2000-03-29 16:38:44 +04:00
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<html><head><title>
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Gadget Box PPS Level Converter and CHU Modem
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</title></head><body><h3>
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Gadget Box PPS Level Converter and CHU Modem
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</h3>
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<p><h4>Introduction</h4>
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<p>Many radio clocks used as a primary reference source for NTP servers
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produce a pulse-per-second (PPS) signal that can be used to improve
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accuracy to a high degree. However, the signals produced are usually
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incompatible with the modem interface signals on the serial ports used
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to connect the signal to the host. The gadget box consists of a handful
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of electronic components assembled in a small aluminum box. It includes
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level converters and a optional radio modem designed to decode the radio
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timecode signals transmitted by the Canadian time and frequency station
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CHU. A complete set of schematics, PCB artwork, drill templates can be
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obrtained via the web as the distribution <a href=
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"http://www.eecis.udel.edu/~mills/ntp/ntp">gadget.tar.Z</a>, or by
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anonymous FTP from ftp.udel.edu in the <TT>pub/ntp</TT> directory.
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<p>The gadget box is assembled in a 5"x3"x2" aluminum
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minibox containing the level converter and modem circuitry. It includes
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two subcircuits. One of these converts a TTL positive edge into a fixed-
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width pulse at EIA levels and is for use with a timecode receiver or
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oscillator including a TTL PPS output. The other converts the timecode
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modulation broadcast by Canadian time/frequency standard station CHU
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into a 300-bps serial character stream at EIA levels and is for use with
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the <code>tty_clk</code> and <code>chu_tty</code> line disciplines in
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the ntp3 distribution.
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<p>This archive contains complete construction details for the gadget
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box, including schematic, parts list and artwork for a two-sided,
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printed-circuit board. All files are in PostScript, with the exception
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of this file and an information file, which are in ASCII. The artwork is
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in the 1:1 scale and is suitable for direct printing on photographic
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resist for each side of the board. While a plated-through-holes process
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is most convenient, it is possible to bridge the two sides using
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soldered wires where necessary.
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<p><h4>Circuit Description</h4>
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<p>Following is a brief functional description of the device. See the
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schematic diagram gadget.s01 for reference. The audio output of a
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shortwave radio tuned to CHU at 3330, 7335 or 14670 kHz is connected to
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J2. A level of at least 30 mV peak-peak is required, such as provided by
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the recorder output on many receivers. The input level is adjusted by
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potentiometer R8 so that the timecode modulation broadcast at 31-39
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seconds past the minute reliably lights green LED1, but the signals
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broadcast during other seconds of the minute do not.
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<p>Opamp U4A provides low-impedance drive for the bridged-tee bandpass
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filter U4B. The filter has a bandpass of about 600 Hz at the 6-dB points
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and a center frequency of about 2150 Hz. It is designed to avoid
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aliasing effects with receivers of relatively wide bandpass
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characteristics. The modem itself is implemented by U2 and its
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associated circuitry. Resistors R4 and R1 are a 40-dB pad which matches
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the filter output to the modem input. U2 is a TTL/EIA level converter
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with integral power supply for bipolar signals. The modem output is
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available at pin 3 (receive data) of DB25 connector J1.
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<p>The TTL PPS signal is connected via J3 to a retriggerable one-shot
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U3A, which generates a TTL pulse of width determined by potentiometer
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R7. The pulse width is determined by the bit rate of the attached serial
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port. In the common case the width is one bit-time, such as 26 us for
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38.4 kbps, for example. This appears to the port as a single start bit
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of zero followed by eight bits of ones and a stop bit of one. The second
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one-shot U3B generates a 200-ms pulse suitable for driving the amber
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LED3 as a visual monitor. The output of U3A is converted to EIA levels
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by U1 and appears at pin 12 (secondary receive data) of J1.
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<p>If only the PPS circuit is required, U2 and U4 can be deleted and the
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gadget box powered from the EIA modem-control signal at pin 20 (terminal
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ready) of J1, assuming this signal is placed in the on (positive
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voltage) condition by the computer program. J1 is wired to keep most
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finicky UARTs and terminal-driver programs happy. If the CHU circuit is
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required, an external 12-volt AC transformer or 9-12-volt DC supply
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connected to J4 is required. Red LED2 indicates power is supplied to the
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box.
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<p>Files
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<p>Following is a list of files included in this archive. All files are
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in PostScript, except the <code>README</code> and
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<code>gadget.lst</code> files, which are in ASCII. The files
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<code>gadget.s01, gadget.s02</code> and <code>gadget.lst</code> were
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generated using the Schema schematic-capture program from Omation. The
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printed-circuit files <code>*.lpr</code> were generated using Schema-
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PCB, also from Omation.
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<p>Files
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<p><code>README</code> - helpful information
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<br><code>gadget.s01</code> - circuit schematic
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<br><code>gadget.s02</code> - minibox assembly drawing
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<br><code>gadget.lst</code> - net list, pin list, parts list, etc.
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<br><code>gen0102.lpr</code> - pcb x-ray diagram
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<br><code>art01.lpr</code> - pcb artword side 1
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<br><code>art02.lpr</code> - pcb artwork side 2
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<br><code>adt0127.lpr</code> - pcb assembly drawing
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<br><code>dd0124.lpr</code> - pcb drill drawing
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<br><code>sm0228.lpr</code> - pcb solder mask (side 2)
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<br><code>sst0126.lpr</code> - pcb silkscreen mask (side 1)
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2000-04-22 20:46:49 +04:00
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<hr><a href=index.htm>Home</a><address><a
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2000-03-29 16:38:44 +04:00
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href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a>
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</address></a></body></html>
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