Application Note: CTR-21 DAA

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LOW COST, CTR 21 COMPLIANT DAA FOR EUROPE

By Joe Randolph, Randolph Telecom, Inc.
December 2, 1999

Preface (added March 7, 2005)

The European regulatory requirement CTR 21 has now been officially withdrawn, and the requirements specified in CTR 21 are no longer mandatory for regulatory approval in the European Union.  In the absence of a formal regulatory requirement, some manufacturers prefer to continue meeting CTR 21.

However, manufacturers who seek to achieve the lowest possible cost without compromising performance can benefit by using the circuit described in TN #98, "Low Cost Telephone Line Interface (DAA,FXO)."

Introduction

This application note describes a low cost telephone line interface (DAA) that can be used for fax machines and low speed data modems. Specifically, this design is suitable for half duplex fax modems at speeds up to 33.6 kbits/sec, and full duplex data modems up to 2400 bits/sec. With some restrictions, this design can also be used for full duplex modems up to 14.4 kbits/sec.

The two key features of this design are its extremely low cost of under $3.50 in 100K quantities, and its compliance with the new pan-European standard CTR 21. CTR 21 is now accepted in about 20 European countries, and additional countries are expected to begin accepting it soon.

Typical Applications

The low cost and European compliance of this design make it attractive for manufacturers of high volume, cost sensitive products. Typical applications include:

• Fax machines
• Set-top boxes that interface to the phone line
• Embedded modems for remote monitoring and control
• Voice messaging equipment

Modem protocols that can be used with this interface include:

• Half-duplex fax up to 33.6 kbits/sec (V.29, V.17, and V.34 hdx)
• Full duplex data up to 2400 bits/sec (V.21, V.22, V.23, V.22bis)
• Full duplex data (with some restrictions) up to 14,400 bits/sec (V.32, V.32bis)

While the scope of CTR 21 includes only "non-voice" equipment, this DAA design can also be used in certain voice products such as hands-free speakerphones and voice messaging systems. The requirements for such products are contained in CTR 37, which largely duplicates CTR 21. For analog voice products that use a handset, the requirements of CTR 38 apply. To comply with CTR 38, the basic circuit presented here would require some modifications.

Background

In July 1998, the European Commission adopted a harmonized interface standard for non-voice equipment that connects to the analog public switched telephone network (PSTN). The introduction of CTR 21 was good news for manufacturers, since compliance with a single standard could now be used to obtain approval in about 20 different European countries. Previously, 20 separate approvals were required, with requirements that were sometimes incompatible among various countries. This led to long approval delays and multiple configurations of the same product.

Unfortunately, the CTR 21 specification contains some unusual requirements that are not easy to meet. These requirements include the provision of a 60 mA current limiter, and tight restrictions on the transient response of the DC loop current.

For FCC Part 68 compliance in North America, the traditional lowest cost DAA implementation uses a "wet" transformer such as the Wurth Electronics Midcom 671-8001. A wet transformer is designed to have the DC loop current flow through its primary winding. This places limitations on parameters such as return loss and distortion, but these limitations are not a problem for half duplex fax modems and low speed full duplex modems.

For applications with more stringent requirements for return loss and/or distortion, the typical approach is to use a "dry" transformer. A dry transformer is designed to have no DC current in its windings, which allows for improved transformer performance. To complete the DAA function, however, a dry transformer must be supplemented with an electronic DC holding circuit that increases the cost of the DAA implementation.

In the past, almost all DAA designs for European applications used a dry transformer to meet the return loss requirements that were imposed in most European countries. In the new CTR 21 standard, the return loss requirements have been relaxed somewhat, which opens up the possibility of using a low cost wet transformer design. Unfortunately, the CTR 21 requirement for a current limiter makes it impossible to simply use the low cost wet transformer design that has been widely used in North America.

For the circuit described in this application note, the goal was to start with the low cost wet transformer design used in North America, and add the additional features required to comply with CTR 21. The resulting design costs slightly more than a standard North American design, but is still somewhat less expensive than a traditional European design that uses a dry transformer.

