Ground loops in analog and wireless design

Ground loops are parasitic paths in a PCB or an IC that can cause a number of bad effects. These are caused primarily by bad layout, circuit design or interconnections either accidentally or because of lack of experience. In order to avoid ground loops one has to understand what they are. A very brief description is given in a recent article by the techteam at Signal Processing Group Inc., and is available for review at their "engineering pages" in the website located at http://www.signalpro.biz.

Half IF spurious response and second order intercept points

An irksome 2nd-order spurious response called the half-IF (1/2 IF) spurious response, is defined for the mixer indices of (m = 2, n = -2) for low-side injection and (m = -2, n = 2) for high-side injection. For low-side injection, the input frequency that creates the half-IF spurious response is located below the desired RF frequency by an amount fIF/2 from the desired RF input frequency. The desired RF frequency is represented by 2400 MHz, and in combination with the LO frequency of 2200 MHz, the resulting IF frequency is 200MHz. For this example, the undesired signal at 2300 MHz causes a half-IF spurious product at 200MHz. For high-side injection, the input frequency that creates the half-IF spurious response is located above (by fIF/2) the desired RF. Note that high side injection implies that the LO frequency is above the RF frequency and low side injection implies that the LO frequency is below the RF frequency.

The second order intercept point is used to predict the mixer performance with respect to the half IF spurious response. For further details please see the article under engineer's corner/engineering pages in our website at www.signalpro.biz.

Load line analysis for RF power amplifiers

The most basic of analyses is the load line analysis for RF power amps ( or for that matter, any power amp). It is true that we all learned this in our formative years. However, it is equally true that we graduated to high performance complicated CAD programs that do so many things in an invisible manner that we no longer want to know ( sometimes) how the tool go to where it got to. A somewhat similar condition is common in digital ASIC design where the designer no longer needs to know how the logic gate works or what its device level parameters are. He or she simply writes the code that enables the design on a high level of abstraction. A brief expose of load line analysis is presented in a newly released paper by SPG and may be found at www.signalpro.biz under engineer's corner for interested readers.

RF Amplifier design: Load pull analysis

In the design of RF power amplifiers it is useful ( and important) to know how the output power of the amplifier gets influenced by changes of the the load impedance under varying conditions. In order to get an understanding of this, a useful technique is "load pull analysis". It is a graphical ( usually) technique that uses the Smith Chart to plot the contours of the load impedance for fixed constant powers. It provides valuable information to the engineer/user about the performance of the amplifier for reasons of assessment of the quality of the amplifier, conditions of operation, design fit or various other parameters. A technical article on the technique has been released by Signal Processing Group Technical staff and is available for perusal by interested parties in www.signalpro.biz>engineer's corner.

Substrates for high frequency design

We spent an absolutely intense two hours in discussions of substrates for RF and high frequency design with a couple of experts. Frequencies from about 1 Ghz to 77 Ghz were in play. The amazing part of the discussions was the level of parameters to be considered, not only in the manufacture of the laminates but also the layout of the interconnect, filters, transmission lines, and heat sinking.For high speed digital the control of the impedance/constant line width was more of a factor, unlike in RF where multiple line widths and shapes are in common use. A multitude of transmission lines are used in a bewildering array of combinations. Other parameters such as the glass weave and its impact on impedance was a discussion worth having. Three laminates emerged as winners for the a large number of applications in design. The venerable FR4 was buried under the the new requirements at 77 Ghz and even at 24 Ghz.The impact of DF and DK ( buzz words of course to be treated in some detail in subsequent posts). The use of materials and their trade-offs were fascinating. The size of the material sold has also gone through revisions and large sizes are now common. Gone are the limits of 18 X 24. The other very interesting issue that surfaced was the role of, and difficulty in, testing of not only devices but also the substrates themselves. The relationships between the thickness and the width of lines changes from the simple expressions we all knew. The difficulty of modeling has increased and very few CAD tools appear to have the capability to do what is needed. Only one CAD tool was mentioned several times as a recommended one for design and modeling at the high performance levels. Some very interesting numbers for insertion loss and actual measured values of permittivity and loss tangents were presented and argued over. Very interesting empirical design equations and data was presented as well. In this discussion the effect of the roughness factor was presented and emphasized. Finally a detailed discussion on the materials of construction such as resins,fillers and reinforcements ended the presentations. In short a very interesting couple of hours. Interested parties may contact us about these subjects through our website at www.signalpro.biz>>contact.

