hf/rf ZONE Products for the week of August 11, 2003
Micrel Semiconductor Says . . .
Micrel Introduces 300MHz - 440MHz IC Replacement For
Discrete RF Receivers
Micrel Semiconductor announced the MICRF008 RF receiver, a low-cost alternative to discrete superregen receivers and one that eliminates the need for production tuning. Micrel Semiconductor is an industry leader in the design and manufacture of integrated circuits for the analog, power management, and high-speed communications markets.
The MICRF008 is the latest addition to Micrel's family of QwikRadio receivers. Intended as a replacement for superregen receivers, performance has been tailored to meet the needs of cost-conscious applications. This new device is the lowest-cost member of Micrel's QwikRadio family and, like all QwikRadio products, it achieves even lower cost of ownership by offering higher integration than all competing IC solutions. By incorporating a patented sweep mode, the MICRF008 has been designed to work with the cheapest, low accuracy transmitters. Sweep-mode enables these superheterodyne receivers to sweep a relatively wide RF bandwidth, in effect emulating the wide capture bandwidth of a superregen receiver.
"All QwikRadio receivers provide higher integration and lower cost
of ownership than competing IC solutions, yet many manufacturers of the
lowest cost applications continue to use discrete solutions despite their
inherent problems associated with production trimming and long-term performance
drift," said Scott Brown, business unit manager. "The MICRF008
is our response to the needs of those manufacturers," adds Brown.
analogZONE Says . . .
This part confirms what I have thought for a few years -- since this sort of receiver product hit the market -- manufacturers don't want to spend money on an integrated transmitter solution when they can cobble an LC transmitter together much cheaper. It isn't stable, of course, and I know from my own experimenting with a faulty garage door opener that you can tune some of them over a couple of hundred MHz! This receiver admits that problem and solves it, and does so at the right price for most of the applications involved.
All frequency control in the MICRF008 is derived from an on-chip Colpitts oscillator that requires an external resonator -- most usually a ceramic resonator for cost -- that is at the nominal RF carrier frequency being used divided by 129. From the PLL the signal drives a synthesizer that produces a sweep signal that has a bandwidth about 3% of the center RF (9 MHz at 300 MHz.) This is a full superhetrodyne receiver with the IF set at 2 MHz and with a bandwidth of 0.8 MHz at 315 MHz or 1.1 MHz at 433.92 MHz. The input RF amplifier drives the mixer -- which is fed by the sweep signal as the LO -- to produce the IF. There is an on-chip IF filter which is driven by an input amplifier and which, in turn, drives an output amplifier to the peak detector. The baseband signal is filtered in a LPF where the bandwidth is set by an on-chip switched-capacitor resistor controlled by the logic state of a SEL0 pin to be either 2.4 kHz or 4.8 kHz. The dc value of the demodulated/filtered signal is data sliced by comparison with the level created by an external capacitor and the effective value of the switched-capacitance resistor -- which changes with the LPF bandwidth and the reference oscillator frequency. The signal from the LPF also controls the level of the dc signal fed back to the mixer and two IF amplifiers for AGC. An external AGC capacitor controls the ripple on the AGC feed and will also determine some response times. The data out pin is a switched current 10 µA push - 10 µA pull stage which is CMOS compatible. The maximum data rate is 4.8 kbit/s.
The swept LO signal overcomes the problem of the low-cost transmitters being used but there are always "effects" from such "causes." In this case the actual signal received does not have the selectivity as a tuned front end could provide, and there is therefore a noise penalty to be taken into account. The receiver sensitivity at the two test frequencies is a typical -95 dBm in both cases so this is unlikely to be a problem in most of the applications envisioned. However, for longer ranges Micrel gives some extremely good matching suggestions (in the data sheet) for the receive antenna to maximize the input performance.
This is a +4.75 V to +5.5 V part with an operating current that varies from about 6 mA at 300 MHz to 13 mA at 440 MHz. With the reference oscillator powered down the current drops to about 2mA. The receiver input overloads at a typical -20 dBm.
This part will be very well received by those who want to stay with their cheap and dirty LC transmitters in the door opening, toy and other control system markets.
The MICRF008 is in production in SOIC-8 and is priced at $1.29 in 1000-piece lots.