Ferrite beads: A useful circuit component

Ferrite beads are a very low cost and easy way to add high frequency isolation loss in a circuit without a power loss at DC and low frequencies. Ferrite beads are most effective at frequencies in excess of 1.0 Mhz. When these are used with the appropriate parallel capacitance, they provide high frequency decoupling and parasitic suppression. A brief paper on ferrite beads has been released by Signal Processing Group Inc and may be found at http://www.signalpro.biz>>engineer’s corner.

Why is power transfer and power quantities used in RF/MMIC circuits?

It is seen that in high frequency circuits, power transfer and power quantities are used. Typically dBm will be a standard unit in use. The question is: why? The answer to this question is found in relative performance of circuits at high and low frequencies. When frequencies are low, a voltage or current signal applied at an input of a circuit or chip is reproduced quite faithfully in the chip or at the operating terminals of the circuit. The same is true at the outputs. The reason is that parasitic quantities do not play as large a role at low frequencies.The situation is quite different at high or microwave frequencies. At these frequencies the voltage or current signal applied to the input terminal of a device package is not what the active device sees inside the package. The reason is of course, the parasitics of the circuit.If instead of input current or input voltage as the signal quantities we use power delivered to the input port then this problem goes away since reactances do not dissipate power. At the output, if the true available power gain of the device is given, we can calculate accurately what to expect assuming no power is dissipated in the parasitic elements. These reasons are why RF/MMIC circuits are almost always designed with power flow or power transfer considerations.

Measuring temperature using thermistors: The Steinhart-Hart coefficients

The thermistor is a useful device for measuring temperature in certain situations. ( See the earlier post in this blog).The issue is that the thermistor has a non linear temperature response and thus may be complicated in use if used directly. The thermistor curve (temperature versus resistance ) has been modeled very accurately by an analytical equation which can be used to advantage in thermistor based temperature measuring circuits. The fixed coefficients in this equation are referred to as the Steinhart-Hart coefficients and can be either obtained from the manufacturer of the thermistor or be extracted from a resistance versus temperature curve. In any case, once these coefficients are known, an analog – digital system of circuits can be used with a microprocessor, to measure temperature relatively accurately. A paper on the extraction of the coefficients and the analytical approach has been released by Signal Processing Group Inc., and is available for interested readers at http://www.signalpro.biz>engineer’s corner.

Analog and mixed signal design: Measuring temperature with thermisters

Measuring temperature fairly accurately can be done using a number of methods. The sensors that are available to do this are the RTD, the pn junction, the positive temperature coefficient thermister ( PTC) and the negative temperature coeffcient (NTC)thermister. Among these options the NTC thermister seems to be used more and more in applications where the temperature rate of change is fast. The advantages and disadvantages of using this type of device are: Advantages, fast reaction time, small size, two wire connections and relatively inexpensive. The disadvantages are: the temperature versus resistance characteristic is very non linear, some kind of excitation is required, the temperature range is limited, subject to self heating and relatively fragile. In spite of the disadavantages the thermister is a choice many design engineers are making. The web has a number of good articles that are very helpful in the understanding of the thermister. Articles from Betatherm, Microchip technology and National Instruments to mention a few. The challenge does not lie in understanding the thermister itself. It is very easy to undertstand, at least as far as the users perspective goes. The challenge is in coming up with analog and mixed signal circuitry that interfaces with the thermister and allows for accurate measurement of the temperature. Signal Processing Group Inc., has developed a number of circuits which can be used with varying accuracies to measure temperature with thermisters. Interested users may contact SPG at http://www.signalpro.biz> contact.