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Designing Maritime Radio #bitx40

 

My name is Qonita, and I am a student i my last semester. I'd like to learn about BITX40 because it is very suitable for my last project.?
So my project is to make a HF communication system for Fisherman Ship and the Coast Station. The frequency is already been stated by the government. It were 8.222 MHz (for the Ship) and 8.746 MHz (for the Coast). The Ship Station can transmit voice and GPS location (simple digital data, just 9 digit of MMSI ship code and location from the GPS Satellite), but can only receive voice. For the Coast Station, can only transmit voice, but can receive voice and digital data.
I think BITX40 can be a perfect example module for the voice communication. Can anyone explain to me what parts of BITX40 that I should change for the required frequency?
?
Thank you very much
?
?
 

Hello Qonita,

One consideration you may need to address is that the bitx family transmits and receives single sideband (SSB) modulated signals. If your application requires AM modulation instead then significant modifications would be required.

If SSB is ok, then in general you would need to modify the tuned circuits in the RF front end to match the new band and edit the Raduino software to set a different local oscillator frequency.

Of course, there will be many other details to address, including software changes to support your specific restrictions and requirements.

Regards,
Chris Brewer, AC9GN

On Feb 28, 2018 8:55 AM, <qonita.salimah@...> wrote:
My name is Qonita, and I am a student i my last semester. I'd like to learn about BITX40 because it is very suitable for my last project.?
So my project is to make a HF communication system for Fisherman Ship and the Coast Station. The frequency is already been stated by the government. It were 8.222 MHz (for the Ship) and 8.746 MHz (for the Coast). The Ship Station can transmit voice and GPS location (simple digital data, just 9 digit of MMSI ship code and location from the GPS Satellite), but can only receive voice. For the Coast Station, can only transmit voice, but can receive voice and digital data.
I think BITX40 can be a perfect example module for the voice communication. Can anyone explain to me what parts of BITX40 that I should change for the required frequency?
?
Thank you very much
?
?
 

Thank you for your reply..

There is no requirement for the modulation itself, I think SSB is good enough and suitable for my project. Did one RF-Front End circuit work for both transmit and receive system?

What about the Crystal Filter Circuit, should I make change on the circuit? Did the Crystal Circuit aim is for keeping the IF Frequency fixed tuning in 12 MHz??

Thank you

?
 


There's a band-pass filter at L1, L2, L3, it is used during both transmit and receive.
You require a fairly broad passband,? but I suspect it will work fine across your 500khz.
As designed, it has input and output impedances of 50 ohms (though it likely sees a load of around 200 ohms at Q1).
I suggest you either simulate that filter in LTSpice or find some other software to model such filters,
and find new values for the inductors and capacitors such that it has a passband from 8.222 to 8.746 mhz.
Or you could just make all the capacitors smaller until you bring it to your operating frequency.
For initial testing (not when transmitting into an antenna) you could probably just short across this entire filter
with a wire, but you will now be receiving an image frequency and the transmitter may have extra outputs
at unexpected frequencies..

The low pass filter at L6, L7? allows through our 7.2mhz signal from the transmitter and blocks all harmonics
(at 14.4 mhz, 21.6 mhz, etc).? Again, would be best to obtain correct values for your operating frequency,
though simply decreasing the value of the the capacitors will probably be sufficient.
If this is not done correctly, you may wind up transmitting at frequencies other than what you intended.
This low pass filter is only used when transmitting, not when receiving.

Crystal filter should be fine, leave it at 12mhz.??
If that is not clear, then you need to read up about superhetrodyne receivers before proceeding with
your hacks to the Bitx40, perhaps in one of the excellent amateur radio publications such as
the ARRL Handbook, RSGB Handbook,? or Experimental Methods in RF Design.
There is likely something similar available in most other major languages.

Up to date firmware for the Bitx40 can be found here:
? ??
either "bitx40" (simpler hardware mods)? or "bitx40-raduino-v2" (allows the bfo to be varied)
Adjusting the tuning range in that software should be a matter of changing a few numbers,
folks in this forum could probably help if you get stumped by this.
You will also need to download the Arduino IDE to a host computer, use that to load
your new firmware into the Nano processor embedded in the Bitx40.

Should be a fun and educational project!
Hope that helps.

Jerry, KE7ER


On Wed, Feb 28, 2018 at 07:42 am, <qonita.salimah@...> wrote:

There is no requirement for the modulation itself, I think SSB is good enough and suitable for my project. Did one RF-Front End circuit work for both transmit and receive system?

What about the Crystal Filter Circuit, should I make change on the circuit? Did the Crystal Circuit aim is for keeping the IF Frequency fixed tuning in 12 MHz??
 

Hi Qonita,

You're quite welcome, and Jerry covered your question very nicely in his reply. ?

