Charles Wenzel
The following circuit is constructed on a Schmartboard 201-0001-01.
This clock circuit generates in phase and quadrature clock signals of both polarities from a simple clock oscillator IC. A noise shunt on the power supply line reduces noise from the voltage regulator, typically a three-terminal IC.
Voltage
noise on the power supply line is amplified by the NPN darlington transistor and
converted to a current that flows to ground. This current to ground is set by
the emitter resistor and has been adjusted to balance the noise current due to
the input noise. The result is that the noise voltage on the circuit side of the
series 10 ohm resistor is about 20 dB below the input. (See:
https://www.techlib.com/electronics/finesse.html) Reducing the noise voltage
by this modest amount is usually sufficient to take noise from regulators out of
the picture, leaving the intrinsic noise of the clock oscillator to determine
the jitter. The photo shows a large sinewave induced on the power source with a
signal generator and audio transformer. The level is about 80 mV p-p at 1 kHz
and both traces are at the same sensitivity. The top trace shows the significant
reduction achieved by the shunt filter.
The oscillator is selected to be twice the desired clock frequency and can be as high as 20 MHz with HC devices. Choose AC devices for clock frequency above 20 MHz. The prototype's clock is operating at 1 MHz, producing 500 kHz signals. The photo shows one of the in-phase and quadrature outputs. Only one of the inverters of the '7404 is used, leaving the rest for other uses. They could be used to further buffer the clocks or to generate a multiplied frequency. If they are unused, the inputs should be grounded as shown.
Parts |
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| Component | Value | Quantity | Notes |
| Schmartboard | 201-0001-01 | 1 | Prototype will be built with leaded parts. |
| Transistor | MPSW45 | 1 | NPN darlington transistor. |
| Capacitor | 100 nF | 3 | Ceramic type; not critical. |
| Capacitor | 10 uF, 16 volt | 3 | Aluminum or Tantalum electrolytic. |
| Resistor | 3.3 ohm | 1 | 1/4 watt carbon film. |
| Resistor | 10 ohm | 1 | 1/4 watt carbon film. |
| Resistor | 1 megohm | 1 | 1/4 watt carbon film. |
| Logic IC | 74HC04 | 1 | use 74AC04 above 20 MHz (Clock Osc.) |
| Logic IC | 74HC74 | 1 | use 74AC74 above 20 MHz (Clock Osc.) |
| Clock Oscillator | 1 Mhz, typ. | 1 | Choose a clock at twice the desired frequency. |