For Further Information

To find out more about packet radio, TNC boards, manuals, and parts kits, you may contact Tucson Amateur Packet Radio at PO Box 22886, Tucson AZ 85734.

There may be a TAPR beta site in your area, if soT talking on one of the local repeaters may lead you to someone with a packet station on the air. Numerous local groups are spring* ing up, and many FM/repeater groups are beginning to incor* porate packet operations Into their systems.

Subscribing to 73 will provide you with other articles on packet radio, and other amateur magazines will also be carrying more and more information on this new mode.

Additionally, the following organizations have regular newsletters with packet information; TAPRt AMRAD, PO Drawer 6148. McLean VA 22106, and SLAPR, 1309 Gloucester Dr, Edwardsvilie IL 62025. Please include an SASE when writing to any of Ihe above groups for information.

Packet promises to revolutionize amateur communications in the 80s much as did SSB in the 50s and FM in the 70s.

figure a frequency counter for calibrating the modem to within a very few Hertz of the target frequencies.

The tone pair most commonly used is 1200 Hz and 2200 Hz for compatibility with amateurs using surplus Bell 202-style modems.

The receive side is somewhat more involved. The desired bandpass characteristic for using the Bell 202 tones is not met very well by unmodified amateur or commercial FM communications equipment. While a demodulator can be made to work by limiting and careful adjustment it has been shown that the XR2211 PLL demodulator used in the TAPR TNC yields a performance improvement on the order of 3 dB when preceded by the switched capacitor fiiter shown in the schematic diagram.

The incoming signal is buffered by an ojvamp section and then passed to a limiter/indicator using a pair of LFDst The I.EDs provide operator feedback for setting the volume of the receiver. The best adjustment is when the LEDs are |ust extinguished with an incoming signal.

A resistive divider then couples the signal to the MF10 switched capacitor filter, A second-order highpass fiiter followed by a second-order low-pass filter then corrects the radio system bandpass characteristic. The filtered signal is then passed to the XR2211 PLL demodulator, where data carrier detection and data recovery are accomplished.

The MF10 fitter has the advantage of not requiring any critical capacitors (nor any caps, apart from bypassing), relying instead on 1% resistors (cheap) and a crys-tal-controlSed clock signal (already needed for the microprocessor). I he result is an easily replicated filter characteristic with no adjustments!

The XR2211 circuit is very standard and reliable The only changes made tn the circuit recommended by the manufacturer are in the area of the data carrier detect filter capacitor It was increased in size to help eliminate chatter and provide greater resistance to false indications due to noise.

Connections are provided to the demodulator to allow calibration by the same onboard frequency counter that is used to set up the XR2206 modulator. The result is an easily calibrated circuit that requires no special equipment.

If you desire, provision has been made for connect ing an external modem for further experimentation and development

Power Supply

The TNC power supply uses full-wave rectifiers and series-pass regulators for stiff, low-impedance power sources. The design is very conventional and uses the 78/79XX series regulators for their excellent regulation, thermal overload protection, low cost and wide availability. +12 V is supplied to the modem and RS-232 circuitry, while -12 V is used only for the RS-2i2 interface and —5 V is used only for the switched capacitor filter in the modem section. + 5 V is handled by the more substantial LM309K, which may be mounted on or off board and supplies every subsystem on the TNC. Extensive bypassing is used (don't cheat and leave any out!), and no problems have been noted with crosstalk on the + 5-volt bus.

Construction

With the foregoing as background, you are now ready to tackle the actual construction of the TNC. Please be warned, however, that the TAPK TNC is a complex piece of equipment and construction by the faint-hearted or inexperienced is not advised.

While a wire-wrap job should yield a perfectly suitable unit, construction will be much easier if you elect to use a PC board. TAPR has blank boards available for a nominal price, and l suggest you write TAPR at the address given elsewhere in this article for details on price and availability. TAPR also has parts kits available, and you may want to take adavantage of this service as well

If you decide to obtain the parts yourself, consult the parts list for a detailed breakdown of the necessary items It ts OK to substitute many of the items (such as IC bypass capacitors) but be

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