2.8-GHz prescaler keeps cost down
Neil Eaton, Emsys Engineering, Peterborough, ON, Canada -- EDN, July 20, 2000
The prescaler in Figure 1 inexpensively extends the range of a frequency counter by dividing the input signal's frequency by a factor of 1000. The guaranteed input-frequency range of the input prescaler, IC1, is 250 MHz to 2.8 GHz, although typical values are 100 MHz to 3.5 GHz. The prototype operates at frequencies well below 100 MHz, but its fastest generator goes only to 1.7 GHz, so you cannot confirm the upper range. The input-voltage range is 400 to 1000 mV p-p from 250 to 500 MHz and 100 to 1000 mV p-p for higher than 500 MHz. IC1 serves as a divide-by-128 prescaler, whose output is a 1.6V p-p square wave. The RC network level-shifts the output of IC1 to ensure that the top of the square wave is above the 2V input threshold of IC5A. The output of IC5A is a 5V, CMOS-compatible square wave with a frequency of 1/128 of the input frequency. Most frequency counters can handle these frequencies, but the submultiple is inconvenient for an operator. A further division by a factor of 7.8125 (1000/128) produces a scaling factor of 1000.
Fortunately, the frequency counter averages its input over many cycles, so the output of the prescaler need not be exactly 1/1000 of the input frequency for every input pulse. The 0.8125 figure is 13 divided by 16. The average frequency ratio is therefore 7.8125 if you divide 13 output pulses of 16 by eight and the remaining three by seven. For best results, the divide-by-seven pulses should be as evenly spaced as possible. The result is a repeating sequence 16 output pulses long with the following pattern:
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IC2 divides the prescaler's output by seven or eight, depending on the state of the B input. IC3 and IC5 both connect to the output of IC2, so they count output pulses, not prescaler pulses. IC5 divides the output by five to generate the divide-by-seven periods. IC4 divides the output by 16 to reset the cycle upon completion. Without IC4, the cycle would continue to divide by seven at every fifth output pulse for a ratio of 7.8. The construction of the circuitry inside the dashed lines in Figure 1 is critical. The MC12079 is available only as a surface-mount device. In the design, it and its associated passive components is mounted on a Surfboard (Capital Advanced Technologies Model 9081, available from Digikey). It is then fastened, component side up, to a bare copper-clad board to create a ground plane. All connections to ground from the high-speed circuitry go directly to the ground plane using short lengths of copper braid. (Desoldering wick is ideal.) The input uses a BNC chassis-mount connector with its shell soldered directly to the ground plane, and the center pin connected to the Surfboard with the shortest possible wire. The rest of the circuit is noncritical. To avoid clutter, Figure 1 shows no bypass capacitors, but you should place them near every IC. (DI #2564)
