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A 7 Bit Barker Code Generator Circuit

This a simple circuit using CMOS logic that continuously produces the 7 bit Barker Code.

 

Figure 1: The scope displays two repetitions of the 7 bit code in yellow
and sync pulses in that coincides with  the generation of bit 1.


Overview






Figure 2. A regulator was included so the generator could be operated
independently of other circuits.The connector on the right side of the
 board makes +5V, ground, the code and sync outputs available connection
 to the circuit that requires the code. 
   

I need a fast, adjustable, and not necessarily very stable or frequency-accurate  Barker code generator for an investigation. I had been using AVR controllers to generate the codes, but merely changing the chip rate meant modifying the assembly language source and programming the chip, what's more as the chip rate increases, the firmware approach gets more difficult until finally, the controller can't quite keep up. That is the reason for this simple circuit. Its easy to adjust the frequency by soldering in a different oscillator component, and externally clocking the generator is also possible.


Circuit

The inputs of a three input NOR gate are driven high by three pulses in a repetitive seven pulse sequence that correspond to the time during which the Barker code waveform is low.

Figure 3. All of the gates are contained in the TC4000B package,
making this a two-chip solution for a seven-chip generator :-)

The chip rate is determined by an RC oscillator. At the moment, with the 10k resistor shown and the "Select" capacitor being 47 pf the chip rate is 609 kHz. Pin 5 of the Toshiba TC4000B, which is similar to the CD4000, is grounded through a 10k resistor. The allows a future option of using pin 5 as an input to switch the code generation on and off or by removing the 10k feedback resistor and the timing capacitor, external clocking is possible. The resistor on pin 5 may be replaced with a connection directly to ground.

It should be noted that with a 5 volt power supply and the 10k feedback resistor, the output on pin 9 is being asked to supply nearly 500 microamps, and that the TC4000B data sheet shows the maximum output low current to be 400 nanoamps. It would be proper to use a chip with a heftier output capability or a slightly larger value feedback resistsor.

The counter chip is a HCF4017B, set up so that when output "7" goes high, the counter is reset. The period that output "7" is high is approximately 100 ns, which has the effect of stretching the one chip period that the output is low by approximately that amount of time. I think that for my purposes, this won't be an issue until past 1 megachip/second and with a faster counter, that limitation can be pushed to higher frequencies.

A sync output that is to be used to trigger an oscilloscope is taken from the counter's "1" output. This output was chosen so that loading on the sync output would have little or more likely no effect on the code output.

A 78M05 is used for the voltage regulator because I have a lot of them, not because I need much current.

A green LED and a 470 ohm resistor were added as a power indicator after the schematic was drawn.


Construction