The T – type attenuator can also be used with unequal I/O impedances to match the input line impedance with the output line impedance, as well as, providing attenuation of the signal. This can be done if proper values of the resistors are chosen . A brief paper on this is available on the SPG website under the “complementary” menu item.
The t type attenuator is a very useful circuit and is used quite extensively to not only attenuate signals but also to match inputs and outputs. This post presents the derivation of the design equations for this type of attenuator. The equations have been derived for a symmetric t type attenuator with equal I/O impedances. Please visit the SPG website and select the relevant post from the complementary menu items.
A javascript script is available to calculate the resistors needed for the t type attenuator in the case of equal input/output impedances. Given the required attenuation and “characteristic impedance” the script will calculate the resistors required. Please visit the SPG website and select the “complementary” menu item and then select the javascript from the list of items.
The T type attenuator can also be made using L and C ( “lossless” components. In this case the defining equations are slightly modified. Please find the equation and info on the SPG website under “complementary” items.
The T type attenuator is a very useful circuit block that has the capability to simultaneously match two transmission lines and attenuate the signals traveling on them. It is a symmetric circuit and when its impedances are chosen correctly the I/O can be exchanged without affecting the signals. Unequal impedances can also be used with appropriate modifications. These and other characteristics are explained in the “T pad” post on the Signal Processing Group Inc, website. Please visit for more info.
The noise factor of a device or circuit is simply SNR ( input)/SNR(output) where these are power quantities. The Noise Figure is the logarithm of the noise factor i.e N.F = 10Log( Noise factor). Please visit our website for more technical content.
We have been using C++/Windows for a long time for writing CAD tools as an adjunct to various industry standard tools. The types of areas we used it was for analog and RF/MW design and products. This process served admirably for a long time and gave us many advantages. Lately it seems we have started replacing some of the programs by applications written in JavaScript that seems to be faster to develop and provides, if not the same functionality or usability as C++ /C Windows etc but close to it. It seems we are writing more and more calculators in it for instance. For more info on this or other ideas please visit the Signal Processing Group Inc. website.
LTSPICE is a wonderful CAD tool developed by Linear technology ( now Analog Devices Inc) that can be accessed from Analog for simulation. It is a very professional CAD tool with many controls. One of these is the calculation of rms values of a waveform ( cntrl>left click). However, we found that the user needs to beware. LTSPICE can give you erroneous results, not because it is not working correhttp://www.signalpro.bizctly but because of the way it calculates rms values. Make sure that you do the hand calculations before using this feature. Please visit the SPG website with more info on other topics of interest.
A quick look at the high input power specifications and linearity specifications that are usually used in RF Power amplifier specifications. These quantities generally are: 1 dB compression, PSAT, IIP3 and OIP3. These are defined and described below.
1.0 1 dB compression: In the linear region of a RFPA operation, as the input power is increased, the output power also increases linearly. However, if the input power is continuously increased a point is reached where the increase in output power does not occur linearly. The output power starts compressing. This means that even though the input power is increased the output power does not respond to that increase. When the output power falls 1 dB below what was expected of a linear amplifier, that point is the 1 dB compression point of the amplifier. In many cases the 1 dB compression point is assumed to be the end of linear operation for the amplifier although in reality the amplifier starts compressing much before this point.
2.0 PSAT: PSAT stands for saturated output power of the amplifier. This means that as the power input to the amplifier is increased even beyond the 1 dB compression point a point will be reached where the gain of the amplifier will become 0 dB i.e. the amplifier is saturated at that point. Sometimes a 3 dB saturation point is also defined as a saturation point of the amplifier for more definition for the user.
3.0 IIP3: As the RFPA starts coming closer to compressing and gain starts changing from linear to compressed ( and in some cases even earlier, second and third order distortion products of the input signal start appearing in its output. A way to quantify this type of linearity distortion is to use quantities such as IIP3. As stated above as the input signal is continuously increased the output power also increases. If the third order distortion product is monitored, it will be found to increase at three times the rate of the fundamental component. If the third order product power is extrapolated it will intersect with the fundamental output power characteristic at a point where the input power is IIP3. It is a limiting power point that is used to understand the linearity of an amplifier as well as the limits of operation of the amplifier.
4.0 OIP3: OIP3 is simply the power being delivered by the RFPA when the input power is IIP3. Different users and manufacturers use IIP3 and OIP3 or both when specifying their amplifiers.
For more information on these topics and others of interest please visit the Signal Processing Group Inc., website.http://www.signalpro.biz
Further to the discussion of V to I converters it seems that the Howland Circuit is a pretty good fit for our needs. It only uses one opamp and some resistors and a few caps ( depending on your application needs) We would recommend this circuit. There is a detailed expose of this block from TI available publicly on the web. For more tech info and info about Signal Processing Group Inc, please visit our website ( under construction) at www.signalpro.biz.