Int. J. of Information Systems and Communications Vol. 1, No. 1, June 2011 Copyright Mind Reader Publications www.ijisc.yolasite.com
Designing a Crosspatch System to Interface between HF and VHF Radios Using RSSI Prabir Banerjee†, Rajorshee Raha* †Professor, Department of Electronics and Communication Engineering, Heritage Institute of Technology, Kolkata
[email protected]
*M.Tech Student, Electronics and Communication Engineering, Heritage Institute of Technology, Kolkata
[email protected]
Abstract—High frequency(HF),(3-30)MHz, and Very high frequency(VHF),(30-300)MHz, radio are the backbones of communication systems from many years. They are playing a major role in country wide communication system. Unfortunately, there is no foolproof direct interface between these two radio frequencies. A lot of research work had been done to create a cross patch between HF radio and VHF radio. But not enough success had been achieved. The search for a complete solution for this cross patch circuit is still in progress. The idea in this paper is to design a two way communicating circuit to interface between these two radio frequency ranges which will not depend on any external signal rather will take the help of an internal signal to provide the interfacing. Keywords — High frequency(HF), Very high frequency(VHF), Radio Signal Strength Indicator (RSSI), press-to-talk(PTT), signal to noise and distortion (SINAD), Dual tone multi frequency (DTMF).
I.
Introduction
The working principles of these two frequency ranges are different. HF radio depends on the phenomenon of sky wave propagation whereas vhf radio uses line of sight (LOS) propagation. HF radio is very useful for long range communication as it uses sky wave propagation. On the other hand, VHF radio is very effective for short range communication where the transmitter and the receiver antennae are in line of sight of each other. Both of these have some advantages and disadvantages. Generally, the HF links are very easy to establish because of sky wave propagation. We also can establish link easily between two remote places using HF radio which in case of VHF is a bit difficult and complicated since the terrain conditions, obstacles etc play spoilsport for VHF links. The set up needed to establish long distance VHF link is costly and also complicated.
A. HF- VHF Patching, The Issues One of the main disadvantages of VHF communication arises from the fact that it is only applicable for relatively short range communication and if we have to establish a connection between two points having large separation we have to connect via another VHF station, i.e., repeater. In case of HF communication, the main disadvantage is noise and it is not useful for short range communication because of skip distance factor. HF link uses amplitude modulation (AM) technique where the carrier signal amplitude is varied according to the amplitude of the modulating signal. The electrical noise signals are vertically polarized and affect the amplitude of carrier signal directly. That is why it affects the Amplitude modulated signals or in other words the HF signal during transmission. On the other hand VHF/UHF uses the frequency modulation (FM) and phase
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modulation (PM) techniques. So VHF signals are less noisy compared to HF signal and the limiter in the FM receivers help the reduction of noise. Although both of these communication systems have some advantages and disadvantages, they play leading role in the field of communication. Unfortunately, there is no direct interface between these two communication systems. If there were a direct interface between these two systems we could utilize the advantages of both HF radio and VHF radio together for more efficient utilization of both of the systems. It would be also helpful to decrease the spectrum burden for VHF/UHF system as frequency spectrum is the most costly thing in the field of communication and it also has a limit. Our idea was to design such a crosspatch circuit so that it can act as an interface between HF radio sets and VHF/UHF radio sets.
