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Frequency hopping transmission

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cwt

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Hi there,

This project is basically to imply frequency hopping spread spectrum (FHSS) method to ensure the security of broadcasting information. The prototype of project shall be designed by using PIC and ready-made transmitter-receiver module. And the hopping frequencies used shall be selected randomly and shown on the transmitter and receiver for verification purpose.

Currently, i am researching on FSK/FHSS system. And there are some doubts arised:

..................................................\/
...1,0 ---> FSK Mod ---> Mixer --->|............................<---Basically, this is the
....................................../\.............................................FHSS Tx. *If there is
.......................................|.............................................any mistake, feel free
.....................PIC-----> Hopping......................................to point out to me. Thx.
..................................Frequency

1. Can the FHSS output obtained by modulating FSK output and hopping frequencies by using normal FM circuit?

2. Will the changing of hopping frequency cause the mixer to operate imperfectly? (Eg. causes the gain to drop.)


Hope to hear from you soon. Thanks in advanced.
 

flatulent

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Anything you can do the other people can do. This means that if you FM the carrier and use an ordinary FM detector at the other end so can the other people.

To do good frequency hopping the signal has to be hopped more than the signal bandwidth and in a random way so that you cannot follow it with ordinary circuits. This also means that the hopping frequencies are not equally spaced.
 

cwt

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Thx for the reply.

I'll send a random sequence generated by the PIC to the receiver side first, before sending the actual data. And this random sequence is only understood by the transmitter and receiver set. So will this approach be ok?

Anyway, can the FHSS be implemented by using 'normal' FM transmitter and receiver with the hopping frequency supply by the programmable VCO?
 

cwt

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Hi there,

Is there any Tx RX module out there which supports FHSS. It is preferable that the carrier signal is feed in by an external source (not from the module itself).
 

E-design

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In early tactical transceiver design I was involved in, we used SSB and a set and sync coding system. The coding was dialed up on a row of thumbwheel switches and one side will transmit the "set", and the other will receive the "sync". What happened basically was that the sets will then frequency hop in a pre-determined sequence according to the coding algorithm dialed up on both sides. All this depended on a very accurate internal frequency reference clock of course.
 

cwt

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Hi jasnoch thx for the site..however, my project need to use PIC to control the hopping sequence..so it might not help. Since the modules you recommend do not required any further hardware or software developement. Thx anyway.

Hi E-design,
What is thumbwheel switch?

I dont really understand this sentence:


The coding was dialed up on a row of thumbwheel switches and one side will transmit the "set", and the other will receive the "sync".


So base on your design you'll be using 2 transmitter and 2 receiver? Each side consists of 1 transmitter and receiver? Just curious, is your design implemented?
Thx for the idea..
 

cwt

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For one direction communication, how am i going to synchronize the hopping sequence if i am using only one Tx and Rx?

I though is one Tx and Rx is used for data communication. And another pair of Tx and Rx is used for synchronizing the hopping sequence. Erm..if i am using this method, in your opinion what do you think? Because for my case, the hopping sequence should be randomize.

By the way, the carrier frequency you generate by using external circuits? If so, for the final modulation part what module or any IC chip you used?
 

E-design

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The RX was waiting to get the sync signal from the TX. At this time both the TX and RX started to frequency hop in a pseudo random sequence which depended on the code selected. It wasn't a very fast hop, only about a hop every 200mS or so. This required a very stable clock to the digital section to ensure that both transceivers stepped in sequence. This was designed for 2-way secure voice communications.

At that time we only had some common logic chips available on the market. Most of the design was done with discrete components and a Z80 processor. There were very few (if any) clever synthesizer IC's available.
 

cwt

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I see..
So you actually build your transceiver from basic components?

I think i'll need your guide throughout my project development time. Hope you dont mind. Thank you so much.
 
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