trx10ghz.pdf

ELECTRONICS’2006 20 - 22 September, Sozopol, BULGARIA

SIMPLE TRANSPARENT RADIOMODEM FOR THE 10 GHZ

MICROWAVE BAND

Boris Yosifov Ribov, LZ1BJR

Space Research Institute, Bulgarian Academy of Sciences, Moskovska 6, 1000 Sofia, Bulgaria,

phone: +359 887 261866, e-mail: ribov@developer.bg

Some legacy systems require remote management using traditional communication

interfaces such as RS-232. The communication flow is low-speed so the use of complex

spread spectrum wireless equipment, based on IEEE 802.11, is unduly. The use of microwave

bands allows high distance coverage between end point stations with relatively low

transmission power. The efficiency of the antennas for the microwave band is very high so the

antenna gain is very high compared to the low-frequency bands.

This paper presents a low-cost simple radio modem for the 10 GHz band, which uses

narrowband FSK technology for transmission.

Suitable applications for this design cover a wide range of industrial systems for remote

management and control. For example remotely control of traffic lights systems, control and

management of dam lakes pumps, etc. The use of narrow-band FSK requires very small

bandwidth, which is important for the licensed bands.

Key words: Microwave, Wireless, Radio modem, Gunnplexer, RS-232

1. INTRODUCTION

Previous successes using 10GHz gunn diode oscillators as local oscillators and

transceivers for narrowband weak signal work [1], brought to mind the possibility of

using these inexpensive units for higher speed digital data transmission. In addition to

being inexpensive, these units - which are commonly used for motion detection (door

openers and burglar alarms), speed measurement (police radar guns), and microwave

receivers (radar detectors) - have all of the microwave circuitry self-contained. This is

important because it makes the equipment more attractive to non-microwave users.

The system block diagram in figure 1 shows the operating principles.

The two ends of a link operate "split." One transceiver oscillator typically

operates on 10.53392 GHz, while the other end is 433.92 MHz lower, on 10.1 GHz.

The difference between the two transmitter frequencies corresponds to the receiver

first IF frequency. The receiver first IF on each end is generated when the remotely

transmitted signal (frequency modulated by the data to be transmitted) is mixed with

the local transmitter. Each end use its own transmitter as a receiver local oscillator,

and each unit transmits continuously. Therefore, each receiver sees the same IF. This

is the same full duplex arrangement used for many years by Amateur microwave

enthusiasts. The transmitters run from 5mW to 10mW of output power depending of

the gunn diode type. The transmitter is frequency modulated as its bias supply is

varied and the frequency/voltage dependency of the gunn diode is used for tuning.

ELECTRONICS’2006 20 - 22 September, Sozopol, BULGARIA

This same technique was used previously to phase lock such oscillators. The 60 cm

dish used in this design has a gain of about 33 dB, or 2000 times at 10.5 GHz. When

driven by the microwave transceiver, the effective radiated power (ERP) is about 10

up to 20W and is the same as that of a 10-watt 2-meter band radio driving a quarter-

wave whip. We selected 433.92 MHz for the receiver first IF, with provision for

tuning +/- 10 MHz to accommodate differential frequency drift with time or

temperature of the free-running microwave transceivers. Using an IF in this range

also lets you do some simple troubleshooting and testing with commonly available

commercial FM 433MHz ISM receivers. No correction is necessary if both ends drift

in the same direction because the IF doesn't change. Automatic Frequency Control

(AFC), implemented by tuning the first IF nominally at 433.92 MHz, is provided to

keep the transceiver tuned correctly. This conversion produces the second IF at the

point, where detection takes place at 10.7 MHz in a Microchip rfRXD0420 FSK

receiver chip [3]. This chip is specified to operate at data rates up to 80 Kbps per

second.

Figure 1 – System diagram

2. TRANSCEIVER MODULES

The structure of transceiver is shown on figure 2. The design looks very

complicated, but in fact it is implemented on 3 low-cost integrated circuits (IC) and 2

special purpose diodes (a gunn diode [5] and a microwave mixer diode). The use of 2

intermediate frequencies (IF) and signal filtering stages ensure very good sensitivity.

The transceiver acts like a transparent modem. That means it bridges the serial

interfaces directly without using special control (AT) commands. The data that comes

to the RS232 interface is buffered. A flow control is supported by the modem using

the RTS/CTS signals. The main processor forms a packet for transmission, when the

ELECTRONICS’2006 20 - 22 September, Sozopol, BULGARIA

buffer gets full. Transmission can be started if the data in the buffer did not reach the

optimal size. That happens when a specific timeout occurs.

