Communication over a TP cable
Communication over a TP cable |
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Signal shaping is the main function of the TPEX -- Twisted-pair Ethernet Transceiver circuit. Original signal generated by the Manchester modulator is rectangular. Applying Fourier transform, we discover that it consists of a number of odd harmonics with non-neglectable amplitudes:
f(x)=sin(x) + 1/3 sin(3x) + 1/5 sin(5x) + ....
Converting the coefficients to dB, the level of 3rd harmonic is -4.77 dB and of 5th harmonic -7 dB. Such a suppression is clearly insufficient since harmonics carry no useful information and only cause unwanted noise. There are two ways of eliminating them:
1) Filtering harmonics
TP cable is driven through a hybrid filter/transformer, for example 20F001N device. In the following picture, the filter is in the top part of the schematic. It is driven by symmetric rectangular signal connected to pins 1 and 3. TP line connects to pins 14 and 16.
Schematic:
Cut-off frequency of this circuit is 17 MHz. Attenuation is shown in the following table:
20 MHz 7 dB 25 MHz 19 dB 30 MHz 32 dB 40 MHz 35 dB Attenuation of higher frequencies is about 25 dB. This filter solves the problem only partially. Signal coming into this filter has two main frequency components depending on the data being transmitted. If a sequence 11 or 00 is transmitted, the modulation frequency is 10 MHz. However, if 01 or 10 is transmitted, the modulation frequency of this sequence is only 5 MHz! Such a signal will pass through the above filter with the third harmonic intact, obtaining the yellow-colored signal at the output.
(filtered signal: 10 MHz / 5 MHz / corrected 5 MHz signal)
This signal is not very suitable for further processing. Since it is difficult to make a filter that would pass 10 MHz and sufficiently attenuate 15 MHz, it is necessary to reshape the signal before filtering. Signal is reshaped as shown, yellow color shows filtered signal. The reshaping is achieved by subtracting about 20% of the signal from Manchester coder delayed by 50 ns. Resulting signal is shown in the following figure.
Another reason for signal reshaping are the properties of the TP cable itself. Since it is mostly capacitive and is driven from output impedance of about 100 Ohms (necessary for termination), it exhibits some "persistence" caused by charging and discharging of the cable. After passing through the cable, the signal looks like:
filtered output signal 10 MHz (data 1111 or 0000) / 5 MHz (data 0101), signal at the end of cable 10MHz / 5 MHz
Equivalent circuit and further details
2) Driving the line with a near-sine signal.
First hint of signal shaping is the method used in communication over coaxial cable. Signal edges have rising/falling times modified to 25 ns. Rectangular signal is converted to trapezoidal, consisting of a number of odd harmonics:
f(x)=sin(x) - 1/9 sin(3x) + 1/25 sin(5x) - 1/49 sin(7x) + ....
Converting to dB, level of 3rd harmonic is -9.5 dB and of 5th harmonic is -14 dB. This is twice as much attenuation compared to the rectangular signal but it is still not enough.
If the TP cable is to be driven through a hybrid transformer only, e.g. Valor ST7011, Pulse PE-65745, YCL 16PT-41, ... it is necessary to use a near-sine shaped driving signal. Filtering or shaping circuits are usually integrated into output drivers. For example, this method is used in CS8900. Besides analog solutions, filtering can be implemented in two possible ways:
- Signal is generated with a table and a D/A converter. Used e.g. in LXT901/907 which uses a 5-bit D/A operating at 70 MHz.
- Signal is reshaped with a FIR filter. Used e.g. in MTD907, which contains a 16-point FIR filter operating at 160 MHz. The following figure shows an example of the filter effect:
The signal shown was obtained by passing a rectangular 10/5MHz signal through a Blackman FIR filter. Although the spectrum is satisfactory, it is necessary to compensate for TP cable characteristics, so the reshaping mentioned at the end of paragraph 1 is also applied.References
Excel sheet for experiments with harmonics filtering
The Scientist and Engineer's Guide to Digital Signal Processing, Second Edition
Dr. Ing. Hans-Jochen Bartsch, Handbook of Mathematical Formulas [Czech edition], Praha SNTL 1983, 832 pages.
Allied Telesyn International CentreCOM Micro Transceivers AT-MX10, AT-MX20, AT210 (2 pg.) [local copy]
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