Circuit Description

The attached Figure 1 shows a schematic of a CTR 21 compliant DAA that uses a specially designed wet transformer, the Wurth Electronics Midcom 82107 . The 82107 transformer provides higher safety isolation (supplementary grade) than what is required in North America. In addition, the 82107 is specifically designed to operate in the circuit of Figure 1 and meet all the requirements of CTR 21. For example, Figure 2 shows that the circuit meets the return loss requirement in CTR 21.

Experienced DAA designers will see that the circuit in Figure 1 has many similarities to a traditional wet transformer design for North America. The primary difference is the inclusion of the current limiter circuit. Some less obvious aspects of the design address other aspects of CTR 21, such as return loss and DC transient response. The resulting circuit has the following key features:

• Fully complies with European standard CTR 21
• Operates on a single +5 volt supply
• Transmit and receive gains set at 0 dB
• Includes ring detect output, switchhook relay, and hybrid circuit
• Survives differential and common mode lightning surges up to 2500 volts, 100 amps
• Total materials cost of under $3.50 in quantities of 100K

Cost Estimate

The attached Table 1 shows a complete bill of materials for the circuit of Figure 1. Component cost estimates are based on quantities of 100,000 purchased directly from the component vendors. Companies with significant purchasing power may be able to achieve a slightly lower total cost.

Possible Enhancements

The circuit shown in Figure 1 is a basic yet rugged CTR 21 DAA. Following are some changes that could be considered for different applications:

Lower Cost

Some additional cost savings could be obtained by using a +12 volt supply and a less expensive op amp such as the LM358. The current design uses a rail-to-rail op amp that allows the circuit to operate from a +5 volt supply. Additional savings could be achieved by cutting back on the degree of lightning protection if the application allows this. Savings could be achieved by using a less rugged surge protector in place of E1, and a less rugged relay in place of U3. Such changes should be considered carefully, since poor lightning immunity can result in a high rate of field failures.

+3.3 Volt Operation

The circuit of Figure 1 will operate on a 3.3 volt supply, but the output swing available across a 600 ohm load on tip/ring will be reduced to about 1.6 volts peak to peak (down from 2.5 volts peak to peak with a +5 volt supply). This amount of swing is adequate for transmitting V.21 and V.22 modem signals at the allowable maximum of –10 dBm, but V.22bis data modems and V.29, V.17, and V.34 fax modems have peak-to-RMS ratios that require greater swing. These modems should be limited to a maximum transmit level of –13 dBm if a single +3.3 volt supply is used. If higher transmit levels or lower supply voltages are desired, the transmit driver in Figure 1 can be converted to a differential driver by adding another op amp. This effectively doubles the available output swing.

V.32 and V.32bis Operation (up to 14.4 kbits/sec)

The circuit of Figure 1 will provide excellent performance for half duplex fax modems up to 33.6 K bps, and for low speed full duplex data modems up to 2400 bps. These modems have only modest requirements for distortion in the DAA, which allows the use of a very inexpensive transformer.

However, the circuit of Figure 1 will also work reasonably well for higher speed full duplex data modems up to 14.4 kbit/sec. The principal limitation with high speed full duplex modems is that they use echo canceling techniques that are susceptible to distortion in the transformer. Typical specifications for a V.32bis transformer require that distortion products be at least 70 dB down for a 600 Hz sine wave transmitted at –3 dBm. The distortion of the Wurth Electronics Midcom 82107 used in Figure 1 is about 60 dB down for this test.

As it turns out, this level of distortion is acceptable for the vast majority of connections through the public phone network. The impairment caused by distortion in the transformer will only be evident on connections where the receive signal level is below about –30 dBm. Receive levels this low are becoming rare, due to the phone network becoming increasingly digital. This means that the circuit of Figure 1 should work at V.32bis speeds of 14.4 Kbits on most phone calls, but will be forced to fall back to lower speeds if the receive level is unusually low. For many embedded applications, this type of behavior may be acceptable.

