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.

# Category Archives: Uncategorized

# LTSPICE and rms value calculations

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 correctly 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.

# 1 dB compression, PSAT, IIP3, OIP3 for a RF power amplifier revisited

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

# Analog Design:The Howland Circuit

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.

# A happy new year to all

2019 has dawned with so much promise. Looks like some really interesting conferences coming up too. The CES and the IMS conference in Boston. At SPG we continue our work on RF/Microwave and specialized APPS. Amplifiers, LNAs, mixers, oscillators, detectors, 902-928 filters ( SAW based) etc. Our service business is in full swing and we are not only designing products but also doing measurements, performance verification, testing etc. Our website is finally under construction as we attempt to modify it. Currently 4 books on Impedance matching have been published and are available on Amazon. All books authored by Ain Rehman. More informational products on RF/Microwave and analog hardware and APPS are in the works. Looking forward to your continued custom like years past. Please visit the website at spg@signalpro.biz.

# Voltage to current converter for a grounded load.

My recent experience in searching for information on this topic on the web was fairly negative. The reason for the search was to use a translinear circuit in my system design. Translinear circuits work on currents so a voltage to current converter is required at the input. ( Perhaps a current to voltage converter on the output. Simplest being a resistance). There is not a lot of information on V to I converters on the web specially if you are looking for a simple one. I found one that seemed to be popular and simulated it on LTSPICE. Its quite complicated and uses an opamp or two. Still searching for a simpler one which would be compatible with device level translinear circuits. Ultimately looks like I may need to design one.

# Volterra series

The Volterra series is a mathematical tool to model RF amplifiers with memory and in general, any non linear system with memory. In many ways the Volterra series is analogous to the Taylor series for systems with no memory. The Taylor series has coefficients multiplying each power of the independent variable. The Volterra series has “kernels”. Volterra kernels are difficult to calculate beyond the third order so most analysis only uses that as the maximum limit in analysis. The Volterra series was used to develop the behavioral model of a RF power amplifier. Please visit the SPG website for more articles and items of interest.

# Impedance matching using single stubs

A new book was published that describes the process of using a single stub to match a load with a transmission line. The method is simple but requires some understanding. This book describes the methodology, presents an example and presents a completely simulated design using easily available public domain tools. The book can be purchased from Amazon. To browse follow this link:https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=impedance+matching+using+single+stub

Please also visit the Signal Processing website for more items of interest.

# Cascaded line matching of impedance with ideal transmission lines simulation.

A cascaded transmission line is one of the simplest ways to match impedances. It is simply a quarter wave transmission line with a characteristic impedance given by : Z0 = sqrt(ZS.ZL) where ZS is the source impedance and ZL is the load impedance. A simulation using a public domain simulation CAD tool was performed for s11. The result is shown below:

From the figure the frequency was 5 Ghz. The schematic of this design is shown below:

Note that the impedance of the line is: sqrt(50*100). The length of the line is a quarter wavelength ( 1 wavelength = 60 mm at 5 Ghz).

For more information on impedance matching please refer to the available book : “VSWR and Impedance matching techniques”, available from Amazon,

In addition a forthcoming book that includes many other techniques and scripts is in preparation.

Please visit our website for more information on our offerings and more technical information/articles.

A low noise amplifier module with NF = 1 dB across the 100 Mhz to 3.0 Ghz band with typical 17.7 dB small signal gain over the band.

# Bipolar switching circuit performance parameters

As analog engineers focus on parameters such as the transition frequency of a bipolar device to estimate frequency performance of a transistor ( which is more of a small signal parameter). We sometimes get confused if we have to use a bipolar as a switching device. Especially in a saturated mode of switching. In this case the base of the bipolar is driven hard enough ( lots of base current) to drive the transistor when it is ON to store charge in its base region. The output voltage at the collector drops to its Vsat ( a few hundred millivolts usually) and when the transistor has to leave this state it has to contend with a storage time. So when we want to estimate the switching speed of a bipolar we need to use the datasheet parameters of the device that apply to the switching mode. These parameters are: td = delay time , tr = rise time, tf = fall time and ts = storage time. Then the maximum switching frequency of the bipolar is given by ( estimate), fmax=1/( td+tr+tf+ts). Note that ft, the transition frequency does not enter into this equation at all! For more technical info and services please visit our website.