Need info on FMA Tetra FM 6Ch Sub Micro RX - RC Groups
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May 16, 2001, 06:27 PM
Senior Member

Need info on FMA Tetra FM 6Ch Sub Micro RX

I just got a plane for a friend that needs to be tested before it sent to him. It has an FMA Tetra FM 6Ch Universal Sub Micro RX in it. I know nothing about these receivers. Can someone tell me what the dip switches are for, and also what shift select it is. I also don't even know if it is PCM or PPM. Any help would be really appreciated. I've got a time crunch on this one.

Thank you in advance!

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May 16, 2001, 08:31 PM
Registered User
Hey Cash: The good job you are doing with the Consumer Advocacy forum is appreciated, so you deserve a quick answer to your question!
The TETRA is a single conversion, PPM receiver that FMA has not produced for about two years since single conversion can no longer meet FCC unintentional radiation requirements unless LO power is lower than the extremely low unintentional radiation permitted by FCC. Low LO output makes such receivers open to interference and TETRA has a strong LO to work well with the passive double balanced mixer in it. The front end of a single conversion receiver just has almost no rejection of an LO signal only 455KHz away from the transmit frequency, so it radiates out the antenna. A dual conversion receiver with a 10.7 MHz separation has beaucoup rejection of the LO to radiation out the antenna.
FMA has replaced TETRA with a number of more recent dual conversion designs that meet the radiation requirements and which are several years newer in design. Fortuitously, these new FMA designs reject the massive spectrum of third order products generated when several transmitters are positioned just a few feet apart instead of the 20 ft recommended by the AMA guidelines. ( One day soon, a picture worth a million words will get into the Support section of the FMA Support section showing a spectrum analyzer with multiple transmitters a few feet apart vs 20 ft apart) Also, these new designs are lighter, smaller, and cost less than TETRA did. However, your friend should enjoy years of service from the TETRA you have acquired and nothing beats "for free"!
You may benefit from reading the FAQ section on our web site for receivers which explains PPM and PCM, the differences between single and dual conversion and the like.
If you look at the TETRA receiver through the clear heat shrink with the connector pins pointing left, you will see a DIP switch with the individual switches numbered one through five , left to right. I refer to that line of numbers as DOWN. The individual switch positions for Futaba, HITECH, et al should be one (1) DOWN (Next to the numbers); two(2) and three (3) UP; and four (4) and (5) DOWN. For JR and Airtronics they are opposite. If they do not match the setting for the transmitter your friend will be using, you may poke throught the heat shrink like I do with a # 11 knife blade and move them or do as Tim and Howie do and use a straight pin to move them to the desired position.
May 16, 2001, 08:47 PM
Registered User

Sent you e-mail with Tetra programming instructions attached.

May 16, 2001, 09:31 PM
Senior Member
Absolutely wonderful! Thank you both for youre responses. I have setup the Tetra with my Hitec Prism 7x (luckily I had a spectra module so finding a TX crystal wasn't an issue) and checked everything out on the plane. It's working fine. Since he has a Prism as well, this should make things easier.

I'll warn him about the single conversion issue, I know about single versus double conversion. I just didn't have a clue as to what the Tetra was since I didn't have any Docs for it. He primarily flies alone in a remote field, so hopefully he won't experience any problems.

Thank you for the kind words about the Consumer Advocacy forum. Thanks again! This has to be why the E Zone leads the way in eflight. It's like a 24 hour, 7 day a week club meeting!