[Coco] COCO 3 for LowFER transmitter?

[jdaggett at gate.net]

John

The 6 bit DAC could be used to generate an adio tone and brought out the sound 
port(RCA jack) or the cassette port.. That will be the audio to amplitude moduale the 
carrier wave. That is no problem. Generating the carrier wave of say 160Khz is a bit 
more difficult. The easiest way to do that is to place some external logic to divide 
down the E clock at 1.78MHz. A divide by 11 will generate 162KHz, divide by 10 get 
about 178KHz. Can take the E and Q clocks XOR them and double the frequency 
and the a divide by 19 to 22 will keep the carrier frewquency within the 160 to 190 
Khz band.  Pass that through a 5 to 7 pole Tschebysheff low pass filter to remove 
harmonics. That will supply the carrier. Modualtion can be accomplished with two 
medium power garden variety NPN transistors and you have an audio amplitude 
modulated signal at about 162KHz. 

All the software has to do is to control the DAC and when to turn it on and off to the 
Morse code being sent. The DAC is selected to the audio out with SelA and B set 
low. Basically is will just like storing data on the cassette but instead of the cassette 
it will be a carrier and modulator circuit. I am not sure if AFSK is allowed on that 
segmemt. 

In all a few transistors, a divider, a few opamps to form the active low pass filter. 
and a coule of 1 watt medium frequency power transistors. The hardware is a nice 
weekend task and the software maybe a day or two.

 
james


On 10 Dec 2003 at 8:34, John E. Malmberg wrote:

> Apparently this message never made it to the list until it was
> reposted here. > -----Original Message----- > From: "Rev. James E.
> Smith" > Sent: Dec 7, 2003 2:14 AM > To: chaplainjones at earthlink.net >
> Subject: COCO 3 for LowFER transmitter? >  > Dear Alan Jones, > I have
> a COCO 3 that is still in great shape. I would like to know if I can >
> use it as a LowFER home-brew beacon transmitter for operation at 160
> to 190 > kHz 1750 meters? As to send Morse code, AM, and use it as a
> transmitter all > in one. Since it has a output of Ch, 3 and Ch, 4
> wide band how could I drop > the frequency and narrow the bandwidth to
> less than 100 Hz without opening > her up? I will be using FCC Part 15
> rules. Any info you can send would be > welcomed.
> 
> Converting the output from Channel 3 or Channel 4 to the VLF band
> would require external hardware, and you can not control the
> modulation, so all you would get out of it would be RFI or nothing
> depending on what you ended up with.
> 
> What you are asking on doing is converting two appoximately 65 Mhz
> signals, one AM and 3.5 Mhz wide and one FM about 15Khz wide to a
> single 160 Mhz signal.
> 
> It is simply not practical to do.  And to even consider indicates that
> you are totally unaware of the relevant radio theory.
> 
> It may be possible to get the DAC that is used to provide audio output
> to cycle at between 160 and 190 Khz, but an external filter would
> still probably be needed.
> 
> 
> Modulation internal to the COCO for AM, except for sounds is also
> limited, as the COCO only has a zero crossing detector for picking up
> audio.
> 
> The output of the COCO is a stair step function, and that can not be
> put directly on the air even if you can get it to generate the correct
> frequencies for CW or find a way to modulate it for AM.  It will
> generate too many harmonics, and thus violate part 15.
> 
> The external circuitry needed with any of these approaches basically
> ends up being the same complexity or even more complexity than if you
> just built a LowFER transmitter.
> 
> If you want to feed the output of a COCO DAC or Stereo Sound PAK to a
> real transmitter, then a filtering circuit is needed to supress the
> harmonics from the square waves.
> 
> -John
> wb8tyw at qsl.net
> Personal Opinion Only
> 
> 
> 
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