In multi-carrier systems ( for example OFDM) the number of sub-carriers generate a high PAPR ( peak to average power ratio) of the generated signal. This fact has ramifications on the design and implementation of the circuits following the modulation and information generating circuits. In addition this also limits the number of sub-carriers that can be used. For example it the RFPA has a limited 1 dB compression point then high PAPR signal would cause the RFPA to be backed off from its most efficient operating point i.e the 1 dB compression point leading to loss of efficiency and thereby high power dissipation and so on. A brief paper released by Signal Processing Group Inc addresses the question of estimating the number of carriers and their relationship to the PAPR. Please visit the SPG website and click on the complimentary items to access this paper.

# Monthly Archives: January 2015

# RF Power Amplifiers and AM – AM and AM – PM conversion

Following on to the recent post on power amplifier specifications http://www.signalpro.biz/wordpress/?s=rf+power+amplifier+specifications&submit=Search another important dual set of specifications is the AM_AM and AM – PM specifications of the power amplifier. These are important for amplifiers that must be used to transmit data constellations which need both AM accuracy and stability and PM accuracy and stability. The result of not taking these into account ( in these types of applications) would result in severe errors in reception of the data ( EVM problems) thereby leading to increased BER ( which is another way of stating the result). Please check on this blog later for a more detailed look at these impairments. Please visit the Signal Processing Group Inc., website for more interesting articles or information about implementing your custom ASICs and modules.

# Peak to average power ratio and its effect on the efficiency of the rf power amplifier

This is a brief post. Peak to average power ratio of a signal is defined by the ratio of the square of the absolute value of the signal to the average of the square of the absolute value of the signal. This ratio has a significant impact on the efficiency of the output RF power amplifier.

In order to understand this effect one has to consider the characteristic curve of the RF power amplifier output power with respect to the input power. Elsewhere in this blog is a clear definition of this curve. The most important point on the curve (for this discussion) is the 1 dB compression point. Typically this is the optimum point to operate the power amplifier. This point has its associated input driver power.

Now if a signal with really low PAPR is used to drive this amplifier it can be driven at its 1 dB compression point fairly robustly with very little distortion. However if the the input signal has a large PAPR then when the signal continues to increase in amplitude because of its high peak power, the amplifier will distort. In order to eliminate this, the user must back off the operating point of the amplifier to some lower input power to accommodate the large PAPR signal. This means that the amplifier is no longer operating at its optimum high efficiency. ( For a discussion of RF Amplifier efficiencies please search the blog with that as a key word and examine the paper on RF Power amplifier design or visit the Signal Processing Group Inc website). It is this effect that causes high PAPR signals to be less efficient when using a RF power amplifier in a transmitter.

# A calculator for the current carrying capability of a PCB trace

A fundamental quantity that is often required in the layout of PCBs is the the estimation of the current carrying capability of a PCB trace given the thickness of the substrate, the copper thickness ( or weight) and where the layer lies. i.e. an inner layer or an outer layer. Signal Processing Group Inc.’s website has a few posts that examine this from a empirical point of view. The expression for the current carrying capability of the trace is given in a companion post/paper. In order to facilitate the calculation of the width of the trace a javascript calculator is now being presented. Please visit the SPG website and using the “complimentary items” link download or use this calculator.

# Output impedance of a fast logic driver

The output impedance of a fast logic circuit driver is an important quantity to know, since when it is being used with a transmission line or being interfaced to another circuit the impedance matching becomes critical. If the impedance is mismatched then ringing occurs and if very severe can even damage the device. It appears that a good way to get a feel for the output impedance of the driver is to use the VOH, IOH and VOL, IOL numbers for the output driver. So to check for the output impedance in the high state use the following: VCC – VOH/IOH and for the low state use VOL/IOL. We recently did this on the Fairchild tiny logic driver NC7SV17 and got 50 Ohms for both levels. Please visit the Signal Processing Group Inc., website for more interesting articles and data.

# A simple temperature stable/independent bipolar current source

It is not unusual to require a temperature independent current source in the design of analog ASICs and chips. These types of circuits take many forms. Some are simple, some are complicated requiring many devices to implement. Signal Processing Group Inc. has recently released a design of such a source that is quite simple in its configuration and easy to understand. The paper may be found in the Signal Processing Group Inc. website under the complimentary items link.