in series with the input to reduce any noise that may be present on the temperature sensor to increase accuracy of your reading. C3 in the feedback branch is also used to reduce noise in the reading. In the Celsius jumper setting, R37 and R27 are used to adjust the gain of our temperature sensor for a Celsius reading. The gain of a non-inverting amplifier is calculated by the formula :

A = 1 + Rf / Ri. Where Rf if the feedback resistor R27, and Ri is the input resistor R37. (Even though the input isn’t at R37, it is considered that for mathematical purposes).

So looking at the values in the circuit:

A = 1 + 1430 / 1000 or A = 2.43

As you can see this is very close to our requirement of 2.422, which is more than close enough for +-1 degree C resolution.

More on the ADC

To stop the count when the output voltage of our temperature scaling amplifier matches the count of our ripple counter DAC, we use a circuit called a comparator. A simple op-amp makes an excellent comparator, as shown by U1:B. When the voltage at pin 5 is above the voltage at pin 6, the output at pin 7 goes as high as possible, in this case about 3.9V. When the voltage at pin 5 is below the voltage at pin 6, the output goes as low as possible, or zero volts. It is called a comparator because it is in reality comparing two voltages, and giving you an output based on the difference. We employ this circuit to stop the count in our ripple counter as soon as its output voltage surpasses the output of our temperature sensor circuit. This allows us to take a reading from the display, and this is what a computer would see as a value for a complete Analog to Digital conversion!

Oh that extra count!

Ok, maybe you’re smart and figured out that for the count to stop, the output voltage of the DAC has to surpass the output of the temperature sensing circuit. This means that if your temperature is zero, and the count is at zero, the output of the DAC does not quite surpass the output of the temperature sensor. This means that the temperature reading will have to increment at least once to have a reading. This means that your temperature readings will at the most be high 0.5 degrees F or 0.5 degrees C depending the range you are in. However, on average it will only be 0.25 degrees high. All ADCs have this problem, and it is related to the bit resolution (in our case 8 bits). This is still within +-1C accuracy.

CT255 • 7

Page 6 Page 8
Page 7
Image 7
Ramsey Electronics CT255 manual More on the ADC, Oh that extra count
Page 6 Page 8

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.
Top Page Image Contents