PLL ( Phase locked loop) design using Analog Devices freeware.

We have been looking at various RF/Microwave design freeware tools recently. One of the tools that we looked at closely is the Analog Device ADsimPLL tools. This allows the design of PLLs and synthesizers using AD's devices which come pre-programmed in the software. The tool is interactive and fairly intuitive and user friendly. There are of course, a few challenges but considering that one pays nothing for its use it is well worth the time spent on analyzing and using it. We designed a 1.83 Ghz loop using the AD4360-7 device. The tool allowed us to calculate the various PLL related component values and provided a quick assessment of the operation both visually and textually. We would have liked to see some additional small features in the tool but all in all our assessment of the tool is quite positive. For further information on this or on PLL design activity at SPG, please visit our website at http://www.signalpro.biz and use the contact menu item for any further discussions or questions on our experience.

Estimating the signal band noise in delta-sigma modulators

Sigma delta modulators are popular devices used in a multiplicity of applications. One of the most prolific of these is the A/D converter. A delta - sigma A/D basically consists of a delta-sigma modulator ( typically first or second order), followed by a decimation filter. The modulator operates in such a way that it generates a high pass response for the noise in the system. This response is known as the NTF or noise transfer function of the modulator. In this way the modulator suppresses noise within the passband but allows the out of band noise components to have a high pass characteristic. A low pass system of decimation filters removes this latter noise also. It becomes imporatnt,in the practical sense, to estimate noise in the passband. An expression can be developed to do this for higher order modulators with fairly accurate results. This subject is dealt with in a recent brief paper released by Signal Processing Group Inc. It may be found in http://www.signalpro.biz> "engineer's corner".

RF/Wireless/MMIC freeware

A survey using search of RF/Wireless/MMIC freeware on the web led to a nice harvest of freeware routines that provide useful tools for those of us who may want to use these types of programs. It is well known that a number of EDA companies sell fairly expensive RF/Wireless/MMIC programs. For many designers it may be difficult to buy these because of the cost. For these users the freeware that is available on the web might be a partial solution. The freeware programs are not as beautifully formatted but appear to be reasonably accurate when compared to results provided by the more expensive packages. An ongoing interest for us is to take a look at these freeware programs and assess their usefulness and price/performance ratio. A useful package distributed free by Agilent is the first on our list. It is called "appcad" and may be downloaded free. Apart from the marketing type information in this package a number of useful tools are included. It certainly deserves a close look.

Logarithmic amplifiers ( LOG AMP); A useful component.

Logarithmic amplifiers or Logamps as they are commonly called are very useful components. They are used in communications, RF and wireless systems, cell phone base stations, audio systems, and power control to name a few application areas.. A typical use in RF/wireless is in the RSSI ( received signal strength indicator) circuit. The logamp can be deceiving in its functionality so a basic description is of help for those who plan to use it. A paper on this component and its basics is available on the Signal Processing Group Inc. website http://www.signalpro.biz under the "engineer's corner" menu item.

SINAD: What is it and why is it important?:

SINAD is figure of merit typically for radio receivers or similar devices. It may also be used in other applications. SINAD compares the signal power, the noise power and distortion power of signals. The specification is usually used in an audio sense. i.e the quantity under consideration is the quality of the received audio. A report on SINAD, its definition and other related parameters is available in the Signal Processing Group Inc., website at http://www.signalpro.biz > engineer's corner for interested parties.

Design considerations for integrated circuit RF/MMIC power amplifiers

Integrated circuit RF/MMIC power amplifiers are getting more and more popular. The PAs can be standalone or part of a larger device. Multiple technologies exist for the implementation of the circuits from CMOS to III-V. For the designer of these circuits different technologies present different challenges. In a brief paper by Signal Processing Group Inc., technical team, some of these issues are explored in a cookbook fashion. The paper may be found in the SPG website at http://www.signalpro.biz>engineer's corner.

SFDR or spurious free dynamic range

Someone asked a question about the significance of the SFDR. The answer to the question was as follows. ( For experienced receiver designers this is old hat of course.)

The SFDR is a specification which allows a reviewer to gauge the range of input/output signals that a receiver can handle while still in a linear range of operation.

The basic mathematical definition is:

SFDR = (2/3) x (IP3 - Noise floor)

The noise floor is defined as:

Pn(output) = kTBGF.

Where k = Boltzman's constant
T = Absolute temperature
IP3 = Third order intercept point at the output
G = Gain of the system
F = Noise factor.

Using this definition the SFDR can be calculated as:

SFDR = (2/3)(IP3 + 174 - 10logB - G - F).

Here the 174 represents the kT noise.

All quantities in dBm.

