Introduction.
Booster-mixer is a specialized RF circuit, that was designed to be used as analogue front-end to common, low-cost digital radio transceivers. It has been designed, prototyped and tested during realization of a scientific grant by a group of university staff members and radio enthusiasts at AGH University of Science and Technology in Kraków, Poland. The main purpose of the solution was to make low-cost radio transceivers even more reliable and immune to external interferences, so they could find new area of application in demanding industrial environment, mainly in Polish underground coal mines. The improvement was tested in laboratory and field conditions and is now being prepared for production phase (patent pending).
The idea.
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Picture 1. The idea of the booster-mixer circuit. |
The idea of booster-mixer circuit is shown in picture 1. The input signal from antenna is fed into booster mixer (directly or by low noise amplifier), where it is converted down by mixer 1 (M1). The intermediate frequency signal is processed by a signal processing block (SPB) which is generally a set of filters and then converted back to RF band by mixer 2 (M2). Such signal is fed into receiver chip (RX). Both mixers operate with the same reference frequency from local oscillator (OSC). |
Typical application.
| The input signal is received by antenna and it's matching circuit (AM), pre-filtered (F1) and amplified by low noise amplifier (LNA). It is then fed into first mixer (M1) where it is down-converted. The signal in the intermediate frequency band is processed by signal processing block (SPB). This block consists of a pass-band filter (F3) and a regulated amplifier, that can normalize the signal amplitude in an automatic gain control loop. The signal is then converted up to the default RF band and fed into receiver chip (RX). In a typical example the chip is interfaced by a microcontroller (MCU). The reference frequency from local oscillator is split (SP) and fed into M1 and M2 mixers through buffers (BF). |  |
Key features.
- The circuit is suited for narrow-band radio communication systems.
- Universal structure working supporting many RF bands (tested at 434 and 869MHz)
- The signal processing block (SPB) can operate in a convenient intermediate frequency band (10.7MHz, 45MHz etc.)
- F3 filter removes unwanted signal components. This filter can be realized in many ways: optimized for performance (quartz filter) or optimized for cost (ceramic, LC)
- The automatic gain control loop with A2 amplifier extends the dynamic range of the silicon transceiver chip.
- Since both mixers operate with the same reference frequency, the stability and precision of the local oscillator (OSC) is less critical.
Results and parameters.
The circuit was built with discrete elements and tested with the following radio transceiver chips:
- CC1020 from Texas Instruments (formerly Chipcon)
- ADF7020 from Analog Devices in GFSK mode.
- AT86RF212 from Atmel (at 868MHz).
Selectivity test.
Spectral characteristics of the booster mixer circuit were measured using Rohde&Schwarz FS315 Spectrum analyzer. The presented results were measured for 3 booster mixer circuit configurations.
Version 1: 434.25MHz, 30kHz signal bandwidth, 18dB gain
Input: -50dBm @ 50Ohm
Output: -31.6dBm @ 50Ohm
Signal processing block with crystal filter
DC power supply: 24mA @ 3.0V
Version 2: 434.25MHz, 90kHz signal bandwidth, 30dB gain
Input: -60dBm @ 50Ohm
Output: -29dBm @ 50Ohm
Signal processing block with ceramic filter
DC power supply: 30mA @ 3.0V
Version 3: 869.5MHz, 100kHz signal bandwidth, 25dB gain
Input: -50dBm @ 50Ohm
Output: -25dBm @ 50Ohm
Signal processing block with ceramic filter
DC power supply: 34mA @ 3.0V
Packet Error Rate (PER) test.
This setup was used to evaluate Packet Error Rate (PER) of a receiver with and without the booster mixer. All tests were conducted in a test system shown in picture 3.
Picture 3. PER test setup.
A test pattern is generated in a PC using Matlab. It is a 37-byte test packet consisting of 6 bytes of preamble, sync word and 30 bytes of data. The data is encoded using simple Hamming (7,4) code with additional balancing bit. This digital pattern is filtered by a Gaussian filter (BT=0.5) and send as a waveform into AFG3252 signal generator. This signal modules the internal FM modulator in SM300 RF signal generator at 434.25MHz with desired FM deviation and adjustable output power. The RF signal is fed into a digital radiomodem, that basically consists of a booster mixer (unit under test), a silicon transceiver chip and a microcontroller. The transceiver is set to transparent reception mode, data clock recovery and sync word detection are disabled. All these functions are carried out by the microcontroller.
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Picture 4. PER test results. |
The microcontroller analyzes not only the effective PER but also distinguishes different causes of packet reception failure, providing statistics for:
Exemplary test results of a booster-mixer working at 869.5MHz with 64kbps are shown in Picture 4. As expected, the packet error rate is better by about 2-3 dB with booster-mixer. |
Patent information.
The booster-mixer solution is patented under polish patent no. PL 215148 B1 dated 31.10.2013:
Sposób korekcji pasma częstotliwości wejściowego sygnału do monolitycznych odbiorników radiowych i układ korekcji pasma częstotliwości wejściowego sygnału do monolitycznych odbiorników radiowych — [Method and circuit for bandwidth correction for monolithic radio transceivers] / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie ; inventor: Cezary WOREK. http://patenty.bg.agh.edu.pl/pelneteksty/PL215148B1.pdf
Summary.
The booster-mixer circuit can be used as an analog front-end to monolithic radio transceiver chips in order to improve overall selectivity and blocking performance while also slightly increasing sensitivity.
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