AD9850 Ebay board types

[Wp100]

Hi,

Looking to get one of these board but note there are 2 different types, one with a blue trimmer one without.

Have not yet been able to work out the use/need for that trimmer or which board to go for ?

Did notice another thread where the poster was having problems with the boards without the trimmer.

Any advice welcome..

 

[JimB]

I have used the board without trimmer, quite successfully on a couple of occasions.
I have had two versions of that board, on with an AD 9850 and the other with an AD9851 (higher clock frequency), both worked OK.

If you look around the website* of the suppliers of those things, there is usually a link to a schematic which should make the function of the pot fairly obvious.

JimB

* Just did a quick google, and the first site I looked at says that the pot adjusts the reference voltage to the comparator in the 9850, and so adjusts the mark-space ratio of the squarewave output from the 9850.

 

[Wp100]

Hi,

Thanks, think I will have to change search engines and my glasses ! looked a several boards/sites and unable to see a schematic or any mention of that pot.

However looking at larger pics of that lower board it does look like there is a small trimmer next to the upper right jumper.

Seems like they are both very similar then.

 

[JimB]

Just had a look at mine and they have the pot also.
I had not taken any notice of it before as I always used the sinewave output.

Please see the attachement here for a very Chinese schematic.

JimB

 

[atferrari]

If I understood it right, to change mark-space ratio in operation you should use a preset instead of a pot?

 

[Wp100]

Just had a look at mine and they have the pot also.
I had not taken any notice of it before as I always used the sinewave output.

Please see the attachement here for a very Chinese schematic.

JimB

Hi,

Thanks for that, did find some diagrams in english eventually, will be pairing it up with an arduino to start with.

Ordered the lower board ( v.small trimmer ) from the Edinburgh based Ebay seller though wonder if we will have to pay import duties next time if some of your folk get their way

 

[JimB]

wonder if we will have to pay import duties next time if some of your folk get their way

Not MY folk, I am a Yorkshireman originally.
Only problem is, when I go back to Doncaster to visit, I think to myself "Don't they talk funny around here!"

JimB

 

[JimB]

If I understood it right, to change mark-space ratio in operation you should use a preset instead of a pot?

Yes indeed, a preset pot...entiometer.

JimB

 

[atferrari]

Yes indeed, a preset pot...entiometer.

JimB

Which is suposed to be used not very frequently. (?)

 

[Wp100]

Hi,

Got my board this morning though have not yet got it hooked up.

This video shows the use of the trimmer

 

[atferrari]

I understand that presets are for occasional adjustments while potentiometers are for frequent adjustments.

In presets datasheets the number of expected operations is much lower than those for a pot.

 

[JimB]

AT
We could discuss all day the meaning of these words, but what is boils down to is:

The electrical function of the device is that of a potentiometer, it divides the voltage.

There are various physical forms of potentiometer.

Some are mounted on the front panel of the equipment and are intended to be adjusted often. EG the volume control on a radio.
Some are mounted on the front panel of the equipment and are intended to be adjusted very rarely. There could be screwdiver slot rather than a knob and ther may be a mechanical locking mechanism.

Some are mounted inside the equipment, probably on the circuit board and are only adjusted when the equipment is calibrated. You probably need a screwdriver to adjust them. There are often referred to a "trimmers", "presets", "trim pots", "preset pots".

But electrically they are still all potentiometers.
EXCEPT... when only one end of the track and the slider are used, then it is a variable resistor.

JimB

 

[atferrari]

Thread closed...!

 

[Wp100]

Thread closed...!

Hi,

Well not quiet , just a bit of info for anyone reading this post who like me is new to the AD9850 and has also purchased that lower pcb will the all smd parts.
Below is a clear pic of the different output pin assignments and some simple code that works at a fixed output with just a basic arduino uno board; no display or keys required.
The 9850 board should have jumpers on J2 and J3 .

There also seem to be a lot of chat about the voltage these board should be run at , seems the oscillator modules used may be 3v3 only devices on some boards.
Running at 5v my board uses 120ma and the oscillator case gets slightly warm, at 3v3 it uses 60ma and runs cool.

 

[atferrari]

How much did you pay it?

I intended to build a sweeping generator using another one from the family but the difficulties (for me) to design the output filter made me to forget the idea.

I later bought a function generator that I suspect is built with that IC. It works like a charm.

 

[Wp100]

Hi,

You can pick those AD9850 boards up from Ebay for about GB£ 4 from China, but I paid £8 to get one from here in the UK; saves waiting 2 weeks !

You might want to look at the many Comments on this site, there is mention of sweep generation, think the guy there will be able to help you further.
Have just built up his project and it seems to work, though just waiting for a rotary encoder to finish it.

The square wave is at 5v but the sine wave is about 1v so a buffer/amp is next.

https://www.ad7c.com/projects/ad9850-dds-vfo/


Edit
Just found this AD9850 / Pic based project with Sweep
https://www.vk5tm.com/homebrew/dds/dds.php

 

[dr pepper]

There are as mentioned 2 diffo clock frequencys.
I have a module with the pot, its for pulse width on the squarewave o/p.

I got it to work with an lcd and a rotary encoder, sweep on the othwer hand proved difficult, the registers that control freq need loading with odd ball values, you need some nifty way of calculating desired freq to the correct values for the registers.

The other issue is that the o/p isnt linear, it drops off rapidly with freq, so you need an auto limiter circuit for a sweep gen, or some self calibrating software on vb or something like that.

 

[augustinetez]

Also, if you are only using the sine wave output, disable the input to the square wave section by removing R12 (1k) on the modules with the blue trimpot. (Looks like R8 on the other, bigger style module)

Certain settings of that pot cause interference/spurs on the sine output.

Terry VK5TM (Yes, that's my website referred to above)

 

[dr pepper]

Hi Terry,

Did you ever get the module to sweep?

I got mine to work via rs232, so it can be used as a sweep, I put the whole thing into a crystal oven too for the heck of it.

I didnt know that about the square o/p, howcome it messes up the sine o/p, is it a power issue?

 

[augustinetez]

Hi dr pepper

I have code (asm) that works as a sweep gen, it's on my website (near bottom of page) https://www.vk5tm.com/homebrew/dds/dds.php
Ver 1.1a is the original and requires a crystal for the PIC and 1.1e uses the internal oscillator of the PIC and can use the AD9851 modules. Other than that, both versions are identical.

Needs some more work to make it sweep faster but all reports back so far have said it is OK for most people.

Not sure why the pot setting causes the problems. Maybe some sort of harmonic interference, haven't really chased it any further to find out the reason.
It appears on both AD9850 and AD9851 modules I have tested. At some settings it distorts the waveform and in others you can see additional spurs or noise on the output.

Terry VK5TM