Thus if IP3 is known and gain is known , the input IP3 is known. The input signal should not exceed this as 3rd order distortion products will emerge from noise beyond this level at the input.

So an obvious conclusion is: Keep IP3 as high as possible and the noise floor as low as possible for high SFDR. Typically IP3 is about 11.6 dB above the 1 dB compression point of an amplifier.

Also it must be stressed that all components in a system, that have the potential of introducing distortion, should be assigned an IP3. Ultimately the final IP3 is the cascade of the individual IP3's.

Two useful impedance matching techniques

For maximum transfer of power from a source to a load, the source and load impedances must be conjugate matched. A number of techniques to do this have been developed. This post looks at two fairly simple and very popular ones. The L - section match and the cascade transmission line match. Simple analytical techniques are used to do this and described in the paper. The calculations can be done with a simple calculator. In order to access the detailed description, interested readers are directed to our website at www.signalpro.biz. Follow the links in the website to engineering pages>engineer's corner and then select the paper from the list on the page.

Receiver spurious response rejection

This is a very interesting specification for which no clear definition seems to exist. Note definition 1.0: Spurious rejection is the ratio of a particular out of band frequency signal level required to produce a specified output to the desired signal level to produce the same output. Definition 2.0: " All superheterodyne receivers have a potential for responding to frequencies other than the desired frequency channel. This needs to be minimized by designing in spurious response rejection by proper choice of the IF frequency and use of RF filters. 70 to 100 dB is achieveable in practical receivers." Definition 3.0: Ratio of desired signal to the total of all spurious signals at an offset of channel spacing in dB. What are these spurious responses being considered? A sample of these signals is described below:
(1) Image frequency/ frequencies.
(2) Half - IF.
(3) Straight IF pickup.
(4) High order spurs result from combinations of harmonics ( m,n) which result in spurious responses so close to the desired frequency response that they cannot be filtered out.
(5) A whole family of spurious responses of type ( 1 x n) is the n x LO spurs which can be troublesome if the RF front end has return responses or re-resonances.
(6) Second image in dual conversion receivers.
(7) Spurious signals present on the LO signal itself.
(8) Transmitted signal in half duplex radios assuming the role of a LO.

These responses are difficult to measure because of signal generator wideband noise.

Nevertheless this is a key receiver specification, and needs to be understood and above all, used and specified clearly.

RF/MW ESD matching using resonant circuits

An interesting technique that finds extensive use in RF/MW ESD circuits and complex matching circuits is the concept of resonating out reactances. Taking the case of the ESD circuit we find that in the most usual case RF/MW ESD circuits ( as other ESD circuits do) use some form of diodes to protect sensitive inputs on an IC. This of course leads to a parasitic capacitance which causes loading and mismatches. In order to eliminate the effect of this capacitance, at a single frequency an inductor can be used in parallel with the parasitic capacitance. The value of the inductor is chosen to resonate with the parasitic capacitor and therefore at the resonant frequency the pair becomes invisible leaving only the resistive part to be matched or considered. This is a simple technique which finds wide application in a number of critical circuits. Obviously the limitation is the single frequency characteristic. However, with some subtle manipulations it can also be used in wider bandwidth applications.

Noise figure versus input referred noise

If we use the specification for a low noise amplifier, invariably the noise performance is a Noise Figure. However, in a particular system design we calculated the input referred voltage that could be a limiting factor for the very first stage LNA. The issue was how to convert from the noise figure of a selected LNA ( from Analog Devices no less) to the input referred noise voltage to make sure the amplifier was being chosen correctly. Well here is the conversion at least in one form.

Note: The noise factor is simply 1 + NA/Ni. Ni is the noise power coming in from a 50 Ohm matched source and is equal to -174 dBm/Hz. ( Pretty standard usage).

The noise voltage being generated by the 50 Ohm source is vni=4.46E-8 Vrms/Hz. This can then be used to compare whether the amplifer will work with a particular noise figure ( from the expression 1 + NA/Ni).

Check and see if the number NA, the noise input referred power generated by the amplifier itself, converted from a voltage to power is acceptable or not. Must remember to use the impedance level of 50 Ohm. Simple?


Example: If the NF is = 0.8, then 1+ NA/Ni = 10**0.08 = 1.2 ( approx). We can calculate vna as above for vni.

Here is a note on input noise. It has been found that the -174 dBm/Hz should be modified to -162 dBm/Hz for the rural environment in the US and to -98 dBm/Hz for the urban environment. The -174 dBm/Hz is therefore a theoretical figure used to specify and calculate noise figures and noise factors!

Yes, another thought; we need to make sure that the derivation for the noise factor is elaborated: Here it is:

Noise factor F = SNRi/SNRo where i stands for input and o stands for output.