Loop Current Detection

In some applications, it is necessary to detect the presence or absence of DC loop current from the central office. For instance, a brief interruption of central office loop current is often used to signal that the far end party has hung up. For voice messaging equipment, detection of this signal can be used to quickly return to the idle state after an incoming message has been completed.

To add loop current detection to the circuit of Figure 1, an opto isolator can be inserted so that a portion of the loop current flows through its input LED. Care must be taken to ensure that the LED in the opto isolator is adequately protected from surge currents, but there are several possible circuits that will accomplish this.

Compliance With North American Regulatory Requirements

The circuit shown in Figure 1 fully complies with FCC Part 68. Thus, this circuit can be used in North America for situations where the manufacturer wishes to use the same product design in Europe and the North America. As mentioned previously, though, the lowest cost design for North America would use the Wurth Electronics Midcom 671-8001 wet transformer and no current limiting circuit. It should be noted that the 671-8001 is pin compatible with the 82107, so a simple jumper provision around the current limiter circuit would allow the same circuit board to be populated for either option.

Another requirement that typically applies in North America is the safety specification UL 1950. Starting in April of the year 2000, the Third Edition of UL 1950 will replace the Second Edition, and some new "power cross" requirements will apply. The circuit of Figure 1 can be made fully compliant with these new requirements by placing an appropriate fuse in series with the tip lead, to the right of the sidactor E1 shown in Figure 1. Several fuses can be considered, but the lowest cost option is probably a 5x20 mm glass fuse, such as the Bussmann C515, 1.25 amp fuse, at an estimated cost of about 15 cents. The use of this fuse will not compromise the 100 amp lightning immunity of the circuit.

Increased Lightning Immunity

In most parts of the world, the phone line passes through a "primary protector" at the point where it enters a building. The primary protector limits common mode and differential lightning surges to less than 1,500 volts peak. The circuit shown in Figure 1 is designed to handle any residual surges that get past the primary protector.

Unfortunately, the primary protector is sometimes disconnected or defective, which allows common mode lightning surges to reach 4,000 to 5,000 volts peak. The percentage of primary protectors that are disconnected or defective has been estimated at about 1% of all installations.

This may seem like a negligible amount, but a manufacturer with 10 million set-top boxes in the field could see 100,000 failures per year if lightning damaged only 1% of the installed base each year. Because of concern about common mode lightning immunity, some manufacturers of high volume consumer products require a very high degree of common mode lightning immunity.

The isolation of the Wurth Electronics Midcom 82107 transformer used in Figure 1 is rated at 1875 volts rms, which translates to about 2600 volts peak. For a slight increase in cost, the Wurth Electronics Midcom 671-8077 transformer can be substituted, to achieve isolation of 5000 volts rms (7000 volts peak).

Designers who seek extremely high isolation should remember that the switchhook relay and the ring detect opto isolator must withstand the same voltages as the transformer. In addition, the circuit board layout must provide adequate physical separation between the circuitry on the phone line side of the barrier and the circuitry on the other side. In general, it is possible to address these other considerations without a significant impact on the cost of the circuit.

Caller ID and On-Hook Monitoring

Many applications require some method for monitoring the voiceband AC signal that is present on the phone line during the on-hook state. Examples include caller ID reception and detection of the CNG tone for automatic voice/fax switching.

For implementing this feature, it is highly desirable to couple the received signal through the transformer. This allows the transformer to provide both lightning protection and immunity from common mode 60 Hz interference. These two impairments often present problems for implementations that use capacitively coupled differential amplifiers.

It is possible to use the transformer to couple the signals in the on-hook state by placing a bypass capacitor across the switchhook relay and making some other minor modifications to the circuit of Figure 1. Care must be taken in the design of such circuits to avoid affecting compliance with other regulatory requirements such as on hook AC impedance. The options that can be considered will depend on whether the monitoring is only for caller ID signals or is for other signals such as CNG tones. Depending on the implementation, the cost for adding on-hook monitoring to the circuit of Figure 1 will be in the range of 20 to 60 cents.

