Preamp and 330 + MHz Prescaler

Published:2012/10/25 21:40:00 Author:muriel | Keyword: Prescaler, 330 + MHz | From:SeekIC

Signals applied to the input connector can be switched either through the AC path which includes the preamp and the prescaler, if switched into the circuit, or the DC path, which routes the signal to a Schmitt trigger buffer that then sends the signal on to the counter.Regardless of how the signal is routed, it must first pass through an input protection network, which includes two schottky diodes and a zener clamp. The 1N5711 schottky diodes prevent the input signal from going more than a schottky diode drop below ground or above the power supply. I used Schottky diodes because they have a lower voltage drop than the PN protection diodes on the CMOS integrated circuit they are intended to protect, and as such, they will draw much more of the current from excessive input voltages than the input protection diodes in the integrated circuits.The two 1N5226 zener diodes in series prevents the power supply from rising above 6.6 volts in case the input is accidentally connected to a low impedance source that is higher than 5 volts. The 47 Ohm resistor limits the input current in case of excessive voltage being applied to the inputs.The input of the frequency meter requires a full 5 volt CMOS logic swing, and the prescaler's output is less than 1 volt peak-to-peak, so the prescaler, when switched into the circuit, the signal goes through the prescaler, then the preamp, and the preamp drives the frequency meter through the Schmidt trigger buffers.The MCT10280 prescaler can be set to divide by 80, 40, 20, or 10, as a function of which pins are tied to the power supply. I set this one to divide by 10 since it is adequate for my needs, and the mental calculation of multiplying the meter reading by 10 is not taxing. One problem with the MCT10280 is that if it doesn't have an adequate input, the output is very noisy, which shows up as counts in the couple MHz range on the frequency meter. This noise shows up if the signal amplitude the signal frequency is too low. For this reason, I only intend to use the prescaler with inputs between 10 MHz and 300 MHz.



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