RF/MW ESD and complex matching using resonance

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.

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.

Impedance matching using two useful 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.

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.

FCC regulations for unlicensed transmitters

What does the term unlicensed transmitter actually imply? What is it that one can do with this frequency band/bands? In fact, to start a wireless system design in the unlicensed band this has to be the first step in the design. To understand this more fully we took a look at the FCC website. It is a vast website and in spite of a search engine it still takes a bit of doing to locate the relevant articles, regulations, rulings, tips etc. We did manage to locate a couple of papers which we believe are helpful for people who may want to understand this concept of the unlicensed frequency band in the US. These can be accessed through our website at www.signalpro.biz ( or, of course, through the FCC website!). Go to engineering_pages>engineer's corner and look for the unlicensed frequency band information. By the way, there are always updates to these, so it is a good idea to also check on the updates.

VSWR, refelection coeffcients, s11, etc

The basic quantities such as VSWR, return loss, reflection coefficients, s11 and s22 etc are all basic to design. Their interactions and relationships are also basic and a brief note to document these relationships can be both informative and useful. There are numerous descriptions of these individually on the web. We prefer to publish the note in our website at www.signalpro.biz>engineering pages>engineer's corner for interested parties.

The relationship of Tf ( forward transit time) and ft in a bipolar transistor model

Someone recently asked how the ft of a bipolar is related to tf, the forward transit time of the bipolar. The tf is a model parameter while ft is not. Yet we always talk about the ft of the transistor. The answer to this question can be found in the spg website ( www.signalpro.biz) under engineering pages>engineer's corner for interested parties.