#DECIBEL SCALE EXAMPLES RF TV#

These factors add to cost and complexity. In addition, the detector must be built in an isothermal chamber to minimize changes in the ambient temperature from affecting accuracy. Care must be taken in the design of the directional coupler such that all of the energy from the take-off port makes its way to the detector. There also other mechanical challenges to building a thermal detection system. The typical thermal detector has a 20 dB dynamic range, which is better than diode detection. Another limitation of thermal bridge detectors is its dynamic range. While this system can work well under controlled conditions such as fairly constant ambient temperature it is difficult to make these systems completely independent of temperature changes. The components enclosed by the dotted line are within an isothermal barrier, which is required for such type detectors to work correctly and not be influenced by external temperature variations. Figure 2 presents a simplified schematic. The resultant DC voltage is fed to a voltage amplifier such as a differential operational amplifier and fed either to a meter movement or analog to digital converter for electronic display. The resultant voltage output of the bridge will vary linearly with the level of RF presented to the bridge.

Heating by the applied RF signal causes the bridge output to change in accordance with the applied signal level. This element is coupled by use of an RF directional coupler to the RF signal. The thermal detector is comprised of a balanced resistive bridge where the resistive element changes resistive value with temperature change. The thermal detector is a linear detector which responds to the heating effect of the RF signal(s) applied making it more suitable for measurement of multi-carrier signals found in television systems, combined FM systems or multi-carrier digital systems such as CDMA and COFDM modulation techniques. Bandwidth is generally limited by the associated directional coupler. The advantage of the thermal detector is that it is inherently more broadband than the diode detector. Thermal bridge detectors are typically used to overcome the problems associated with measuring power of complex modulated and multi-carrier systems. There has been a lot written about the legacy style thermal detectors so the discussion will be limited to a brief description of thermal bridge detection methods. These types include electromechanical thermocouples and thermister types.Ī newer style of thermal detector is the thermal bridge detector. There have been several different types of thermal detectors used for RF power measurement over the years. This makes the diode detector unsuitable for multi-carrier systems. If a multi-carrier set of signals (such as those in the broad band line of a combiner system) are presented to a diode detector, errors are created due to the non-linear mixer effect inherent in diode action. In this case a 1.24 dB error will result in peak power measurement.Īlso keep in mind that a diode is a non-linear device by definition of its transfer function, and when used in a power measurement system they operate in the non-linear portion of their dynamic range. If the bandwidth were restricted to just 1 MHz, the resultant carrier would be attenuated by half or 3 dB. Restriction in bandwidth of the measurement system will not affect the average power but can have an effect on peak power measurement. The peak power would be 3 dB higher than this – giving a peak to average ratio of 3 dB and a crest factor of 2. The average power in this case is equal to the sum of the power of each carrier. The diode detector output provides a DC component average of the two signals.

For example, consider two equal amplitude RF carriers separated by 1 MHz. Now let us look at the case of peak measurements. The diode detection method shown above will work well for CW-type signals and fairly narrow-band signals with low crest factors. The detected output voltage which is an average of the RF sample is fed either directly to a meter movement or conditioned before being fed to an analog or digital display. The diode detector system consists of a pickup loop, diode, and smoothing network usually consisting of a resistor and capacitor. Diode detectors offer a simple, inexpensive means to measure CW (continuous wave) and relatively narrow-band type signals where an average of the RF sample is produced.

#DECIBEL SCALE EXAMPLES RF TV#

The Diode Detectorįor decades the average responding diode detector has been the mainstay of FM broadcast and TV aural power measurement. To get an accurate measurement of RF power is a challenge, requiring an understanding of the both traditional and newer, improved techniques of power measurement. The measurement of RF power has gotten more complicated with the introduction of complex digital waveforms and multi-carrier formats. Richard Haskey's: The Worst I've Ever Seen!.

Decibel scale examples rf