About the Author

Joe Randolph is a consultant who specializes in the design of telecommunications equipment. He has extensive experience with the design of analog PSTN interfaces for worldwide applications, and has product designs approved in over 100 countries. His experience with international product design includes telephones, modems, voice messaging equipment, fax machines, PBXs, and internet telephony. Questions about the circuit described here can be directed to:

Joe Randolph
Randolph Telecom, Inc.
781-721-2848 (voice)
781-721-0582 (fax)
jpr@randolph-telecom.com (internet)
www.randolph-telecom.com

TABLE 1

CTR 21 DAA COST ESTIMATE

(100K QUANTITY)

	Reference			Unit	Ext.
Quantity	Designation	Description	Vendor	Cost	Cost
1	C1	Aluminum cap, 2.2 uF, 16 V	Generic	0.03	0.03
1	C2	Aluminum cap, 10 uF, 50 V	Generic	0.03	0.03
2	C3,C8	Ceramic cap, 100 pF, 25 V, NPO	Generic	0.03	0.06
3	C4,C7,C9	Ceramic cap, 0.1 uF, 25V, Z5U	Generic	0.03	0.09
1	C5	Ceramic cap, 0.1 uF, 25 V, X7R	Generic	0.03	0.03
1	C6	Ceramic cap, .33 uF, 250 V, Z5U	Generic	0.20	0.20
1	C10	Ceramic cap, 390 pF, 25 V, X7R	Generic	0.03	0.03
1	C11	Ceramic cap, 0.33 uF, 25 V, Z5U	Generic	0.05	0.05
1	D1	Signal diode	Generic		0.03
1	D2	Zener diode, 33V	Generic	0.04	0.04
1	D3	Diode bridge, CBRHD-02	Central Semi	0.12	0.12
1	D4	Bidirectional zener, 5.1V, CMPZDA5V1	Central Semi	0.10	0.10
1	D5	Surge diode, 600W, P6SMB33CAT3	Motorola (ON)	0.15	0.15
1	E1	Sidactor, 310V, P3100SC	Teccor	0.30	0.30
1	Q1	NPN Darlington, 120V, FZT604	Zetex	0.21	0.21
1	Q2	NPN Transistor, 65V, BC846	Motorola (ON)	0.05	0.05
1	R1	33 K, 5%, 1/10 W	Generic	0.01	0.01
1	R2	11.5 ohm, 1%, 1/8 W	Generic	0.01	0.01
1	R3	20 K, 5%, 1/4 W	Generic	0.01	0.01
5	R4,R7,R11,R12,R13	100 K, 1%, 1/10 W	Generic	0.01	0.05
1	R5	20 K, 5%, 1/10 W	Generic	0.01	0.01
1	R6	392 ohm, 1%, 1/10 W	Generic	0.01	0.01
1	R8	41.2 K, 1%, 1/10 W	Generic	0.01	0.01
1	R9	48.7 K, 1%, 1/10 W	Generic	0.01	0.01
1	R10	620 ohm, 5%, 1/10 W	Generic	0.01	0.01
1	R14	86.6 K, 1%, 1/10 W	Generic	0.01	0.01
1	T1	Transformer, Wurth Electronics Midcom 82107	Wurth Electronics Midcom	0.58	0.58
1	U1	Opto-isolator, NEC PS2801-1	NEC	0.15	0.15
1	U2	Op amp, LMV822	National Semi	0.40	0.40
1	U3	Solid State Relay, AD6C111	Solid State Optronics	0.65	0.65
	U3 Alternate	Solid state relay, PS7342C-1A	NEC
37	Total Parts Count		Total Parts Cost:		$3.44

 

 

FIGURE 2

RETURN LOSS OF CIRCUIT IN FIGURE 1

MEASURED PER THE REQUIREMENT IN CTR 21