adjacent channel power ratio (ACPR)

In multicarrier systems, the carriers can be spaced quite close to each other. When this is the case a quantity referred to as the adjacent channel power ratio or ACPR becomes important. As mentioned above, multicarrier systems have a number of carriers which may generate signals whose power may add in phase. As more tones or signals start interacting, the peak additive power will increase. The average power of these signals may well be within the dynamic range of the system. However, the peaks of power may exceed the dynamic range. This will cause non linear odd - order distortion in the system. When this happens it results in adjacent channel power output or ACP. The ACPR is the ratio of the system output power at an offset frequency with respect to the power of the channel of interest. This can be considered one measure of linearity of a transmitter ( or RFPA). If the transmitter or the PA generates unwanted sidebands at an offset frequency that lies within the passband of an adjacent channel. For a given modulation scheme, the relationship between third order intermodulation products and the ACPR at a given power level is: ACPR = IMR2-tone + 10*log[ n**3/(16X + 4Y)].For a given modulation scheme, the relationship between third order intermodulation products and the ACPR at a given power level is: ACPR = IMR(2-tone) + 10*log[ n**3/(16X + 4Y)]. Here X and Y are given by:

X = (2n**3 – 3n**2 – 2n)/24 + [mod(n/2)]/8.0

And

Y = n**3 – {[mod(n/2)]/4.0}

All ratios here are in dBc. i.e. the ratio of the two tone intermodulation to signal carrier IMR and ACPR. Check out our website and engineer's corner. Go to http://www.signalpo.biz.

De - embedding in high frequency measurements

High frequency measurements for circuits such as MMICs and high speed digital circuits are made using some kind of Vector Network Analyzer ( VNA) or some kind of TDR instrument. In most cases the DUT ( device under test) is mounted on a test fixture which probably has an input connector and microstrip and an output connector and microstrip. The measurements are to be made on the characteristics of the DUT. To do this the test fixtures have to be de-embedded. This technique and its basics form the subject of the latest brief paper from the technical team at Signal Processing Group Inc. It can be found at http://www.signalpro.biz in the Engineer's Corner.

The wavenumber β or the phase constant

β is an important quantity used in understanding transmission lines and waveguides. It is not intuitive so this treatment presents a brief explanation of the quantity in the analysis of transmission lines, waveguide and other wave systems.

Sometimes β is referred to as the phase constant of the line or guide. If the cartesian coordinate system is used and a coordinate, say “z” is used as the direction of wave propagation then βz measures the instantaneous phase at point z on the line with respect to z =0.

In addition, voltage or current on the line is the same at any two points separated in z such that βz differs by multiples of 2π. Since the shortest distance between points where voltage or current is at the same phase is a wavelength, then:


βλ = 2π

( replacing z by λ),

β = 2π/λ

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Wireless design: electrical length

Sooner or later, the design engineer who is working in microwave or high frequency electronics, is going to come up against the concept of electrical length. In order to understand this concept lets work out the following arithmetic:

1.0 The wave number or phase constant = β = 2π/λ

For those unfamiliar with this, we recommend looking up the description of this quantity in the SPG blog at (http://signalpro-ain.blogspot.com/).

2.0 The electrical length is defined by θ = βl where l = physical length

3.0 θ = βl = (l/ λ) *360 degrees

Here λ is the wavelength of the signal in the applicable dielectric ( or sometimes called the guide wavelength).

4.0 For a frequencies in Ghz, this becomes: [360 * fGhz * l(cm) * √εeff]/30 cm


In this case frequency is in Ghz, physical length is in centimeters.

