7
$\begingroup$

I'm close to my EE. BSc finale and I'm looking to play around a little bit with the stuff I've learned. To be honest, I'm a little bit upset that after 4 years of learning communications theory, I'm not able to choose the hardware I need.

My idea is to build a half-duplex walkie-talkie with the following reqs:

  • Low voltage operation. Till 5V.
  • Range about 1.5 km LOS.
  • ISM frequency use. (I prefer 433 MHZ UHF bands.)
  • Single chip that doesn't require extra elements such as crystal or filters.

The idea is to install 2 walkie-talkies, one on each house. House are about 900m range. One has a direct window outside while the other is an interior office.

The application is for voice only. Considering the FM advantages I'll prefer it, but if there is a big difference of value then AM could be an option.

The chip I'm looking for must be the smallest that can be.

I've seen many chips (such as the following links) but I'm not sure that their digital buffers support voice applications.

http://www.hoperf.com/rf_fsk/fsk/21.htm
http://www.melexis.com/Wireless-Multi-Market--Sensing/RFIC-Transceivers-27-to-950MHz/TH7122-121.aspx

$\endgroup$
  • $\begingroup$ I see you posted a follow-up as an answer. While it's nice to get feedback on one's answer, that's not how we do things on Stack Exchange (actual self-answers are another matter entirely). This site is a questions-and-answers site, not a forum. $\endgroup$ – a CVn Dec 13 '13 at 9:04
  • $\begingroup$ If you want to request clarification on a post, you should leave a comment on it. If you want to express thanks, you should upvote (by clicking the up arrow next to the vote score) and possibly accept (by clicking the checkmark button). Accepting an answer generally tells other users that you feel that that particular answer properly addresses all your concerns. You can hover over the vote arrow buttons to see a short description of when you might want to upvote and downvote a post, respectively. Note that vote down takes 125 rep; by making a few quality posts, you should get there quickly. $\endgroup$ – a CVn Dec 13 '13 at 9:04
3
$\begingroup$

The chip you've linked is capable of analog FM transmission and reception, from 2.5kHz to 80kHz bandwidth. In the frequency range you're looking at it has a frequency resolution of 12.5kHz. It's very low power, with the right antenna it should reach your target LOS. Check out Figure 4.2 in the TH7122 and TH71221 Cookbook for an example showing how to use the chip as an analog FM transmitter, and figure 6.7 for an example of how to use it as an FM receiver. By combining the two circuits with the typical application circuit you can build a single-chip analog FM transceiver.

Note that your requirement, "Single chip that doesn't require extra elements as crystal or filters" is going to be hard to meet with any radio design. There are several inductors and capacitors used as filters outside most "single chip" transceivers. If you want any frequency stability you're going to need a crystal.

The Philips SA58646 is also a good choice, but again you're going to have to deal with a few inductors, capacitors, and a crystal.

Also note that with such low power devices your antennas are going to have to be very, very good, and probably very large. If you don't want to spend a lot of time and effort on antennas, or need small antennas, then you'll have to trade off some additional effort adding an RF amplifier to these devices and consuming more power.

Are you sure you don't simply want an off-the-shelf transceiver? If you're trying to avoid filters and crystals while building your own transceiver, it seems you might need a higher level solution than a DIY circuit. The radio linked is a cheap HT for the UHF band, runs on 3.7V, and has a 3W output. It's under $20, and if you remove the battery, casing, controls, connectors, then the one or two PCBs that are left are very small. It does appear to have a low power TX setting, so you should be able to set it to 1W output rather than the full output power if you have power consumption concerns beyond requiring low voltage. You may even consider modifying the radio to remove the PA and use the very low output prior to the PA, saving significantly more power.

$\endgroup$
0
$\begingroup$

There is a large amount of low powered hardware that can be shipped in from Europe on the 433MHz band, as there is an EU wide treaty which allows for Low Powered Devices (LPD) to operate on 69 channels between 433.075 MHz to 434.775 MHz with a licence.

Apart from the the required of low power, there is little restrictions on what can be used, so you can get LPD433 devices for model control, analogue voice, digital voice and key less entry systems.

If you are in the US, there should be little issues, but in the UK, channels 1 to 14 are UK Amateur repeater outputs and channels 62 to 69 are UK Amateur repeater inputs. Although HAM's are secondary users anyway as the Ministry of Defence has it as Primary.

remember reading that the UK government campaigned against it as the background QRM was so bad in this band, something which apparently wasn't a issue in the test of Europe.

As a result, low powered walkie talkies are available, although ones of 446MHz are more common, which allows for up 500mW.

10mW is the EU restriction of power regarding LPD433, however this may not be a massive problem if you are building it yourself from scratch.

Good Luck!

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.