SSB is preferable for voice communications anyway: it's more efficient than AM (amplitude modulation) and uses less bandwidth to transmit the same information. I only mention it because radio services typically have regulations specifying the allowed modulation methods. If you're working on a theoretical project then this doesn't matter.

Good luck with your project!

Regards,
Chris Brewer, AC9GN


On Wed, Feb 28, 2018 at 10:31 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

There's a band-pass filter at L1, L2, L3, it is used during both transmit and receive.
You require a fairly broad passband,? but I suspect it will work fine across your 500khz.
As designed, it has input and output impedances of 50 ohms (though it likely sees a load of around 200 ohms at Q1).
I suggest you either simulate that filter in LTSpice or find some other software to model such filters,
and find new values for the inductors and capacitors such that it has a passband from 8.222 to 8.746 mhz.
Or you could just make all the capacitors smaller until you bring it to your operating frequency.
For initial testing (not when transmitting into an antenna) you could probably just short across this entire filter
with a wire, but you will now be receiving an image frequency and the transmitter may have extra outputs
at unexpected frequencies..

The low pass filter at L6, L7? allows through our 7.2mhz signal from the transmitter and blocks all harmonics
(at 14.4 mhz, 21.6 mhz, etc).? Again, would be best to obtain correct values for your operating frequency,
though simply decreasing the value of the the capacitors will probably be sufficient.
If this is not done correctly, you may wind up transmitting at frequencies other than what you intended.
This low pass filter is only used when transmitting, not when receiving.

Crystal filter should be fine, leave it at 12mhz.??
If that is not clear, then you need to read up about superhetrodyne receivers before proceeding with
your hacks to the Bitx40, perhaps in one of the excellent amateur radio publications such as
the ARRL Handbook, RSGB Handbook,? or Experimental Methods in RF Design.
There is likely something similar available in most other major languages.

Up to date firmware for the Bitx40 can be found here:
? ??
either "bitx40" (simpler hardware mods)? or "bitx40-raduino-v2" (allows the bfo to be varied)
Adjusting the tuning range in that software should be a matter of changing a few numbers,
folks in this forum could probably help if you get stumped by this.
You will also need to download the Arduino IDE to a host computer, use that to load
your new firmware into the Nano processor embedded in the Bitx40.

Should be a fun and educational project!
Hope that helps.

Jerry, KE7ER

On Wed, Feb 28, 2018 at 07:42 am, <qonita.salimah@...> wrote:

There is no requirement for the modulation itself, I think SSB is good enough and suitable for my project. Did one RF-Front End circuit work for both transmit and receive system?

What about the Crystal Filter Circuit, should I make change on the circuit? Did the Crystal Circuit aim is for keeping the IF Frequency fixed tuning in 12 MHz??


 

¿ªÔÆÌåÓý

Hi Qonita,


This sounds like a good project, most of what you are doing will involve programming. Hardware wise you¡¯ll need to setup the output filters as either lowpass or bandpass. All Maritime and commercial services use Upper Sideband (USB) and they are also channelized. You will need to set your mode to USB and you could setup the encoder as a rit/clarifier. If you are in need of more than one channel the encoder switch can be used to change or step through channels. Also you could program the display to readout the channel number and frequency. This would streamline your code a lot and maybe leave room for decode program for the GPS position stream and DSC functions.?

Have fun with it and let us know how it goes.?


Skip Davis, NC9O?
 

Thank you for all of your reply

In this picture below, the Band Pass Filter is using a tunable inductor. But in the schematic files in hfsignals.com using a fixed inductor and capacitor.
?

Is there any advantage and disadvantage for these two types of passive components?

Thank you so much :)
 


Sorry this is the schematic picture
 

I could not see either picture on this chromebook.

You plan to be transmitting and receiving at a frequencies of 8.222 and 8.746mhz.?
The Bitx40 bandpass filter is used for both transmit and receive, so the simplest solution would
be to have a single bandpass filter using fixed components with a 500khz bandwidth.

Making this filter tunable during manufacture to properly align the rig may be a good idea
if the fixed parts are not precise enough to give the same center frequency for each rig built.
But this makes building and aligning rigs more complicated.

If you wind up using filters narrower than 500khz for some reason, it may be necessary to?
have one bandpass filter to transmit and one to receive, selected perhaps by relays.
A significant complication, best to avoid this.

Allowing the user to tune the filter seems unnecessary, as this will be operating on fixed
transmit and receive frequencies near 8mhz.

The transmit low pass filter can almost certainly be left about as it is with slight
adjustments made to inductor and capacitor values, no need for that to be tunable.
?


On Thu, Mar 1, 2018 at 03:59 pm, <qonita.salimah@...> wrote:
In this picture below, the Band Pass Filter is using a tunable inductor. But in the schematic files in hfsignals.com using a fixed inductor and capacitor.
Is there any advantage and disadvantage for these two types of passive components?

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