Posted on Feb 4, 2014

Three-stage active Butterworth bandpass input filter is used in radioteletype demodulator to separate RTTY tones from each other and from noise. Filter is centered on 2200 Hz, and has bandpass of about 260 Hz to allow reception of some of audio sidebands produced by keying and allow for small drift. Five pots serve for trimming center frequency of

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each stage andQ of end stages. Article gives step-by-step design and alignment procedures. Use two 10K resistors between V and ground to get V/2 for bias when operating from single supply. -P. A. Stark, Design an Active RTTY Filter, 73 Magazine, Sept. 1977, p 38-43. (View) Triggering is accomplished by turning transistors on, whereas in most similar circuits the transistors are turned off. Trigger pulse merely has to lower point o below ground for fraction of microsecond. Almost any diode and transistor can be used. Speed can be up to 10 Mc with high-speed transistors. Output fall time is fast. -Binary Flip-Flop Tums On, Electronic Circuit Design Handbook, Mactier Pub. Corp. , N. Y. , 1965, p 214. (View) Simple circuit for Signetics NE565 PLL locks to input frequency and tracks it between two values used, to produce corresponding DC shift at output. Values shown are for 1070-Hz and 1270-Hz FSK signals. Three-stage RC ladder filter removes sum frequency component. Band edge of filter is chosen to be about halfway between maximum keying rate (150 Hz) and twice input frequency (about 2200 Hz). Output is made logic-compatible by connecting voltage comparator to pin 6. - Signetics Analog Data Manual, Signetics, Sunnyvale, CA, 1977, p 845. (View) Cross-coupling of two basic inverters gives low-cost lip-lop using 2N711 germanium pnp mesa switching transistors. Flip-flop can be set and then reset, or run as Gaunter using combined...

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