I am developing an analog RF link for a purpose not related to part 15 "broadcasting". I have 900 MHz. transmitter and receiver modules and just got them working together.
Just for testing, I connected the tx. to a CD player and Beethoven's Piano Concerto #5 sounds great through the link even though it is mono.
I am developing an analog RF link for a purpose not related to part 15 "broadcasting". I have 900 MHz. transmitter and receiver modules and just got them working together.
Just for testing, I connected the tx. to a CD player and Beethoven's Piano Concerto #5 sounds great through the link even though it is mono.
My goal is to get a range of 150 feet through walls which seems as if it will be easily accomplished with these modules. The manufacturer claims that ranges in the neighborhood of 1000 meters are possible but I have yet to verify this. I have not checked range an will report here when I do.
Operation in the 900 MHz. band is governed by FCC Rules Part 15.249 and the allowable field strength is 50,000 uV at 3 meters which is 200 times the limit for the 88-108 MHz. band. A 1/4 wave ground plane antenna is only 8.2 cm long and I built one using #12 wire, some coax, solder, and epoxy. I thought some who like to build their own equipment might be interested in this for a STL or control application.
The Linx Technology modules I have are:
TXM-900-HP-II and RXM-900-HP-II and I got them through Digikey.
Neil
Sounds like fun!
That concept breathes a bit of life back into the idea of having a central master 900 MHz transmitter and a number of Part 15 FM slaves operating at different freqs to cover several different locations.
What was the total cost for your modules to get operational, including power supplies, connectors, enclosures needed, etc.?
Did you need to do anything with pre-emphasis for the 900 MHz section or did the bandwidth of the modules take care of that?
Were they listed here?
http://dkc3.digikey.com/PDF/T063/0387-0388.pdf
App Notes
TXM-900-HP-II
http://rocky.digikey.com/WebLib/Linx%20Technologies/Web%20Data/TXM-900-HP-II.pdf
RXM-900-HP-II
http://rocky.digikey.com/WebLib/Linx%20Technologies/Web%20Data/RXM-900-HP-II.pdf
Experimental broadcasting for a better tomorrow!
Too bad the evalution kits are so expensive. I'll have to look at the data sheets to see if there might be a place for this in a couple of the things that I'll need to do in the future.
I haven't run tests but the design guide states a frequency response of 50 to 25 kHz.
It has been a while since I dealt directly with Linx and at the time their policy was that you had to buy the evaluation kits before you could buy the modules if ordering from them. When I got my modules, Digikey would sell them without the eval kits. I don't recall the exact price but I think it was in the $30 range for each module.
As per power supplies, etc., the modules work from 2.7 to 16 volts and draw tens of milliamps so nothing too fancy is required. For prototyping, I am using a lab bench supply at 9 volts.
Again, I have very little data to date on the actual range on these units nor do I know how quiet the 900 MHz. band is here. There is a lot of spread spectrum stuff (phones, data links, video links) produced to operate in this band which could cause problems. So far, I have had none but I am just getting started with this.
If for nothing else, the app. notes and design guides are interesting reading. I'll post here when I have more actual data.
Neil
SCWIS et. al.,
Were they listed here?
http://dkc3.digikey.com/PDF/T063/0387-0388.pdf
I believe they are on page 388 under the HP3 series, 900 MHz. 8 channel tx and rx. It appears they changed the catalogue numbers from when I got mine (about 4 years ago) but the prices and specs look right.
You asked about packaging. I am using this for a doppler radar telemetry link which is battery operated. The tx will fit easily in an Altoids mint tin which will attach to or inside the existing doppler unit. These things are really small and can fit almost anywhere. The tx is about 2 postage stamps in size and the rx is about 3 stamps. As I mentioned, the antenna is home brew, built from scraps, and the tin was surplus from my wife's purse, so I really can't cost this out except for the module costs.
(Sorry about the edits, but distractions kept me from finishing this on the first try.)
Did you need to do anything with pre-emphasis for the 900 MHz section or did the bandwidth of the modules take care of that?
I did a frequency response check on the link and without external preemphasis and what I put in is what I got out from 1 kHz. to 5 kHz. The low cutoff was 1 kHz. and this is probably due to the coupling cap I used. My application does not require low frequency response or preemphasis so I didn't pursue it. It would seem that pre/de emphasis would be of benefit for an audio link but other than what Linx vaguely states in the design guide about bandwidth limiting I don't think there is any processing except for a low pass filter on the audio. I also observed that the output from the receiver is about 1/3 the signal voltage into the receiver.
Since my application is different from audio linking I have not experimented to say for certain how this will work for a part 15 audio link.
Neil
What about these 900 mhz. wireless speakers? I have a couple of sets laying around. The little xmtr transmits on 2 frequencies.. For L. and R. and each speaker has a rcvr. in it..
We'd be very interested in a system that would get a mile or two. How much would you charge for one maxed out 900mhz transmitter- preassembled in a weatherproof case and 3 receivers in cases? We're buying...
Dave
Surfside 1640
If you're just trying to get audio to a transmitter why not use something like this?
http://sewelldirect.com/Intelix-Audio-Balun-2-RCA.asp
Any cable based solution is great if you can run the wire. The problem sometimes is that in the idea environment of ultra high density audiences like multiple apartment buildings it might be possible to get two ro three rebroadcast locations set up, but running all that cable would be a tough sell. THe 900 MHz wireless option removes one hurdle.
THe 900 MHz wireless speakers might be interesting, depending on range.
Experimental broadcasting for a better tomorrow!
I'm thinking more along the lines of IFB units in our TV studio, seems like it might be a lot cheaper than similar devices that are already built.
Dave,
I am not offering this or anything else for sale. I just wanted to let others know about these particular Linx modules in case they wanted to experiment.
I once presented a lecture about communication fundamentals at a meeting of the engineers of the Ohio Dept. of Transportation. They were interested in getting signals over short distances to control traffic signals without the need to bury cable ($17 per foot). One of the other speakers described the use of commercially available license free spread spectrum wireless links which are used for traffic control synchronization. They work for audio, control signals, and video but are pretty pricy (he said in the $3000 range for each end but that was eight years ago). I believe such links are available for surveillance and data linking and spread spectrum would offer better noise immunity and security than the Linx modules I cited. You might find suitable units if you do a web search.
Neil
Quote..
"THe 900 MHz wireless speakers might be interesting, depending on range."
I figure with a couple of these babys, (and a little matching) The range could be extended..
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=LPY2
Neil-
Thanks for the tip. Now if we could just convince Keith Hamilton to create a part 15 STL that would send synch...
Take care.
Dave
SuperCircuits has some nice equipment that could easily be used for a STL application. Here's their website.
https://www.supercircuits.com/index.asp?PageAction=VIEWCATS&Category=472
The link is working as intended. The tx. was placed near my mailbox and the receiver was placed underground in my basement lab area. Both aerials were 1/4 wave ground planes and the distance traversed is 150 feet through tons of dirt (maybe some reflections).
The signal is full quieting at the receiver.
I intend to do some drive arounds and will report the range results here. I am trusting that the law of maximum inclemency will prevail since Ohio winters are not the most pleasant for these kinds of tests.
In case you wonder, the rule of maximum inclemecy states that the performance of an antenna is proportional to the inclemency of the weather.
Neil