DAC Digital to Analog Converter | Ramsey Electronics CT255 instruction

The DAC (Digital to Analog Converter)

The resistor values in this ladder are chosen to give us nice even steps in the voltage range we are able to work with. The outputs of the counter can’t actually achieve 5 volts with the LEDs on the output, but really close at 4.67V. We then figure what the voltage will be across R38 when all outputs of the counter are at 4.67V and we will have the maximum output of our DAC. In this case it is 3.10V. Of course the minimum works out to be 0V. To find our minimum resolution we find the number of steps the counter has, which is in this case 2^8 which is 256 possibilities. 0 is one of them, so take 3.10V and divide by 255 to come up with 12.16mV per step or count. This means this is the smallest change in voltage that we can measure with our DAC.

When the counter is counting up, you can take a volt meter, probe R38 and see the voltage “ramp” up with the count. On an oscilloscope, you can not only see the voltage ramp up, but also take 255 steps on the way up to 3.10V, each step being 12.16mV! It’s worth it just to check this.

The ADC (Analog to Digital Converter? In a sense…)

Well, you may wonder how this ramp from our counters and resistor ladder can be used to display temperature. This will be done in a way many analog to digital converters used to make conversions. ADCs use much more complicated methods now to increase conversion speed, but that is beyond this manual. Now you know that each count is equal to 12.16mV, so if you have a count of 100, you should have an output of 1.216V from our DAC. Well this means that the display should also be indicating 100 degrees Celsius, right? This also means that our temperature sensor should be putting out 1.216V at 100 degrees Celsius to get this count of 100, but it doesn’t since it only outputs at 10mV per degree Celsius, which would only be 1.00V. This means we need to scale the output of the temperature sensor to meet the needs of our DAC so the count display and the voltages match.

Scaling to Celsius

To scale the output, we just have to figure out what it will take to make 1.00V become 1.216V. Not too tough here, we just multiply our 1.00V by 1.216. But wait, we’re going to make things complicated. We want to display temperature in 1/2 degree increments. This means that we have to multiply the sensor output by 2 to multiply our reading by 2. In this case we need 1.216 x 2 or a gain of 2.422. Since the display has a potential count of 255, and we will never find an air temperature of 255 degrees Celsius (and like it), we can divide the display by 2 to scale it to the range of 0-127.5 and use the last LED for 1/2 degree steps.

U1:A is set up as a non-inverting amplifier, and the output of the temperature sensor is sent to the non-inverting input (pin 3) to be amplified. R39 and C6 are

CT255 • 6

CT255 specifications

The Ramsey Electronics CT255 is a versatile and advanced communication tool designed for radio enthusiasts and professionals alike. As a 2-meter FM transceiver, it is known for its robust performance and extensive feature set, making it ideal for both casual users and serious operators.

One of the standout features of the CT255 is its compact design, allowing for easy integration into various environments such as home stations or portable setups. The unit is built with user-friendliness in mind, featuring an intuitive interface with a clear display and easily accessible controls. This ensures that users can quickly tune into their desired frequencies and modify settings without extensive technical knowledge.

The CT255 operates on a frequency range from 144 to 148 MHz, providing access to the popular 2-meter amateur radio band. This frequency capability is complemented by an impressive output power of up to 25 watts, ensuring robust communication over extended distances. It also includes a built-in automatic repeater shift, which simplifies contacts through repeaters, making it an excellent choice for those looking to enhance their communication range.

In terms of technology, the CT255 employs advanced digital signal processing (DSP), which significantly improves audio clarity and reduces background noise. This makes communication clearer, even in less-than-ideal conditions. Additionally, the transceiver features an array of scanning functions, including memory scan and priority scan, allowing users to monitor multiple channels effortlessly.

The CT255 comes equipped with multiple memory channels, enabling easy access to frequently used frequencies. This feature is particularly useful for users who participate in various nets or have multiple repeaters they commonly contact. Moreover, the unit supports CTCSS and DCS tones, which adds an extra layer of privacy and reduces interference from unwanted transmissions.

Safety and durability are also key characteristics of the CT255. The robust construction ensures it can withstand the rigors of regular use, making it suitable for field operations. It is designed with heat dissipation features to prevent overheating during prolonged use, ensuring reliability during critical communications.

In conclusion, the Ramsey Electronics CT255 is a well-rounded transceiver that brings together advanced technology, user-centric design, and excellent performance. Its range of features makes it an appealing choice for radio amateurs seeking a dependable and efficient communication solution. Whether for casual chatting or emergency communications, the CT255 stands out as a reliable tool that meets the demands of modern radio users.