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.A 900MHz FM Transceiver Project
This project design and supplemental notes by Rick Bandla (VE3CVG)
This transceiver project makes use of the Linx RXM-900-HP-II and the TXM-900-HP-II modules. See www.linxtechnologies.com
The usual FM calling frequency for the 35cm band is 904.500 MHz. The Linx modules are channelized and the best frequency to use as a calling channel would be 906.37MHz on Channel#2. Note that the WCARC 35cm CW beacon is on 903.37MHz CW and should be audible with this transceiver on channel #1. The 900MHHz Amateur Radio Band Plan may be viewed at http://www.rac.ca/opsinfo/900plan.htm
Still missing from this page is the overall wiring diragram of how I tied these modules together. There is certainly no magic in how I did it. Its straight forward point-to-point wiring. I used a lot of "ugly" construction. I cut PC board into narrow .125" strips then clip off squares and cement them to the main board (chassis). I then solder components to the little squares. It usually looks ok and works. I use a product called shoe-goo for cement. I really like pc board chassis construction especially for higher frequencies. Simple and inexpensive and I can make it excatly the size I want. Sometimes its not pretty. I also like those $5 micro-relays. They work great for RF and for power transfer.
I used a $7 computer headset with a boom microphone. It came with 2 stereo plugs, a black one for the stereo earphone and a blue one for the microphone. Both the earphone and microphone require amplifiers to work with the LinX modules.
I did not include an Rx audio level potentiometer but its easy to add if you need one. I took complementary outputs from the Rx amp and fed one side to the left ear and the other side to the right ear. Works well.
The electret microphone requires bias to produce a usable signal. I experimented trying resistors from +5v to the ring connection on the stereo jack (which is not stereo at all). I tried several values from 1.2K to 15k and the 15k obviously worked best so that is what I installed permanently. Feel free to experiment.
Looking at the front panel, you can see 3 indicators on the top of the transceiver. The indicators from left to right are a green Power On LED, a red Tx LED which illuminates for manual TX, push button TX, or CW TX, and a yellow Sync LED.
From left to right the controls are: Rx channel, Tx channel, Power ON when up, TX or CW when up, under the TX switch is a red push to TX pushbutton, the to the right of the switch is the earphone stereo jack, microphone stereo jack, under the microphone connector is the CW stereo key jack.
Looking down on the transceiver with the controls facing you, you can see the Rx module at top centre of the picture. On the left you can see the thumbwheel switches and the 2N2222 level inverters. These are needed because the thumbwheels that I used have diodes in them making a + voltage the active voltage. Unfortunately the Linx modules follow the 1=high and 0=low convention but they require a true Zero to sink the internal bias voltage. Ideally I would have added another stage of inversion and my thumbwheels would have been a little easier to use.
Immediately to the right of the Rx module, you can see the Rx earphone amp with the 2 caps at the back which are the outputs to the earphone jack. To the right of the earphone amp is the mic amp, with the white orange wire going towards the left to the TX module (hidden by the Rx module).
The black object between the Tx LED and the Sync LED is the power transfer relay. I could have used an astable flip flop with power transistors, but the relay was handy.
The 5 volt regulator is just visible immediately below (and partially hidden by) the power ON LED and power ON switch.
Note that I mounted my LinX modules on a small prefab pc board. This stabilized the modules, and allowed me to mount an RF TR relay to switch the antenna between modules and keep the leads very short. I separated the 2 modules by a half inch or so, but only one or the other is powered up at any time. This follows the manufacturers recommendations - these devices work best in half duplex mode, like conventional amateur gear.
I finally finished packaging the 900 Mhz xcvr. The project took more fiddling with packaging than I anticipated. The total construction time could be dramatically shortened by not using the thumbwheel switches. Building up the amps and connectorizing the Linx modules takes an hour or two.
I hooked my 900MHz plumber's delight antenna to the transceiver, See the link to it elsewhere on this page. I can hear the Tx audio in my handheld and I can hear the signal generator in the earphones of the transceiver, so I think all is fine. Final results are still to be determined as of this date.
Click here for more info on The Infernal Tower.
Click here to view my Antenna Area and my 900MHz Plumber's Delight.
Click here to go to VE3CVG Home Page