So = Si X G ( G = Gain)
No = [Ni noise power from the 50 Ohm source + NA, noise power generated by the amp].

F = [Si/Ni] / [GSi/G(Ni+NA)] = 1 + NA/Ni.

Also for other items of engineering interest go to our website at www.signalpro.biz.

The printed inverted F antenna

The printed inverted F antenna ( as opposed to the planar inverted F antenna) is a useful antenna capable of being printed right on the PCB of a wireless product. In an attempt to understand this antenna in more depth, the technical staff of SPG researched the topic. The result was that, there seems to be almost no information on this type of antenna in any of the typical texts on antennas. The only viable source for information on this antenna is the web. This too, is fairly sketchy. Our technical staff has now prepared a white paper containing the type of information on this antenna needed by practising engineers, and will be releasing it shortly via this blog, and in the engineering pages of the website at www.signalpro.biz.

More on the inverted F antenna analysis

An analysis of the printed inverted F antenna was carried out using the NEC2 program. This program is available in the public domain. It models antennas as wires.
One can set the wire radius. The disadvantage of the program is its inability to model dielectrics as substrates. However, in spite of this, with a bit of smarts a lot of information about antennas can be obtained from it. In case of a printed strip, Balanis's book provides the conversion between the wire radius and the width of the strip for those who may be interested in further analysis. Our experience has been that no matter how much modeling is done ( as we did follow up with ADS MOMENTUM)in the end the antenna ends up being tuned by somewhat of a trial and error method. In our opinion both procedures are important. We need a quick way to assess the antenna operation using a program like NEC2 which is surprising fast and a more refined means of simulation like ADS or FEKO. Check out the article in the SPG website ( http://www.signalpro.biz) under engineer's corner for printed antennas. ( Note: "Engineering pages"
has now been changed to "Engineer's corner".

A first pass success: Silicon proven RF Amplifier

A first pass success is always welcome. When the sucess is a high frequency device it is doubly so. The latest addition to the high frequency, silicon proven ASSP portfolio, is a high frequency, wideband amplifer fabricated in a 0.35um SiGe process.

It is fairly general purpose and can be used as gain block, an LNA etc. The basic features are as follows:

Features:

Usable frequency gain = 100 to > 2500 Mhz
19 dB typical ac gain at 900 Mhz, VCC = 2.7V
NFMIN = 1.2 dB at 900 Mhz
NFMIN = 1.5 dB at 2500 Mhz
1 dB compression point at 900 Mz = 2.9 dBm
1 dB compression point at 2500 Mz = 0.9 dBm
OIP3 at 1.5 Ghz = 15.0 dBm
OIP3 at 2.5 Ghz = 10.0 dBm
Power supply from 2.7 to 5.0 Volt
Power supply current typical = 4.7 mA
Reverse isolation s12 = -48.0 dB min.

The device was tested from -55 Degrees C to 125 Degrees C. An extended frequency test was also done at 5.0 Ghz. The gain dropped to 17 dB. Other parameters were also slightly affected.

Anyone with interest in this device and its development may contact the author via the website located at www.signalpro.biz.

RF ASIC design:Second order system analysis

Second order systems appear frequently in the design of analog systems as well as digital systems. In most cases these types of systems are difficult to understand analytically and designers must resort to simulations and empirical assessments.
Examples of these types of systems ( or circuits) are PLLs, switching power supplies, analog equalizers, mechanical servomechanisms, filters etc. There are some expressions available to do approximate analysis and design before resorting to long simulations or empirical data gathering. These are mainly based on the second order characteristic equation. Solution of this equation yields at least two very useful quantities. The natural damped frequency and the damping ratio. Use of these parameters can greatly facilitate the analysis and design of second order systems. For a brief cookbook style treatment of this analysis please read the article in our website www.signalpro.biz under engineering pages.

Analog ASIC and RF ASIC success factors

An analysis of several success factors in analog/mixed signal/RF ASIC design and manufacture turned up a number of interesting facts. There were many reasons for success that have been already described elsewhere in this blog. However, the interplay of relationships and their impact on the success of ASIC design and development was not touched. Much to our surprise the analysis of over 100 ASIC projects executed in SPG indicated that when significant success was achieved, not only were the obvious success factors present ( see the blog entry) but a key factor was the customer interface. (1) The customer interface was a technical person who was really closely involved in the design from the system side; (2) the technology that was being used to implement the ASIC was an excellent fit to the requirements; (3) the fabrication vendor relationship was strong and close with SPG; then the probability of clear success was over 99% (conservatively). We did not find a single failure in our list of 100 projects when these conditions were also met. ( In addition to the success factors quoted elsewhere in this blog. The very first entry in the blog since its inception). Thus the objective of this entry is to add this success factor to the list. The search for success in the analog/mixed signal/RF ASIC design and development is critical for our success.

