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Old 1st May 2019, 5:29 pm   #1
roberthewitt
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Default Arduino based DDS with 898 Eddystone dial

Hi All

Just looking for any suggestions . Currently contemplating build a ham band rx using a combination of DDS VFO the frequency of which is controlled by a mechanical dial (I have a mint condition Eddystone 898 which is collecting dust and seems a shame not to use) I already have the DDS up and running with LCD and Arduino sketch. I am thinking of connecting a pot to the 898 to provide an analog reading to the arduino to indicate physical position of the dial, then use that value to give "startup frequency" to the code. The encoder, also connected to the 898 would then move the dds frequency up and down. The rest of the reciever would be valve based.

Does anyone have any better ideas on how to use the 898 dial to control the dds frequency ?

Just think it would be nice to make an accurate stable receiver using old and new technology and a nice vintage dial with the option to switch to lcd.
Robert
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Old 1st May 2019, 5:38 pm   #2
MrBungle
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Default Re: Arduino based DDS with 898 Eddystone dial

Synchronising it might be a problem. You could connect a rotary encoder to the dial and a stepper to the needle. That would allow you to have a memory function as well!
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Old 1st May 2019, 5:44 pm   #3
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Default Re: Arduino based DDS with 898 Eddystone dial

How about a rotary encoder switch, you are obviously competent at the arduino stuff. Although the disadvantage would be indexing it to the eddystones analogue scale. And I wouldn't have a clue if its possible to get round that.

Oooops mr bungle beat me to it!
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Old 2nd May 2019, 5:44 am   #4
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Default Re: Arduino based DDS with 898 Eddystone dial

Off the wall thought, why not arrange for the Arduino to control a stepper motor as well as the DDS to adjust the dial according to frequency and use what would have been the tuning shaft to control the encoder?

Will still need a limit switch at one end to provide a "Home" indication to the software.

(Thought I better look up the dial. Could be somewhat of an involved job)
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Old 2nd May 2019, 9:56 am   #5
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Default Re: Arduino based DDS with 898 Eddystone dial

I think a stepper motor to drive the dial is the way to go.


I am not familiar with Arduino but using a PIC microcontroller, it is possible to automatically save multiple values to the EEPROM when the power is removed(switch off or power cut etc.)
I found that a reservoir capacitor of around 2200 uF gives enough time after power is removed to do this..
This could be used to store the last position of the dial so its position is known the next time the radio is switched on.

Frank
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Old 2nd May 2019, 10:08 am   #6
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Default Re: Arduino based DDS with 898 Eddystone dial

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Originally Posted by Terry_VK5TM View Post
Off the wall thought, why not arrange for the Arduino to control a stepper motor as well as the DDS to adjust the dial according to frequency and use what would have been the tuning shaft to control the encoder?
Noooo! The Eddystone 898 is a thing of wondrous beauty with the most silky flywheel tuning. You have to put your hands on it, not let some robot get all the pleasure.

Use an optical encoder driven from the flywheel to pass tuning data.
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Old 2nd May 2019, 10:14 am   #7
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Default Re: Arduino based DDS with 898 Eddystone dial

I tend to agree. How about just build an analogue VFO and use a huff-puff stabiliser?

http://www.hanssummers.com/huffpuff.html

Then use the DDS to generate a signal to mix the VFO with.
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Old 2nd May 2019, 10:23 am   #8
Wendymott
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Default Re: Arduino based DDS with 898 Eddystone dial

Hi Robert. I have two Eddy EC10's that are also looking for a new life. So although not an 898 dial.... had a similar problem. Adapting a rotary encoder to a linear scale dial.
My PIC controlled DDS modules can be driven by an optical encoder, and I was thinking of a gearing assy. Interesting times.
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Old 2nd May 2019, 1:02 pm   #9
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Default Re: Arduino based DDS with 898 Eddystone dial

Quote:
Originally Posted by FRANK.C View Post
I think a stepper motor to drive the dial is the way to go.
Are you suggesting driving the dial in reverse via its output spindle ?