423.22 MHz

f/ 16

BUFFER

GunnPlexer

10.7 MHz

Mixer

26.45125 MHz

433.92 MHz

433.92MHz

10.7 MHz

10.7

422.92

FSK

DEMOD

10.7

LIMITER

IF AMP

LNA

MHz

10.59992 GHz

MHz

BW= 500 kHz

BW= 2MHz

BW= 180 kHz

DATA IN

RSSI

10.1 GHz

FIFO

controller

PIC16F877

LM317

AFC

DATA OUT

Gunn

OSC

RS-232

Figure 2 – Transceiver high-level diagram

2.1. Audio channel

We added the audio channel as an afterthought. It provides for human

communication, particularly while debugging the link and operating it with digital

data the first time. An electret microphone produces the audio signal for the

transmission. The signal is amplified and limited also filtered by high and low pass

filters before modulating the transmitter. Levels were selected to provide only small

deviation compared with that of the digital channel. This allows the audio channel to

operate without significantly interfering or degrading the digital data. A volume

control and speaker amplifier sufficient for driving headphones or a small speaker are

provided on receive.

2.2. Main Microcontroller

Design uses a low-cost microcontroller from Microchip as main processor for the

modem. The microcontroller is PIC16F877A [2] and integrates ADC and UART

modules. The UART module is used for the intercommunication with the host.

Microcontroller is based on high performance RISC architecture. Working on 20MHz

it is able to supply 5MIPS. Also it has in-build 4K-program memory and 384 bytes of

RAM memory. Two of the ADC inputs are used for the RSSI and AFT measurement,

while another one measure the current value of the voltage that supplies the generator

gunn diode.

2.3. RF part and modulator

The rfRXD0420 [3], manufactured by Microchip is low cost, compact single

frequency short-range radio receiver requiring only a minimum number of external

components for a complete receiver system. The rfRXD0420 covers the receive

frequency range of 300 MHz to 450 MHz. The rfRXD0420 can be configured for

Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), or FM modulation.

The rfRXD0420 is compatible with rfPIC™ and rfHCS series of RF transmitters.

Functional diagram of the receiver is shown on figure 3.

Modulator consists of linear regulator LM317, which supplies the gunn diode.

The loop back input of the regulator is used for the modulation input. Thus ensure

ELECTRONICS’2006 20 - 22 September, Sozopol, BULGARIA

voltage variation at the power supply, which comes to drive the gunn diode

proportional to the modulation signal. The variations of the gunn diode supply

ensures frequency offset (frequency modulation) in the frequency generated by the

gunn diode. Circuit diagram of the modulator and AFT sections is shown on figure 4.

Figure 3 – Receiver functional diagram

Automatic frequency tuning process acts directly to the modulator to compensate

temperature frequency offset. Because of the continuously work of the both

transceiver modules the intermediate frequency of 433.92MHz is always available in

both receivers. The automatic correction of the frequency is made in one of the units.

It is enough to make the first intermediate frequency steady. The correction from one

side compensates the frequency drift from the other side of the link because

instability of the TX frequency in both modules cause first intermediate frequency

instability, which is the same for both modules.

+12V

OUT

LM317

+12V

100nF

100k

1N4148

100k

10k

120

R13 220

1N4148

10k

R10

51ohm

10k

U2B

LT1365

10pF

DATA_IN

10k

510

100n

R11

2k4

10n

R12

2k7

100n

+5V

3A703A

GUNN OSC

U2A

LT1365

+12V

240

100n

U2C

LT1365

100n

R14

100k

AFT_REF

100n

MIC

AFT_IN

Figure 4 – Modulator circuit diagram

2.4. Microwave part and Gunnplexer

The outdoor unit consists of 60 cm parabolic antenna and a gunnplexer [4]

module. The gunnplexer module is a mechanical construction shown on figure 5. It

has two sections; one is for the mixer diode near the open end focused to the

parabolic antenna and a generator gunn diode [5] in the closed section. The gunn

diode is AA703A or 3A703A . It acts as generator that supplies the RF power to the

ELECTRONICS’2006 20 - 22 September, Sozopol, BULGARIA

antenna; also it acts as local oscillator for the mixer diode in front of the generator

diode. Mixer diode is a microwave mixer diode.

Figure 5 – Gunnplexer construction

We used ɛ 605 for the mixer diode. At the output of the mixer diode comes the

first intermediate frequency, which is about the difference between generator diode

frequency and the reception frequency. In our case the first intermediate frequency is

433.92MHz.

3. GOALS IN THE PROJECT

Fortunately, microwaves and high-speed communication fit together very well. In

fact, if the data rate is increased significantly it is absolutely necessary that wider and

wider bands be used. As frequency is increased, antennas of reasonable physical size

are better able to focus the transmitted beam without wasting signal in different

directions. The Amateur microwave bands, through 24 GHz, offer the best available

performance and cost for such communication. In order to be widely useful our link

needed several attributes:

•

To be inexpensive - competitive with present TNC/radio combinations

•

Moderate speed - significantly faster than current alternatives of

1200/2400/9600 baud

•

Medium range - at least 35 kilometers to be effective

•

To use readily available parts

•

To be simple to build and maintain

•

To be reliable - a variety of applications may depend upon it

4. SYSTEM PERFORMANCE

The system performance depends of the gain of the antennas, free space signal

propagation, cables loses and TX power. The transmitted power is limited to 10mW,

which is 10dbm. The system operation margin depends of the environment

parameters and is in the range of 10-15dB for safeguard work. Another parameter for

the calculation of the distance between the nodes is the receiver sensitivity. Measured

sensitivity of the presented demodulator is about –96 dbm. So, the link performance

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