FM transmitter hacking..(Tunecast II)

[Guest]

Hum,No FM section here?

Here's an FM transmitter I built around the Belkin Tunecast 2 as the base/exciter.
I added some filtering and an output amp/buffer. Output (so far) is ~30mw.
I thought I'd post it here also for reference,etc. Incase anyone was interested.

(Sorry for the messy hand drawn schematic!)
http://www.phatbong1.50megs.com/cgi-bin/i/Stuff/Transmitter2.JPG

L1 is a few turns of the input wires through a small ferrite core from a monitor cable..
L2 is a small iron choke,from a car-stereo "noise filter"
L3 is a small radial coil..looks like a black electrolytic cap..(just to further filter/isolate the Tunecast's supply)

(The TC2's shielded supply wire is soldered right across the diode near the power input jack. The TC2 has it's own 7805 regulator,so you can feed 12V straight to the diode.)

RF-wise,I've got: TC2-->filter-->power adjust-->MMIC amp-->filter-->2N4401 amp-->Output

I've also got a homebrew copy of the Ramsey LPF1 on the output,so I didn't bother with filtering after the 2N4401 stage,the LPF1-clone handles it.

The INA-10386 MMIC amp is run from a regulated +8V supply (7808). If you change this,you'll need to adjust the 50ohm resistor.The coil (L4) is the standard 10uh inductor suggested on the datasheet.
(The datasheet will tell you all ya need to know to setup the INA-10386 stage.)

The filter values depend on your operating freq,I think mine are ~113mhz,I could lower them a tad..but atleast I shouldn't be spewing too much crap up in the Airband.(but I also wanted "full" covereage up to 108Mhz.)
Values from: http://www.mycal.net/old/projects/mpr/fm10faq.htm about halfway down.

I put a simple LPF on the audio input of the TC2 to help with overmodulation/siblance issues..It seemes to have helped alot! I've got treble now,and no "Essing"! It needs a bit higher signal input now,I may add a simple op-amp gain stage,just so I don't have to crank the source's output.
I originally had the Volume/Level pot installed..but removed it..

I also added the NiCd pack(+ diode) in there so that if you were going to use it in the car,and went to start the car,the 'brownout" from the starter wouldn't kill your broadcast! Just a bit of a buffer,and it allows a bit of run-time while mobile,etc..
Mostly just to help prevent "dead air" and unexpected transmitter-resets..

Also,all the wires between components/boards are shielded..I didn't want *ANY* RF-feedback issues...
Everything is well bypassed (.001uf caps everywhere.)..ferrite beads,etc.
The amp stages and most of the power filtering is all built on a chunk of double-sided copper PC board. (with pieces of copper board soldered as vertical shields between sections to prevent feedback/oscillations.)

Despite all the filtering/shielding there is STILL a bit of a hum.. I can not,for the life of me track it down. I've lifted grounds,added grounds,changed cable routing,etc.. I'm thinking it's something "internal" to the TC2,perhaps the "multiplex/refresh" frequency for the LCD,I noticed it flickers a bit. (not noticable with the old backlights,cause they were so dim,brighter backlights make it more visible!)

One thing not shown on the schematic,The RF-power meter..Just a basic 1N34,small cap,and a trimmer..it's not very linear/accurate..but it gives me an idea of the power output...good enough for now.

I removed the original backlight LED's on the TC2,they were nearly useless(too dim),and I think they cause some kind of noise in the audio (very faint high-pitch squeal) when they were on..(they don't stay on all the time either.) So I glued a couple green LED's in behind the TC2 and the power meter..
The other two LED's are "External power" and "Power on" indicators.

The whole works is built into an old external SCSI HDD case..I left the small fan in there,and ran it from a resistor to slow/quiet it down a bit..
Not really needed at this point,but it could be good when I get the rest of the amp finished,since it may run warm.

Now if I could get the output up to say 1W i'd be happy.
(any suggestions?)

[Guest]

Uhm,Yea..the battery and LED's are drawn backwards..
I was tired when I drew it..

[Glowbug]

Awesome stuff!
_________________
Glowbug
"Tubes are the spice of life"
(Unless specifically stated, my posts are my opinion and not as an Admin)