Half IF spurious response and the second order intercept point

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.

Rf power Amplifiers: Load line analysis

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 got 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.

Image frequency in RF and wireless circuits

The image frequency in Rf/wireless receivers is an issue that has to be understood by radio designers and tackled for robust design. The image frequency is a so-called spurious signal which can cause a number of bad effects.Its origin lies in the mixing of multiple frequency signals in the receiver mixer. A paper released recently by Signal Processing Group Inc., describes this effect in simple terms so that an understanding of the effect may be obtained by interested designers. The paper can be accessed in the engineer's corner at http://www.signalpro.biz.

The role of the heat sink in power and RF IC design

As power circuit designs and devices proliferate in products such as LED drivers, HID lighting, motor control and electric vehicles it is becoming important to understand themal effects in active devices. All active devices dissipate power, and power active devices dissipate lots of power. This power dissipation creates heat which must be removed by some means to prevent excessive heat buildup inside a package or module which ultimately would lead to destruction of the appliance, circuit or device. One of the ways devices can be made safer, thermally that is, is the use of a passve heat sink. The role of the heat sink in active device thermal management is explored in a recent report released by Signal Processing Group Inc.'s technical staff and may be found in: http://www.signalpro.biz>engineer's corner>heatsink.pdf.

Random signal generation in PSPICE/SPICE

The SPICE programs we use for circuit simulation do not have a direct way to generate random waveforms. i.e. there is no voltage or current source which can be attached to a circuit node and which can generate a random signal for analysis. As a result we had to develop code on MATLAB and C++ to allow us to generate a PWL random waveform of as long a length as required. It is used as a piece wise linear signal and can generate the random signal as required.Please contact us through our website located at http://www.signalpro.biz for more information about this circuit simulation tool.

What is the difference between, an ASIC, an analog ASIC, a rf ic or a MMIC?

There are a number terms that are used frequently to describe the various forms of semiconductor devices used in the industry. An ASIC is one of them. Other terms are rf ic/ wireless ic, an analog ASIC, a MMIC and so on. So what are the differences between these devices? Ultimately all of these devices are semiconductor integrated circuits, indicated by the ic ( or integrated circuit abbreviation). However there are some differences between the various forms of these integrated circuits that lead to the differing nomenclature. Generally if there is no descriptive abbreviation before or after the "ic", then an ASIC is considered to be a digital device consisting of logic gates, digital memories, microprocessors or microcontrollers and associated circuits all integrated on to a relatively large piece of silicon. An analog ASIC or an analog ic is usually made up of mostly analog circuitry such as amplifiers, comparators, A/D converters, D/A converters,operational amplifiers, analog comparators, voltage references, regulators, etc. A pure analog ASIC or ic operates in the analog domain. It is usually smaller in size than a typical digital ASIC. An rf ic is distinguished by integrated circuitry such as transmitters, receivers, PLLs, modulators, frequency multipliers, rf amplifers, rf power amplifiers, mixers, inductors, transformers, baluns etc. An rf ic also operates at higher frequencies ( typically up to 1 Ghz). A mmic is a monolithic microwave integrated circuit. It may have the same type of circuitry as an rf ic but the frequencies of operation are much higher that rf ics. Currently mmics may be found operating at frequencies in excess of a 100 Ghz. A further type of device referred to as a mixed signal ic has both analog and digital circuitry on it. Sometimes the digital ciruitry is dominant sometines the analog circuitry is dominant.

What ia an ASIC? Why use it?

ASIC stands for "Application Specific Integrated Circuit". This means that an ASIC is designed to be used in very specific applications. Another term for it is "custom" chip. For ASICs a customer specifies certain requirements and specifications and the ASIC supplier then designs to those specifications and supplies the ASIC to the buyer. As such, ASIC has become a generic term. The usage of ASICs has become popular owing to a number of features that ASICs offer. First, the ASIC is usually very small. So it saves space on a board or enclosure that the customer may be using. In some cases there is no way out for the customer,but to use an ASIC. This may be the case, for instance for hearing aids, watches, cell phones. ( Although, strictly speaking a cell phone uses a lot of standard devices in addition to ASICs).An ASIC can be a great cost reduction tool.Insertion costs, discrete device costs, manufacturing costs and other associated costs csn be saved by using ASICs. An ASIC is a very useful tool when it comes to hiding intellectual property ( IP). Most of the circuitry is inside a very small package and can be made to be almost impossible to reverse engineer. Obviously a well designed ASIC can also increase reliabity significantly. No wires to come loose, nor connectors to fail etc. All in all an ASIC can be very useful indeed. To read more about designing an ASIC for yourself please see the article in this blog.