LINEAR Programmable Baseband Filter Design Note 432 - Philip Karantzalis Design Note

Programmable Baseband Filter for Software-Defi ned UHF RFID Readers Design Note 432 Philip Karantzalis Introduction clock frequency. The lowpass division ratio options are

Radio frequency identifi cation (RFID) is an auto-ID tech- 100, 300 and 600 and the highpass division ratios are, nology that identifi es any object that contains a coded tag. 1000, 2000, 6000. Figure 1 shows a typical fi lter response A UHF RFID system consists of a reader (or interrogator) with a 90MHz internal clock and the division ratios set to that transmits information to a tag by modulating an RF 6000 and 600 for the highpass and lowpass, respectively. signal in the 860MHz–960MHz frequency range. Typi- A sharp 4th order elliptical stopband response helps cally, the tag is passive—it receives all of its operating eliminate out-of-band noise. Controlling the baseband

energy from a reader that transmits a continuous-wave bandwidth permits software defi nition of the operating (CW) RF signal. A tag responds by modulating the refl ec- mode of the RFID receiver as it adapts to the operating tion coeffi cient of its antenna, thereby backscattering an environment.

information signal to the reader.

An Adaptable Baseband Filter for an RFID Reader

Tag signal detection requires measuring the time interval Figure 2 shows a simple LTC6602-based fi lter circuit that between signal transitions (a data “1” symbol has a longer uses SPI serial control to vary the fi lter’s gain and band- interval than a data “0” symbol). The reader initiates a tag width to adapt to a complex set of data rates and encoding. inventory by sending a signal that instructs a tag to set (The backscatter link frequency range is 40k Hz to 640k Hz its backscatter data rate and encoding. RFID readers can and the data rate range is 5kbps to 640kbps.) operate in a noisy RF environment where many readers

For fi ne resolution positioning of the fi lter, the internal

are in close proximity. The three operating modes, single-

clock frequency is set by an 8-bit LTC2630 DAC. A 0V

interrogator, multiple interrogator and dense-interroga-

to 3V DAC output range positions the clock frequency

tor, defi ne the spectral limits of reader and tag signals.

between 40MHz and 100MHz (234.4k Hz per bit). The

Software programmability of the receiver provides an

lowpass and highpass division ratios are set by serial

optimum balance of reliable multitag detection and high

data throughput. The programmable reader contains a high L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

linearity direct conversion I and Q demodulator, low noise amplifi ers, a dual baseband fi lter with variable gain and bandwidth and a dual analog-to-digital converter (ADC). The LTC6602 dual, matched, programmable bandpass fi lter can optimize high performance RFID readers. –10

The LTC6602 Dual Bandpass Filter –20

The LTC6602 features two identical fi lter channels with –30 matched gain control and frequency-controlled lowpass,

and highpass networks. The phase shift through each

channel is matched to ±1 degree. A clock frequency, either internal or external, positions the pass band of the fi lter –60 at the required frequency spectrum.

The lowpass and highpass corner frequencies, as well

FREQUENCY (k Hz)

as, the fi lter bandwidth are set by division ratios of the

Figure 1. Filter Response for a 15k Hz–150k Hz Passband