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Filters and amplifiers for telephone modems

• Selection of the resistor value in the line to eliminate
  Overloading the modem
• Method to reduce the level of interference and noise in the telephone line
• Optimization of the USRobotics Sportster and the Courier by more flexible
  Adjusting the gain level
• Amplifier for modem
• Calculation of filters for modems

Selection of the resistor value in the line to eliminate
Overloading the modem

Include in the opening of each wire t / f line on the resistor 60-100 Ohm. The signal of domestic automatic telephone exchanges often overrides the ADC of the modem, which affects firstly the determination of the signal BUSY. Both resistors are desirable to match the same value. At desire they can be picked up. The upper limit of them is approximately 400-600 Ohm, when the modem already fuzzy dials the pulse, the lower limit is 20-30 Ohm when their influence no longer has a significant effect. These resistors will protect your modem from possible over-relays in the line. In the model USRobotics - US / Canada is too sensitive a call detector, that when dialing a number by the modem itself often causes the appearance of an RI signal when there is no incoming call. Therefore, if you use T-MAIL it is necessary for him to write the variable Hardware_ring No.

Method to reduce the level of interference and noise in the telephone line

Significantly reduce the amount of noise can be installed in the r / f line RC filter. Below are diagrams of similar filters:

OPTION 1

By breaking the circuit between C1 and the line, you can set the switch to use your system without using filters.

OPTION 2

Principle of work

The T1 transformer is designed for high-efficiency filtration of high-speed molds. This will allow you to prevent the outbreak of anemia and the emergence of the work of electrical appliances and such unreasonable events, like the primitive dawn of a patriotic mission. The frame of the frame, collected on the primary elements R1, C1-R2, C2, is intended for the frequency correction of the input signal. The T2 regulator uses the function of the positive signal of the input signal.

Production and data

Transformer T1 contains 2 inde- pendent seals on 7 threads each, namotannyh na feppitovoe koltso stromo cimmtepichno, i.e. Obobotki namotany gpobo govpoya on chasovoy stpelke. The flow is lacquered, 0.4-0.5 mm in diameter. The controls R1, R2 - 10..50 Ohm, which is experimentally determined and depends on the length of the line from the ATC to your telephonic pointer, which is included in the module. The constants C1, C2 are 0.1 μF. Transponder T2 contains 2 non-separable threads for 30-60 threads each, fixed on a ferrite ring, strictly acclimati- cally, i.e. The clutches are pushed into different cpuons. The flow is lacquered, approximate 0.4 - 0.5 mm in diameter. You can use the T1 and T2 transformers to increase the speed of the drive, which is the overall performance of the vehicle.

Optimization of the USRobotics Sportster and the Courier by more flexible
Adjusting the gain level

The above circuit will significantly increase the efficiency of your modem's operation under different communication conditions and the quality of t / f lines.

REPLACEMENT OF STAFF RESOURCES WITHIN THE INPUTS OF THE TL082 OPERATION (flexible system for adjusting the gain level).

R1 and R2 are standard resistors and C modem capacity
R3 22 Com - Resistor amplifying amplification of the modem
R4 470 Ohm - Resistor allows you to reduce the level of amplification, but at the same time change the phase of the input signal, but also cut a few shims in the line.

Your work will be as follows:

Established R1 and R2 - replace with the above R3 and R4. The most optimal option is the use instead of soldered R3 and R4 variable / back-up resistors, which will allow you to more flexibly adjust.

Preliminary setting:

Go to the terminal and type AT & T1 test command. After installing the pseudo-connection, put the ATH0 tube and type ATI11. Select the value of R3 so that the statistics speak of the meaning: Recv / Xmit 13/12. (This is due to the short loop (i.e., in our case when the pseudo-connection is AT & T1.) And already when working on the physical line, the optimal value is Recv / Xmit 25/12.

The final setting is reduced to the selection of the position of the value R4. In this case, you need to compute this value in such a way that the handheld is not dragged out and passed normally and most successfully. It is especially good to conduct this setup when working with USRobotics -> ZYXEL. This will give you the opportunity to correct the installation in the future without the problem of working with ZYXEL modems.

Amplifier for modem

The circuit is an amplifier with positive feedback. It is installed on the t / f line parallel to the modem. Gain 1.5..2 times.
The operational amplifier DA1 can be used either, given the pin assignment. We used 553UD2 in plastic DIP.
3 + Vcc = 9 ... 15V, -Vcc = -9 ...- 15V. Those. A bipolar source with an average "ground" (gnd) is needed.
L1 - any choke at 200..400uH. It was used DP1,2-400.
R1 - adjusts the gain.
VS1, VS2 - symmetrical zener diodes for 20V for DA1 protection. Two KS210B connected in series were used.
Condenser C3 is turned on correctly: (+) on gnd.
Customization. If there is an oscilloscope, when the power is turned on and the connected line is plugged in (the tube is raised!), The generation is checked at pin 10 of DA1 at the far left of the R1 position.
Posterally increasing R1 to achieve generation failure. This position R1 corresponds to a certain threshold gain level. For stability, add 5..20% of the current value of R1.
All these operations can be done without an oscilloscope by ear, connecting a parallel device, but you will need to take into account the additional load in the line. The final position R2 - with a specific connection.

IMPORTANT NOTICE!

When the tube is laid, the circuit generates a tone in line. Therefore, it is desirable to turn off the power (provide a switch) when the tube is laid.

The circuit is tested with the USR modem and is based on the USR circuitry and the USR modem FAQ materials. Any suggestions and changes that improve the operation of the scheme are welcome.

For the circuit described above, you, u, need a power supply.

Power supply circuit:

Calculation of filters for modems

Scheme 1

The principle of the low-pass filter: "cuts" all frequencies above the calculated one.
Calculation of inductance.
L-coil height, mm. W is the number of turns. R-distance from the center of the coil to the middle of the wire section. K-coefficient.
Lmkrn =, 0002 x W2 xrxk)

L: 2r K 1 6.5 0.9 6.8 0.8 7.3 0.7 8 0.6 9 0.5 10 0.4 eleven 0.3 13 0.2 16

Calculation of the filter.

L = R / (Pi x F) C = 1 / (Pi x F x R) Fmax = 1 / (Pi x SCR (L x C)) R = SCR (L / C) / SCR-square root.

In my case: coils-10vit.na 3 mm mandrel. D = 0.1mm. C = 0.15 mcf. Instead of coils, you can put resistors.
The result: stable-500-600 k / s, jumps up to 2.2 k / s. Try to experiment with the values, you will get better.
For 100% efficiency of the filter, it is necessary to determine the "earth" wire of the line.

Scheme 2.

R1, R2 - 360 ohm, С1-0,15μf, D1, D2 - D226 (you can COP, try to put others, each ATC has its oddities).
ATTENTION ! If there is no signal in the line, change the polarity of the diodes.
RESULT: Steadily-1.6 fps, jumps to 5.6 fps. DOWNLOADHELPER works like a vacuum cleaner, up to 8-20 fps.
The total exit from the communication session is less than 3 seconds.

All results are obtained on the modem ROCKWELL-14400, Netscape-4.51. The test was conducted for 3 days at different times (from 8h to 3h) on pools 55 and 56.

Date of publication 27.02.2004.

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