Modem Circuits

To align the modem, using an oscilloscope, connect it to the receiver and open the squelch on an unused channel and watch the waveform on pin 6 of the 74HC04. Adjust the 47K pot so that the noise is centered on the screen and not all at the top or bottom of the screen - i.e. the noise should be just as much positive 5v as it is 0v. If you do not have an oscilloscope then adjust the 47K for a DC voltage of 2.5v on pin 6 of the 74HC04.

The 2400-bps modem in Figure 1 makes it easy to connect a Basic Stamp (Parallax Inc, www.parallaxinc.com) to a telephone modem. Using this circuit, you can call home via your PC and find out if the house is still there. The Basic Stamp2, BS2-IC, has the necessary programming space, and a 64-kbyte StampMEM memory-storage board enhances site data collection.

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The above diagram is a standard low-pass telephone line filter (L1,C1,L3,C2). L2 and L4 are needed since we're dealing with the 'Tip' and 'Ring' of a phone line.

I have recently done quite a lot of work on RTTY, using the HamComm program and been so impressed that I made a PCB to support the program. I think that I will continue to use the software, so I have registered it with the programs author: W. F. Schroeder, DL5YEC has obviosly put so much work into it. The program will send and receive RTTY up to 300 bauds as well as sending and receiving CW.

This application note describes the V.34 Modem Interface and the benefits obtained when the DS8933 and DS8934 Line Drivers and Receivers are employed. The following sections are covered. # V.34 Standard # DS8934 and DS8933 # Async Application # Sync Application # Driver Characteristics # Receiver Characteristics # RS-232 and RS-423 # Summary # References

The KD2BD Pacsat Modem is a low cost, high performance 1200 bps BPSK modem designed to interface between a packet radio terminal node controller (TNC) and an amateur satellite ground station, and allows full-duplex access to the 1200 baud "Pacsat" constellation of amateur satellites.

The LT1886 is a high speed dual power amplifier with excellent high frequency distortion performance. The LT1886 features a 700MHz gain-bandwidth product, 200V/ms slew rate and over 200mA of output current drive in a small SO-8 package. Using only a single 12V supply, the LT1886 is the best choice for the line driver in ADSL CPE modems (customer premises equipment).

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The modem-access adapter (MAA) in Figure 1 has features that reduce problems when modems and phones share the same line. The design also overcomes the shortcomings of modem interference protectors. The design eliminates mutual interference between phones and modems, is compatible with the line-usage indicators on multiline phones, can search for an idle phone line in multiline installations, draws minimum power, and requires no changes to a PC modem's hardware or software.