitude ZSEC angle, theta, read directly from HP4192A analyzer displays. Then ZPRI calculated. With ZPRI known then calculated. analyzer measurements done power condition then power on-transmitter inhibited, condition. Each will yield different parallel circuit. With equivalent circuit minimum self-impedance Isolation Transformer, minimum Terminal input impedance calculated frequency between 75KHz 1MHz. Alternately measure magnitude angle with analyzer frequency between 75KHz 1MHz. must "zero out" transformer self-impedance frequency measured Transceiver's input impedance then apply Equation calculate Transceiver's input impedance frequency between 75KHz 1MHz, once parallel network known, following formula used:
1-Cin Zpri
Equation
atan
Equation
minimum Terminal impedance then calculated each frequency with following formula: Minimum Terminal input impedance ZSEC (ZPRI TR2) 4Kohms, angle theta. Where ZPRI Transceiver's calculated impedance frequency theta calculated angle ZSEC transformed primary impedance. sample calculations follow using 4487 4454 hybrids. 4487 transceiver input supply utilizes transformer with 1.414:1 turns ratio direct coupled stubs turns ratio transformer coupled stubs. 4454 transceiver single supply utilizes transformer with 1:2.5 turns ratio direct coupled stubs 1:1.7675 turns ratio transformer coupled stubs.
APPLICATION NOTE #113
Released 11/99
Sample Calculation Transceiver Input Impedance ACT4487, power
Using circuit Figure following measurements were made after "zeroing out" Isolation transformer's self-impedance 1MHz.
Zsec 13.6 0.349 radians
0.70
From above equations:
Zpri Zsec
Equation
Zpri 2.721
Equation
-tan
Zpri
Equation
2.895
power condition 4487 Transceiver looks like 28.95Kohm resistor parallel with picofarad capacitor 1MHz. minimum Terminal input impedance with power ZSEC 4Kohms 13.6Kohms 4Kohms 3.09Kohms 2Kohm minimum calculate Transceiver power input impedance 75KHz equations change frequency variable omega 75KHz.
4.712
2.895
1-Cin Zpri Equation
Equation
Zpri 2.894
atan
0.027 radians
Therefore Transceiver's power input impedance, 75KHz, 28.94Kohms angle -0.027 radians -1.547 degrees. minimum Terminal input impedance, 75KHz, with power 28.94Kohms (0.707)2 4Kohms 14.47Kohms 4Kohms 3.13Kohms
APPLICATION NOTE #113
Released 11/99
Sample Calculation Transceiver Input Impedance ACT4454, power
Again using circuit Figure following measurements were made after "zeroing out" Isolation transformer's self-impedance 1MHz.
Zsec 1.36 radians
From above equations:
Zpri Zsec
Equation
Zpri 2.56
Equation
6.079
-tan
Zpri
Equation
1.223
power condition 4454 Transceiver looks like 2.56Kohm resistor parallel with 60.8 picofarad capacitor 1MHz. minimum Terminal input impedance with power ZSEC 4Kohms 16Kohms 4Kohms 3.2Kohms 2Kohm minimum
calculate Transceiver power input impedance 75KHz equations change frequency variable omega 75KHz. 4.712
60.79
1.223
-Cin Zpri
Equation
Equation
Zpri 1.155
atan
0.337 radians
Therefore Transceiver's power input impedance, 75KHz, 11.55Kohms angle -0.337 radians -19.3 degrees. minimum Terminal input impedance, 75KHz, with power 11.55Kohms (2.5)2 4Kohms 72.2Kohms 4Kohms 3.79Kohms
APPLICATION NOTE #113
Released 11/99
Conclusion
above measurements made with power with transmitter inhibited mode. equivalent parallel circuit found, Transceiver input impedance, under this condition. equations valid either transformer coupled direct coupled databus connection circuits. only parameter that changes from circuit connection next turns ratio Isolation transformer. general MIL-STD-1553B specification hard violate since Isolation transformer usually much better than it's minimum specification 4Kohms from 75KHz 1MHz. most cases manufacturers measure this parameter only 1MHz case since represents worst case frequency input impedance. Transceiver, with Isolation transformer turns-ratio, minimum Transceiver input impedance reflected Isolation transformer secondary, "ZSEC", must equal greater than 4Kohms make minimum Terminal input impedance specification stated paragraph 4.5.2.2.2.3 (2Kohm) direct coupled case, must equal greater than 2Kohms make minimum Terminal input impedance specification stated paragraph 4.5.2.1.2.3 (1Kohm) transformer coupled case.
Prepared
Michael Consi Engineering Manager, Databus Aeroflex Circuit Technology November 1999
APPLICATION NOTE #113
Released 11/99

APPLICATION NOTE #113 Transceiver Input Impedance APPLICATIO

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