FAQs on Basics of Microchip RF Solutions

RF stands for Radio Frequency, but it often used in the sense of “ anything related with Electromagnetic signals”.

RF is the wireless transmission of data by analog / digital radio signals at a particular frequency. It maintains a two-way, online radio connection between a mobile terminal and the stationed host terminal.

The different RF Frequency Bands are :

HF: High Frequency, 3 MHz to 30 MHz
VHF: Very High Frequency, 30 MHz to 300 MHz
UHF: Ultra High Frequency, 300 MHz to 3 GHz
SHF: Super High Frequency, 3 GHz to 30 GHz
EHF: Extra High Frequency, 30 GHz to 300 GHz

Wavelength: the distance a radio wave will travel during one cycle.

λ=c/f

c = 3x10 pow(8) = speed of light (meters per second)
f = receive frequency (Hertz)
λ = wavelength (meters)

Basically the power level of RF world is expressed in dB / dBm. The common standard is to refer powers to 1 mW (0.001 Watts). Such power ratio, expressed in decibels, is called dBm.

P dBm=10log(P watts/1mW)

The advantage of using decibels instead of Watts to express the power of a signal along an RF chain is that instead of dividing or multiplying powers to take care of amplifications and attenuations, we just add or subtract the gains and the losses expressed in decibels. For link budget calculations, the dBm convention is more convenient than the Watts convention.

Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.

The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency. Continuous phase forms of FSK exist in which there is no phase discontinuity in the modulated signal.

There are three types of antennas: Big antennas, thin antennas, and very small antennas.

Big antennas are half a wavelength or bigger. These antennas are expensive, fragile, and predominantly used for longer range fixed point-to-point applications. The advantage with this type of antenna is that it has a gain factor of 10 or even higher. Big antennas are rarely used in license-free radio products.

Thin antennas are typically a single thin wire (or PCB track) structure between a half and a quarter wavelength at the frequency of operation. They are efficient and have a fairly omnidirectional nature, which is excellent for mobile type applications, and are often used on the receiver side, for instance, as a quarter wavelength wire dangling from the receiver box. The gain for this type of antenna is about 1.5 to 3. Of all antenna types they generally provide the best performance to price ratio, but larger size and relative lack of robustness (compared to small antennas) keep them from dominating short range radio applications.

Very small antennas are typically small loops printed on a printed circuit board. These antennas are also of an omnidirectional nature. Very small antennas have an extremely narrow bandwidth, and generally need a post-manufacturing step to tune them to the exact transmit frequency. However a typical small PCB loop antenna in a key fob converts 95% of the RF energy generated by the key fob into heat, radiating only 5% of that precious milliwatt of generated RF power and results in an antenna gain of 0.075 (-11 dB).

In spite of all these shortcomings, small loop antennas are extensively used when products require pocket-sized product dimensions, robustness, and low cost. Application note AN831 from Microchip describes the design and matching of a small printed loop antenna in detail.

To work on the antenna design you may simply download the EZNEC Antenna Designer free demo from www.eznec.com and play with it. This program will almost instantly give you a good idea of any antenna’s performance.

The rfPIC microcontroller devices integrate the power of Microchip’s PICmicro® devices with UHF wireless communication capabilities for low power RF applications. The devices offer small package outline and low external component count to fit the most space-constrained applications.

The following are the device in the Microchip product range :

On chip RF Transmitters:

rfPIC12F675F - 290-350 MHz – Supports ASK/FSK Modulation
rfPIC12F675H - 380-450 MHz – Supports ASK/FSK Modulation
rfPIC12F675K - 850-930 MHz – Supports ASK/FSK Modulation

Stand Alone Receivers:

rfRXD0420 - 300 MHz to 450 MHz - Supports ASK/FSK Modulation
rfRXD0920 - 800 MHz to 930 MHz - Supports ASK/FSK Modulation

For more details and update on Microchip RF products refer the link,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en010060

Microchip offers rfPIC Development Kit 1 – The Kit provides design engineers with an easy way to evaluate unidirectional remote sense and control wireless links based on the rfPIC12F675 and rfRXD0420/0920 devices. The kit is based on the popular PICkit™ 1 FLASH Starter Kit and consists of modular building blocks for different transmitters and receivers that can be utilized for prototype systems or to evaluate different options using Microchip products.

For more details on rfPIC Development Kit 1 refer the link,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en010060

With the UHF ASK/FSK Transmitter and the available Integrated crystal oscillator, VCO, loop filter and power amp for minimum external components the baud rates supported are :
• ASK data rate: 0 – 40 Kbps
• FSK data rate: 0 – 40 Kbps by crystal pulling

The maximum data rates supported by the Microchip RF receivers are :
• ASK: 80 Kbps NRZ
• FSK: 40 Kbps NRZ

The baud rates can be changed using the Low Pass Filter Capacitor selection on RSSI signal path. Please read the APPENDIX C: RSSI LOW-PASS FILTER CAPACITOR SELECTION from the application note AN860 (rfRXD0420 ASK Receiver Reference Design) from the linkhttp://ww1.microchip.com/downloads/en/AppNotes/00860b.pdf

RF Transmitter Module - Contains the RF Transmitter which is on chip to the rfPIC12F675 has memory as follows :

- 1024 x 14 words of FLASH program memory
- 128 x 8 bytes of EEPROM data memory
- 64 x 8 bytes of SRAM data memory

RF Receiver Module - The RF Receiver which is supported by PIC16F676 has the following memory capability:

- 1024 x 14 words of FLASH program memory
- 128 x 8 bytes of EEPROM data memory
- 64 x 8 bytes of SRAM data memory

The RF Microcontroller can be programmed using any of the Microchip Programmers like PICKIT2 / MPLAB ICD2 / MPLAB PM3 through Microchip MPLAB IDE.

For details please refer the Microchip Development Tools web link,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=81

Below are the few application notes which Microchip can offer:
AN868 Designing Loop Antennas for the rfPIC12F675
AN710 Antenna Circuit Design
AN678 RFID Coil Design

You may look into the antenna design application notes from the following link,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1488

Yes. The operating voltages and currents are well suited for even miniature batteries such as lithium coin cells.

Radio Frequency products are of comparable cost, size, supply voltages, and operating current to PIC microcontrollers. In addition, they are specifically designed for easy interfacing to embedded PIC microcontrollers.

No. Most countries have a number of license-free frequency bands. Products operating in these license-free bands do not require end users to obtain a license.

However, such products must still conform to performance standards required by the country of operation, and it is the responsibility of the product manufacturer to ensure that such standards are met. Compliance is usually confirmed by testing in a government certified lab that provides a third party test report that governments then accept.

These standards are not difficult to understand, but are not always reader-friendly or concise. Much of Microchip’s various RF support documents attempt to help users understand potential engineering issues relating to regulations, which are not always clear in the various government publications, and also explain why these regulations exist and how to design products that may be certified in multiple countries with little or no hardware differences. Once these regulations are better understood, many features of system and circuit design that may have initially seemed puzzling will become clearer.