I would have thought with this sort of dial, the best move would be to set it up with the usual variable capacitor to make a precision & stable VFO, then to deploy the Arduino to make a frequency counter with the IF offset applied. Then you will have the lovely dial and your tuned frequency readout.
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Old 2nd May 2019, 8:52 pm   #10
roberthewitt
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Default Re: Arduino based DDS with 898 Eddystone dial

Thanks to all for input . I endorse the comments about the "silky flywheel tuning" and will stick with manual tuning. Using a simple pot as voltage divider a 0 -5v swing gives input values of 0 -1023 to the Arduino . The pot can be tuned through 180 deg in parallel with the main shaft which" tunes" the encoder. The position of the pot wiper (ie between 0 -1023) would then always indicate the position of the dial even after switch off/on.
Not sure if 200(khz bandspread) / 1023 increments will be acurate enough though (200hz /increment) ??

Can anyone recommend a source for single turn precision pot around 1 - 5k ohms rather than just using a bog standard pot ?
Robert
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Old 2nd May 2019, 8:59 pm   #11
FRANK.C
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Default Re: Arduino based DDS with 898 Eddystone dial

Hi Argus25
The main point that I was trying to make was that if using a stepper motor to drive the dial, the dial position can be saved even if the power unexpectedly fails. Negating the need for the dial to be returned to the end stop every time the the radio is switch on.



I have never seen the type of dial in question so don't know how practical it is to use a stepper motor with it.

Frank
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Old 3rd May 2019, 2:44 am   #12
John KC0G
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Default Re: Arduino based DDS with 898 Eddystone dial

To see how the 898 dial works, see the article by Gerry O'Hara on the HBR-13 receiver at: http://eddystoneusergroup.org.uk/Tec...20Receiver.pdf

The logging wheel has 100 divisions. There are markings of 0, 100, ..500 on the dial, and so there are 500 divisions. The spec states that the 898 has a 110:1 reduction ratio. I have just tried an unused 898 and it took ca. 55.75 turns of the knob to go through 500 divisions.

Thinking out loud, if the 898 covered a bandspread of 250kHz, you would have a tuning rate of ca. 4.54 kHz per turn of the tuning knob. If you wanted a tuning increment of say 10Hz, a 128 ppr encoder with quadrature detection (giving 512 ppr) might work. You would need to provide a scaling factor in the Arduino. The encoder would not be cheap. And you would have to set the Arduino to the dial, eg by setting the dial to mid-point on power-up, and pressing a sync button.

Good luck and 73

John
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Old 3rd May 2019, 9:01 pm   #13
buggies
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Default Re: Arduino based DDS with 898 Eddystone dial

Quote:
Originally Posted by roberthewitt View Post
Thanks to all for input . I endorse the comments about the "silky flywheel tuning" and will stick with manual tuning. Using a simple pot as voltage divider a 0 -5v swing gives input values of 0 -1023 to the Arduino . The pot can be tuned through 180 deg in parallel with the main shaft which" tunes" the encoder. The position of the pot wiper (ie between 0 -1023) would then always indicate the position of the dial even after switch off/on.
Robert
You would only get about 0 - 680 count if only rotating the pot 180 degrees of its full 270. Perhaps use a higher (stabilised) supply voltage with a preset at each end for setting the 0 and 1023 points. Bit like tracking with trimmer and padding caps...
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Old 3rd May 2019, 9:05 pm   #14
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Default Re: Arduino based DDS with 898 Eddystone dial

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Originally Posted by buggies View Post
You would only get about 0 - 680 count if only rotating the pot 180 degrees of its full 270. Perhaps use a higher (stabilised) supply voltage with a preset at each end for setting the 0 and 1023 points. Bit like tracking with trimmer and padding caps...
It's easy to map the actual range to a virtual one. There is a MAP command in the instruction set.
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Old 3rd May 2019, 9:41 pm   #15
buggies
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Default Re: Arduino based DDS with 898 Eddystone dial

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It's easy to map the actual range to a virtual one. There is a MAP command in the instruction set.
Agreed - but I think you cannot recover the lost resolution that way.
Too late this evening to do calculations.
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