Active filters that use operational amplifiers depend on the performance of the amplifier for their performance as well. Recently  a brief study was made at Signal Processing Group Inc. using circuit simulations to identify and quantify these effects. A test bandpass filter was set up that used a MLF ( multi-loop feedbck) RC/ operational amplifier based second order bandpass filter centered at 1 Mhz. Simulations were done to assess the effects of varying gain and operational amplifier bandwidth. Results are available for interested parties on the SPG website located at http://www.signalpro.biz. The link can be found under the free articles page of the website.

In most high performance PCB designs a backplane ( or power plane/s ) are usually recommended. However there are reasons not to use these under components sometimes if better performance is to be obtained ( or oscillating amplifiers are to be avoided). So do not use backplanes ( ground or power) under components that may be sensitive to parasitic capacitance. For example a high speed opamp will oscillate if a backplane is used under it ( or its inputs). The reason is that the backplane leads to the generation of a parasitic capacitance under a pin which in  turn causes a loss of phase margin. Additionally parasitc capacitors cause loss of bandwidth or high frequency or high speed performance. Therefore it is best to evaluate where and ( where not) to place a backplane. Some early consideration to this can avoid headaches after the board is fabricated.

A CMOS Schmitt trigger circuit is a really useful block in many ways and has many applications in mixed signal design including oscillators, ( connect a resistor from input to output and a capacitor from the input to ground and oscillations begin easily) signal delay circuits, comparators, logic edge speed up circuits and so on. In a random search for design equations a useful paper turned up written by I.M Filanovsky and H. Bates that provides a set of design equations for Schmitt trigger design. Interestingly this treatment goes beyond the ordinary textbook analysis and provides a more detailed ( in our opinion ) analysis. The design equations were tested using PSPICE simulations and turned out to be quite accurate, The title of the paper is Schmitt Trigger Design, published by the IEEE.. ( IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-1:FUNDAMENTAL THEORY AND APPLICATIONS, VOL. 41. NO. 1, JANUARY 1994)

Frequently a simple frequency modulator circuit comes in really handy in RF design, test, modules etc. Signal Processing Group Inc., recently released a PCB based FM modulator using a 200 Mhz to 400 Mhz VCO. ( Other frequency ranges may be available). The board has the VCO, a DC bias input to set the rest frequency of the VCO, a high pass RC network for the modulating signal and input and output BNC connectors. The supply voltage is 3.3V to 5.0V. Interested parties may contact Signal Processing Group Inc, either through the website or directly by email at spg@signalpro.biz.

Signal Processing Group Inc., released a design of a 5 Ghz equalizer for digital signals like USB3 physical layer signals. It is an integrated version that uses a high frequency BiCMOS process. The schematic and layout is available for interested parties as IP. The simulation was done using SPICE and the layout database is in GDS2 format. In case of serious interest please contact Signal Processing Group Inc., either through the website or directly at spg@signalpro.biz.

Recently we received the first silicon of an IF chip designed in 0.6um BiCMOS process. Among other functional blocks is a RSSI circuit. It is composed of three logamps that provide the gain for the IF signal and three RSSI blocks that provide the RSSI signal current output. This current is subsequently converted to voltage through the addition of a resistive/capacitive load ( external) and an optional buffer. The frequency of operation that has been used in tests is 70 Mhz and 280 Mhz. The supply voltage for the chip varies from 3.3V to 6.0V. The IP can be used in other applications if desired. It is also available for purchase and porting as needed. Interested parties can contact Signal Processing Group Inc, through the website or directly at spg@signalpro.biz.

90 degree bends in microstrip are common. These may be chamfered or non chamfered. A generally accepted accurate set of expressions for the inductance and capacitance of the equivalent circuit of these bends is provided in a brief paper on the Signal Processing Group Inc.’s website. Please access these by first registering for “free stuff”.