Square wave

Traditionally, the reception quality of a digital receiver is expressed in terms of BER (bit-error rate). This figure is the proportion of received bit errors in a given period. Typically, you measure the BER in the lab by applying an RF signal, modulated by a pseudorandom code, to the receiver under test. This Design Idea suggests an alternative method based on the use of a simple square wave. This method may not be superior to the usual technique, but it is simple to implement and gives a reliable result...

In operation, the reset signal first clears IC3 and IC4 to all zeros. Signals 1 and 2, both of which are clipped so that they resemble square waves, beat against each other in XOR gate IC1. If Signals 1 and 2 are both the same value at the instant the clock signal CK goes low, counter IC3 counts. The clock signal`s frequency must be high enough that the clock signal takes five to 20 samples of the IC1`s output before IC1 changes state. You have to determine the appropriate clock rate for your application...

The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply (SMPS) which employs a CMOS Schmitt Trigger square wave oscillator, 10 mH inductor, high voltage switching transistor (TIP47 or other high voltage, 1 amp transistor) and a driver transistor (2N3053). The inductor should have a low DC resistance of 1.5 ohms or less. The switching supply must have a load connected to prevent the voltage from rising too high, so a 22K resistor is used across the output which limits the voltage to about 120 DC with the phone ringer disconnected and about 90 volts DC connected. The output voltage can be adjusted by changing the value of the 150K resistor between pins 10 and 11 which will alter the oscillator frequency (frequency is around 800 Hz as shown). The supply is gated on and off by a second Schmitt Trigger oscillator (pins 12/13) so that the phone rings for about 2 seconds and then the circuit idles for about a minute between rings. These times can be adjusted with the 10K and 300K resistors connected to pin 12...