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Offline trash

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Re: FM boad
« Reply #4 on: July 24, 2012, 01:36:37 PM »
That chip looks interesting.
I was going to suggest using a TDA7000 with a 10.7MHz IF.
It's a common chip that you can buy for a couple of dollars. It needs a few extra external components but it's easy to set up.
You can even buy a kit from jaycar which has everything you need ready to go, all you need to do is change the tuning coil for the lower 10.7MHz IF.

Interfacing it into the radio is not particularly hard either. Again, you'll still need the radio circuit diagram.
The FM demod board is constantly running and the IF is taken from the 10.7MHz IF buffer amp output.
If you want FM, then just switch the audio amp's input from the AM/SSB detector to the FM board.

For transmit mod, the radio is put into AM mode. The audio removed from the high level modulator and it's held ON.
The audio drive is then applied to the VCO in the PLL loop. Picking the right place and method to inject the audio without effecting the VCO will take a little bit of consideration.



Offline SSL-100 mike

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Re: FM boad
« Reply #3 on: June 12, 2012, 04:41:43 PM »
[quote]Now for the hardest part: finding someone to talk to.[/quote]
agreed...fm mainly used in the euro zone and South America...i myself have only heard a handful of operators use fm in the southern hemisphere but do agree its a mode that should be legal for cb bands in nz and australia ....the fm repeaters(10m) in Australia can be often triggered from here in new zealand when skip is in..a good guide to how propagation is going nz-aus ;)
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Offline hpssb

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Re: FM boad
« Reply #2 on: February 14, 2012, 02:40:19 AM »
It's pretty straightforward if you know what you're doing.  We did dozens of these conversions back in the 80's and 90's.

FM'ing the transmitter is the easy part.  You just need a source of low level audio preset on TX only.
Most radios enable the speaker by grounding one side of it on RX - no ground on TX = audio on TX only.
Just connect this point via a preemphasis network (a fancy way of saying a capacitor and resistor in series) to the VCO control line to transmit FM.

Receiver is a bit harder.  You need to add a small circuit like this to decode the incoming FM:


The hardest part is switching the FM in and out.  Best done with a 4 way switch to change AM/USB/LSB to FM/AM/USB/LSB.
You'll need a schematic of your radio to do the wiring mods unless you are a qualified radio expert...

Easier to forget about the last part.  Buy an old AM set for $5 (or get one for free from the kerbside hard rubbish collection / local rubbish dump).
Do a permanent conversion to FM, nobody used AM these-days.

Easier still: Buy a "10 meter CB" that can be easily modified (cut one diode / join one wire / etc).
26 to 30 megs AM/FM/SSB / High power pirate pleasure for $150 to $300 on eBay :-)
Hey, converting a CB or using FM on 27 megs is illegal, so why not go the whole hog?

Now for the hardest part: finding someone to talk to.
Here (Sydney, Australia) one can listen for hours or sometimes days without hearing anyone on AM or SSB, let alone FM.


Offline BuNtEr

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FM board
« Reply #1 on: January 17, 2012, 10:10:12 PM »
Anyone out there know how to make up FM board to work on CB we Have AM, LSB, USB like to have FM as i know a few epople that use fm on cb,

FM is cleaner than SSB and AM
What can i say but just that