Micrel Semiconductor has developed a single-chip radio transmitter for unlicensed bands in the 280 to 460MHz range that automatically corrects for differences in a connected loop antenna so the board does not need to be tuned on the manufacturing line with a trimmer pot.
The transmitter marks the latest move in the company's foray into radio designs, following on from a series of receivers.
"We fought shy of building transmitters before but we realised that customers wanted both," said Scott Brown, linear and RF product manager.
"Our part automatically compensates for any manufacturing tolerances. There is a CMOS varactor onboard with a DAC [digital-to-analogue converter] to vary the capacitance that the device sees across its pins," said Brown.
The varactor control circuitry uses the output from an on-chip phase-locked loop (PLL) synthesiser and from the power amplifier to work out the correct capacitance to tune for deviations in the loop antenna, typically implemented as a PCB trace.
"It looks at the phase on the output and alters the varactor setting accordingly," said Brown.
Because the tuning is dynamic, it can correct for the hand effect, which causes the resonant frequency of the antenna to change as a person's hand or other conductive object comes near it.
"The device gets a little unhappy if it sees no antenna: it just sweeps up and down," added Brown.
An external crystal or resonator feeds the PLL with a reference to generate the output frequency. A further level of programming is handled by implementing a voltage divider across two input pins. These set the operating voltage of the transmitter and can be used to implement a way of cutting the effective range of the radio. An external DAC can be used in place of the divider to let designers program in the range dynamically.
"Our part senses the output voltage and ensures it stays consistent over time. This is very useful as the battery dies," said Brown.
The transmitter is made on a standard 0.5um CMOS process. The company aims to use its transmitter and receiver cores as the basis for a range of more highly integrated devices for the remote control, keyless entry and home-automation markets.
Bijan Mohandes, European sales director, said: "We don't have to make it in our fab, it could be made at a foundry."
The company is working on designs for 868MHz, the incoming unlicensed band that has been tagged as the main frequency for cheap, low-power radio systems in the home. One target market is in wireless controllers and pads for games consoles.
"The video games controller market is going to explode. Those controllers will use 868MHz," said Brown.
Micrel is also planning to bring its silicon germanium (SiGe) process online later this year, letting the company move into the market now opening up for 10Gbit/s and faster communications devices. The process has been in development for about two years.
Bijan Mohandes, European sales director, said: "We will be able to make our own OC-192 class products. The process will allow devices with an frequency of 45GHz."
The company said in mid-1999 that it was working with Ottawa-based SiGe on developing a module that could be added to its existing silicon fab in Santa Clara, one of two the company owns.
Mohandes says the company plans to combine SiGe with CMOS logic. The company's approach to processing lets it bring together CMOS, bipolar and high-voltage DMOS on one die.
"Because of our foundry heri-tage, we started off with all three processes," said Mohandes. "We are going towards digital integration with SiGe. We intend to use SiGe as an overlay on our base arrays."
He adds that many of the company's products are implemented on transistor arrays because they are amenable to custom designs, which form a significant part of its business: "They just need a metal mask to change the design."