AD9850 DDS Generator

[abbarue]

I would like to build a DDS Generator using the AD9850 IC.
But I don't want to use a cpu or controller chips.
To keep it simple, I would like to set the Freq. using DIP switches.
The following website has a circuit that does it.
He is using the AD9851 but the AD9850 is very similar.

http://www.hanssummers.com/radio/dds/index.htm

But he is only using the 24 MSB.
I want to use the full 32 bits.
I don't know how to modify his circuit to include the other 8 bits.
Does anyone understand this circuit well enough to make that mod.
using the same 74HCXX chips he is using.
I'm thinking that the 74HC08 may be setting 4 of the 8 LSBs but I don't know.
In any case I want to set them using dips.

This would make an awsome Frequency generator, I can get the AD9850 IC for $20.
Thanks.

 

[JimB]

If you want to use the AD9850 in this circuit, you need to be aware of a MAJOR difference between the '50 and '51.
The AD9851 has a clock multiplier circuit built in, where as the AD9850 does not.
The implication of this is that if you want to use the full frequency range of the '50, you will need a 125Mhz clock oscillator rather than the 10Mhz used in the article.

As far as adding an extra set of DIP switches goes, that is quite easy.
First, remove the circuitry associated with the 74HC08, that is just there for adding a frequency shift for a beacon application.
Also remove the diode from the Q0 output of the 74HC4017. That is used to put the AD9851 into clock multiplier mode. It will confuse an AD9850.

Now to add the extra DIP switches, create another bank of switches and diodes connected to the D0 to D7 inputs of the AD9850, and connect the common point of the switches to the Q1 output of the 74HC4017.

Job done! Easy!

JimB

 

[abbarue]

Thankyou very much JimB.
I know about the clock multiplier in the AD9851. And that I would need a higher frequency oscillator.
After studying the circuit I figured it would be as easy as connecting the dips
in the same way as the others are.
But I wasn't sure if I would still need that 74HC08.
I also didn't know anything about the diode at the Q0 of 74HC4017, or which pin to use for the 4th bank of dips.

One puzzle still in my mind though.
The AD9850 requires 32 bits for the frequency setting.
But it also requires 8 bits for the control register and I don't see those
8 bits being sent anywere in that circuit.

Thanks again I think I can do the mod now.

 

[mvs sarma]

Hai Abbarue,
If you are yet to purchase, why not 9851brsz.
the cost difference won't be much. you may also see the DDS projects at projects section of www.hamradio.in

VU3WIJ has even a PCB and associiated front panels to offer.

 

[JimB]

One puzzle still in my mind though.
The AD9850 requires 32 bits for the frequency setting.
But it also requires 8 bits for the control register and I don't see those
8 bits being sent anywere in that circuit.

The 8 bits for the control register are set by the Q0 output from the 4017, only one bit is set to a "1", and that is the clock multiplier bit which is set by the single diode, all the rest of the bits are held at "0" by the pull down resisitors at the left of the DIP switches.

JimB