TAPR TNC1 or TNC2 modem

 TNC2.2


Via VE3FJB: 1291 From VE3KLW Rcvd 861120/0636z, Sent 861120/0931z Orillia, Ont.

At VE3PAK: 3641 From VE3KLW Rcd 19/11/86 12:48:07ET sent 20/11/86 01:17:47ET


Here's something for those folks who are experimentally inclined and own a

TAPR TNC1 or TNC2 (or one of the many clones).  In the front end of the modem

is a bandpass filter which uses an MF10 switched-capacitor filter.  This

filter is designed to have a sloping amplitude response characteristic which

provides approximately 6 dB more gain at the high tone frequency (2200 Hz)

than the low tone frequency (1200 Hz).  It was never clear to me why this

filter was needed.  Its response is more or less complementary to the 6

dB/octave deemphasis characteristic found in many FM receivers; however, in

most cases, the FSK signal from the TNC is injected into a microphone jack or

other low-level input of an FM transmitter.  Therefore the signal is likely

passing through preemphasis circuitry in the transmitter, and the overall

response of the signal path through transmitter and receiver should then

already be more or less flat without further filtering. 

CONTINUED IN FILE TNC2.10




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