Microsat Bi Modulator Interfaces with any Vhf Tnc

By John C. Reed, W6IOJ 770 LaBuena Tierra Santa Barbara, CA 93111

Uplink access to the Microsats and the Japanese satellite digital mode F0-20 requires a two-meter FM signal input keyed with Manchester biphase (Bi$). The following describes an assembly that transforms the normal VHF AFSK (audio frequency shift keyed) output from any unmodified TNC into the Bi4> format. As an example, I use it with my old PK64. The assembly is relatively simple and all parts, including the ICs, can be purchased from a single mail-order supplier (All Electronics Corp, PO Box 567, Van Nuys, CA 91408, tel 800-826-5432). Alignment/testing of the assembly can be performed using the TNC together with an oscilloscope and audio oscillator.

Manchester Bi$ Keyed

The Bi4>-keyed method was used in the first Japanese digital-mode satellite, FO-12. The resulting backlog of stations having the necessary uplink instrumentation made an important influence in selecting Bi4> keying for the Microsat uplink. Bi<i> is simply a continuous carrier that is synchronous with the NRZI input data and having a frequency equal to the bauds, in this case 1200 Hz. Data modulation is a 180-degree phase shift of the carrier at every NRZI-levei transition. NRZI recovery in a Bi<i>-keyed signal is simply the process of detecting carrier phase shift, each shift representing a zero bit. It is like the Microsat downlink PSK (phase shift keyed) method except for the synchronous feature.

The conventional manner of developing Bi4> is to obtain both the NRZI and the clock directly from the TNC, processing this data to make the Bi$ format. However, in some equipment, like the PK64, this is not possible without digging into the TNC circuit board. The described method includes an AFSK-to-NRZI converter, and a clock recovery circuit. The only data required from the TNC is the conventional VHF 1200/2200-Hz AFSK output and the PTT (push-to-talk) switch. Admittedly, reconstructing the NRZI and clock is a potential source of error. However, the error probability is small when considering there is no source of external noise. It's comparable to going through a digipeater in a closed loop configuration.

Block Diagram Description

Referring to the block diagram, Fig 1, and the waveform diagrams, Fig 2, the following describes each waveform diagram. The bit-level sequence indicated at the top of the diagram was chosen to illustrate two typical conditions; two successive-level changes (00), and a single-level change (0).

® Input—The standard VHF TNC 1200/2200-Hz output

0 0

Post a comment