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help: what is RF circuit? (Read 2776 times)

thlcak New Member Offline Posts: 4	help: what is RF circuit? Feb 28^th, 2007, 8:10pm Hi all, Please don't laugh. I am a digital circuit design engineer and the only thing I know about RF is the wide spectrum from DC to 300GHz. Since this is RF design forum, would anyone explain to me what exactly is RF circuit designer? Is Op-amp a RF circuit? PLL? counter? Thank you.
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mg777

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Re: help: what is RF circuit?
Reply #1 - Feb 28^th, 2007, 8:54pm

The only person I'd laugh at is myself.

RF typically means wireless. Most RF carriers are in the 100 MHz to 5 GHz range owing to antenna size, modulation bandwidth, and atmospheric propagation. At the high end USB type wireless interfaces are being designed as high as 60 GHz. The wireline Gbps interfaces (like the SATA hard disk cable) are usually called high-speed to distinguish them from RF.

While RF signals used to be essentially sinusoidal, today's ultra wideband (UWB) RF signals have modulation rates approaching the carrier. Both amplitude and phase modulation are employed, with massive digital layers for modulation as well as error correction. The modulation is typically performed by a DSP while the error correction is either hardwired logic or a specialized embedded processor. The analog part of RF mainly deals with the antenna interface, up/down conversion, carrier synchronization, and analog-digital conversions. Other important analog pieces supporting the basic RF interface are power management, battery chargers, lighting/display drivers, and cameras.

The distinction between analog and digital is one of design paradigm. Analog designers think physically - in terms of voltage, current, impedance, and even fields. Stuff like noise, crosstalk, and jitter are staple analog bugbears. Then there's the design process itself. Where a digital designer uses a 'flow' for Verilog, RTL, synthesis, P&R, timing closure...., an analog designer has transistor schematic, transient SPICE, custom layout, PEX.... What distinguishes analog designers is that to date no one has succeeded in automating analog design to pose a challenge to the archaic but effective human-mediated flow. Generally excellent job security.

But, as Alice said, it's ultimately all much of a muchness.

M.G.Rajan
www.eecalc.com

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aaron_do

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Re: help: what is RF circuit?
Reply #2 - Feb 28^th, 2007, 11:18pm

Here's my piece :) I'm explaining in very basic terms but i don't mean to be rude or anything...

basically you start out with a piece of digital data which we will call a digital signal. Now you want to move it from one piece of digital equipment to another. Unfortunately, there's air (read as free space) in the way. So you need an interface between the digital circuit and the air. That in general is a RF circuit. It does not change the data. It does not compress the data. What it does is takes the digital signal and converts it to a form which can be transmitted and received efficiently then converts it back.

At the transmitter end you are concerned with outputing enough power so that when the signal finally reaches the receiver it is strong enough. You also want to avoid transmitting unwanted signals.

At the receiver end, you will receive the digital signal plus a whole bunch of unwanted signals, and a little noise to top it off. Bascially anything unwanted can in some way or another be made to look like noise (using IIP3, Image rejection, phase noise...). The job of the receiver is to make sure the ratio of the wanted digital signal power to the noise power (all unwanted power) is large enough to recover the wanted data; this is the SNR (this is akin to being in a noisy room...if its too noisy, you can't hear what your friend is saying, unless they speak louder than the noise).

So with all the transmitters signals flying around how do we ensure our SNR is high enough? Basically all electronic devices must conform to certain standards to ensure other transceivers can operate properly. These standards control the output power of the transceivers (and in fact all circuits).

If you want to know why we don't just transmit a digital signal, i suggest you read about: fourier transform, bandwidth, fractional bandwidth, and probably filtering.

I also want to point out that more and more the analog circuits are being replaced by digital ones. In the future (not too far) we will have fully digital transceivers (except antenna, PA, LNA...a few basic RF blocks). However I also believe there will always be a place for analog RF designers. When the digital guys get 2.4 GHz, we'll go up to 60 GHz and design PANs and then 100 GHz and put our transceivers on cockroaches... They'll never catch us... Grin

got a bit carried away there...

Aaron

there is no energy in matter other than that received from the environment - Nikola Tesla

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