B. The Implementation It has to be a two way communicating crosspatch circuit as shown in the block diagram figure1. It need to have two sections, one for interfacing with the transmitter and receiver of the HF set and another one for the same action with VHF set. Our laboratory limitations had to be kept in mind also. SKYWAVE Propagation
HF SET
LOS Propagation
CROSS PATCH
VHF SET
Fig. 1 Basic blocks of HF-VHF Crosspatch
As HF sets and VHF sets are not available in our college laboratory, the final decision was to emulate the situation taking standard values for the radio signal strength indicator(RSSI) as a reference to generate the transmit or press-to-talk(PTT) command to the concerned units. The main function of the crosspatch/patch would be: • It has to decode/extract the received signal coming from HF network. • It has to feed this extracted/decoded received signal to the modulation input of the transmitter set for transmitting to vhf network. • Switch on the transmitter when we receive a reasonable signal from HF network. So we have to think of something very unique to give the command to the duplexer for transmitting the signal. When we are transmitting from VHF set to HF set then the crosspatch will first receive the signal from VHF set through the duplexer which is set for communicating with VHF set. In this case, when we get a reasonable signal from the VHF set, we can easily send command with the help of a squelch circuit to the other duplexer which is set for communicating with HF set to switch on the duplexer for transmitting the signal to the HF set. The main challenge was how to automatically switch on the transmitter in the VHF set when we get a reasonably good signal from the HF set through the duplexer which acts like a receiver when we are transmitting from HF set to VHF set. Reasonably good signal signifies a signal which has a signal to noise ratio (SNR) value greater than a threshold for the given link. So the main purpose of our project was to design crosspatch circuit for HF to VHF interfacing. The reverse interfacing is easy with the help of a squelch circuit. Squelch circuit could not be used to give that command to the transmitter as it do not respond in HF radio because of too much noise. The function of a squelch circuit is that, it acts like a muting circuit when the amount of noise in the received signal is too much. Then it stops the signal transmission or blocks it. It is generally used VHF and UHF communication. In case of HF Communication the amount of noise is very high so, squelch circuit does not work in HF radio.
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Designing a Crosspatch System…
RECEIVER (Towards VHF
TRANSMITTER (Towards HF Message signal from VHF set
VHF Signal
PTT command
Squelch/Muting Circuit
Fig. 2 Cross patch block (From VHF, towards HF)
So it had to be a signal which will indicate reliably the link signal quality which will give the command to the duplexer for transmitting the signal through the cross-patch to ensure that noise is less in the finally transmitted signal. Radio Signal Strength Indicator (RSSI) was taken as a reference as it is being widely used in the field of wireless communication because of its reliability. For example, it is used in cellular system as a reference for hand off decision. It also has a calibrated and true relationship with the signal to noise and distortion (SINAD) value of the signal. RSSI is a measurement of the signal quality in a receiver. So, standard RSSI value for an IF processor IC was taken as a reference for the circuit. After going through Various IF Processor datasheets (companies like HP, TOKO, Motorola, Sanyo), it was found that the range of RSSI is 0 volt to 4.5 volt. We decided to take a reference value in between this range to correspond to a reasonable value of SINAD(12 dB) and compare that value with the received HF radio RSSI value with the help of a comparator, as RSSI value is a DC parameter. We used a tone generator circuit to generate a tone when the received RSSI value is above the reference value to generate the press-to-talk (PTT) command for switching on the transmitter. A dual tone multi frequency (DTMF) generator was used for this part instead of taking single tone generator as it works better in a noisy condition. As DTMF generator will generate two simultaneous tones, the chances of corruption of both of the tones are less which is not possible in case of single tone generator. The tone, in turn, was decoded using DTMF decoder to produce the PTT signal. The full crosspatch is shown in figure 3. We implemented the cross patch circuit in the laboratory considering the limitations. As already pointed out, RSSI was emulated using a DC power supply. An op-amp as a comparator was used with reference voltage 2 volt. It will compare the RSSI value of the received signal with respect to the reference voltage. After getting the switch on command from the comparator via electrical switch it will start generating DTMF signal. Normally DTMF IC take inputs through keypad connected alongside them to generate the corresponding tone. That’s why we have used an electrical switch in between the comparator and DTMF generator IC to make the circuit fully automated. One reference row-column matrix combination has been set through the electrical switch which will activate it when the RSSI of the received signal is greater than the reference voltage level (indicated by the comparator). The electrical switch is connecting the comparator and DTMF generator IC. When the RSSI of the received signal is greater than the reference voltage level then it will give the switch on command to the DTMF generator for generation of the tone.