For example:

Let frequency be 1 Ghz.
Let λ = 0.8 λ(air) or √εeff = 1.25
Let l = 0.1 meters = 0.1E2 centimeters

Then :

θ = [360* 1*0.1E2*1.25]/30 degrees

θ = 150 degrees

Why is power transfer and power quantities used in high frequency design

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.

Definitions of the Q factor

Definitions of the Q factor


1.0 Unloaded Q : Energy stored in the component/Energy dissipated in component.

2.0 Loaded Q: Energy stored in component/Energy dissipated in component and
external circuit./load.

Ferrite beads are useful components

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.

Oscillator noise

Oscillators are very important components of any electronic system, be it in communications, signal processing, data acquisition or power electronics. In short, one always bumps up against oscillators in electronic design. Among other things that an engineer faces when designing oscillators or VCOs is the problem of ever present noise. It becomes important to understand the basics of noise sources and quantities in oscillators. Recently Signal Processing Group Inc., released an interesting paper on just this very subject. The paper may be accessed from the SPG website at http://www.signalpro.biz under the engineer's corner menu item.

Distributed element microwave filters

As frequencies increase in filters, lumped elements no longer satisfy the requirements for various reasons ( parasitics, accuracy etc). At this point the designer may choose to convert the lumped element filter to a distributed element filter. One of the techniques used is transmission line stubs in the conversion. This technique is described in a white paper released from Signal Processing Group Inc. recently. The paper may be found at http://www.signalpro.biz >> engineer's corner by interested readers.

Very Low power wireless design

Power has become a limiting factor in many applications. Specially in portable products, wireless applications and temperature limited products and systems. Therefore it is useful to investigate and develop really low power, "ultra" low power devices ( Integrated circuits and modules) that can be used to build more complex systems. techniques such as low voltage devices, sub-threshold CMOS circuits, energy harvesting etc. all play a part in the design of such devices. A number of low power designs can be done now with state of the art technology, specially for wireless ( RF amplifiers, mixers,detectors, LC oscillators, etc) that were not possible even a few years ago.It is a fascinating area of design effort. Over the past few years the techteam at Signal Processing Group Inc, has been involved in the design of low power circuits for use in RFID, 1.2V battery powered systems and short range wireless sensor based "data tubes". Some of this work is available for discussion. For details or for help in the design of these type of devices please contact SPG at http://www.signalpro.biz.

Radio wave path loss calculations and considerations

Radio signals suffer a path loss in free space ( as well as in other media). Recently a brief article was released by SPG technical staff which provides some simple expressions for the calculation of this free space path loss These expressions are useful in quick calculations of received power at a close in distance for an antenna. Using these expressions, and the formula for calculating the induced voltage in an antenna as a result of the received power, is a starting point for more sophisticated calculations. This paper can be found at http://www.signalpro.biz >> engineer's corner.

RAKE Receiver for multipath communications

Multipath in wireless systems is what happens when a transmitted signal travels along multiple paths ( e.g. as a result of reflections from surfaces). At the receiver these received signals can add or subtract depending on the amplitude and phase of the signals causing what is known as frequency selective fading. Another name for multipath interference. Obviously this is a real problem for cellular systems and many different types of solutions have been proposed and are being used. One of these solutions is the so-called RAKE receiver. The RAKE receiver has a number of "sub-receivers" called fingers, each assigned to a different multipath component. Each finger independently receives a single multipath signal. Subsequently, the contribution of all fingers are combined in order to make the most use of the different transmission characteristics of each transmission path. The important parameters that need to be estimated as accurately as possible are, the time of arrival of each finger signal, its amplitude and phase. Knowing these, the signals are combined in what is known as a maximal ratio combiner or a MRC. In this combiner each individual finger signal is weighted by the complex valued channel gain. The effect of this weighting is to compensate for the phase shifts in the channel and the change in amplitude. As this is done successfully significant improvement in signal reception is obtained. For details on a RAKE receiver design please contact Signal Processing Group Inc., through the website located at http://www.signalpro.biz.