RECEVER (Towards HF set)
TRANSMITTER (Towards VHF set) RECEIVED HF SIGNAL
RSSI (In DC) IF PROCESSOR IC
Fig. 3 Cross patch block (From HF, towards VHF) MESSAGE SIGNAL FROM HF SET CONTROL SIGNAL TO SWITCH ON DTMF GENERETOR
COMPARATOR (VREF = 2VOLT)
ELECTRICAL SWITCH
PTT COMMAND
DTMF GENERATOR
DTMF DECODER
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For emulating the situation we have to analyse and understand the real scenario. For that purpose we have done lots of search work in various websites, books, paper and went through few data sheets which are mentioned in the reference section.
II. Testing A. Components used Op-amp (IC 741), Electrical Switch (IC 4066), DTMF Generator (IC UM91215B), DTMF De-Coder (IC 8870-01), IF Processor IC [14]for emulation purpose], Transistor, Resistance, Capacitor, Led, Voltage regulator (7805), Crystal (3.58 MHz), Vero Board and connecting wire.
B. Apparatus used CRO, CRO probe, Multi-meter, Dc regulated power supply [(0--5) volt and (0—12) volt], Soldering iron, Soldering wire, Cutter.
C. Troubleshooting Troubleshooting is one of the most important parts while testing a new circuit. While testing the crosspatch circuit we also faced few problems and had to troubleshoot them. We gained some real valuable experience during the testing process. Some of those situations which we had to troubleshoot during testing of the crosspatch circuit are given here. •
The circuit connection was complete but we were not getting any satisfactory out, we had to look that what went wrong. Ultimately we found that one part of it had a loose connection.
•
After this another problem came up, the crystal oscillators which have been used in DTMF generator and Decoder was not getting connection properly because of its thin legs as we were testing them on a breadboard. We had to fix it also after detection.
•
The biggest problem was still waiting. Although the total connection was proper the DTMF generator IC was not functioning. Later on we found that it was a damaged IC. So we had to change the IC. After this the circuit started giving satisfactory results.
III. Results
No Tone generation from DTMF generator IC
VHF (RSSI) is connected to 0 volt
Fig. 4 DTMF decoder output / PTT command When VHF (rssi) < Vref
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Designing a Crosspatch System…
VHF(RSSI) is connected to +5volt
Generated Tone from DTMF Generator IC Fig. 5 DTMF Generator output When VHF (rssi) > Vref
IV. Application The interfacing between HF and VHF radios can benefit communication by great means. It would be also helpful to decrease the spectrum burden for VHF/UHF system as frequency spectrum is the most costly thing in the field of communication and it also has a limit. It can be used to connect remote places using HF-VHF interfacing as HF links can be used to connect places having large separation and relatively close places can be connected using VHF link. It can also be used in case of Disaster Warning and Relief Content Distribution in Humanitarian Relief when the UHF/VHF networks are down.
V. Conclusion We used a standard signal from an IF processor IC namely, RSSI signal. This signal is a calibrated signal and therefore, the signal can be used confidently. So our circuit designed with RSSI as an input for the comparator will work reliably for the patching purpose. The RSSI Signal will also track the variation in SINAD of the HF link continuously. It will ensure that the patching between the HF link and the VHF link is always reliable. The basic idea has been implemented successfully as the crosspatch circuit (from HF, towards VHF) is giving satisfactory outputs during testing. The circuit is responding very well when the VHF(RSSI) is varied from 0v to +5v with respect to the reference voltage. We have thought of implementing the crosspatch circuit in real conditions using HF and VHF sets and IF processor IC. We have also thought of designing similar crosspatch circuits by using some different mechanism and compare them to find which mechanism is giving the best results in terms of performance, cost and reliability.
Acknowledgement The authors are grateful to the management of Heritage Institute of Technology for allowing them to use the facilities in the design laboratory of ECE department.
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[4].
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