DDS VFO

[pwdrive]

If this thing arrives I'll be launched into the digital world  

http://www.ebay.co.uk/itm/ZL1CVD-DDS-VFO...xyBjBTRL8R

Thought I might fiddle around with one and incorporate it into valve receiver......  

Lawrence

[ppppenguin]

Looks like it uses the AD9850 DDS chip. http://www.analog.com/media/en/technical...AD9850.pdf

This does all the hard bits. The rest is probably a PIC processor to control the programming and display. From the photos they may have bought in the PCB with the AD9850 and designed the rest around it.

[pwdrive]

From the description it looks like everything's ready assembled, I hope so 'cos that's why I chose it, I wanted one that I can just connect to a supply, enter a few parameters via on board controls and use straight off.

The plan is to use it for some mixer osc experiments/fiddling around and if it turns out ok incorporate it into a hybrid receiver.

We shall see.

Lawrence.

[Radio Fixer]

Guess you will be hit with Customs + handling charges which is a shame. But you may get lucky ?

Gary

[ppppenguin]

Don't be surprised if it emits a lot of RF muck. The DDS output ought to be OK but there a fair amount of other digits rattling away. You may need "tin box technology" to keep the digits out of the sensitive RF.

[pwdrive]

Guess you will be hit with Customs + handling charges which is a shame.  But you may get lucky ?

Gary

Bugger....'twas so Gary, never mind, it arrived today It needs a 12 volt supply, none to hand but have a PP9 battery, might hook that up later on and see what's what and hopefully also see what a 30Mhz sine wave displays like on that new 30Mhz 'scope I got the other day with the time base flat out...

Lawrence.

[pwdrive]

DDS VFO hooked up to 9volt supply (supposed to be 12 volts) IF offset set to 0Hz, VFO spun up to 30Mhz, scope displays 30Mhz ok at max timebase (.2usec) and on X10 magnify, 'scuse poor pics, had to turn off one of the lounge lights as it was masking the camera with glare, also  had to reduce the brightness in file to get below the max forum upload size plus I'm a crap photographer.

All appears to be well but bear in mind what's shown is without any proper termination on the VFO output just the scope probe set to X10, the displayed output increases somewhat at lower frequencies, according to the specs it's supposed to, part of that is probably due to the scopes rated bandwidth or the probes, I don't think the probes are the best.

It will be used for supplying a mixer for either a Regenerodyne or a Direct Conversion receiver.

The VFO's step rate can be quickly altered from 10 Mhz steps down to 1 Hz steps.

It's output frequency will go higher but at the moment I've set the max frequency to 35Mhz and the min frequency to 500Khz, the 'scope and the VFO seemed to have past the test:

               


Lawrence.

[Cleggy]

That would have been sooo useful ~40 years ago when everybody on 2m was using ex-radio telephone rigs based upon 8MHz crystals. Attempts to build an 8MHz VFO had to take in to account that any drift would be multiplied by a factor of 18, and few did that well.

Perhaps this is a daft question, but in the absence of a temperature-controlled environment, I wonder if the DDS is more stable than a crystal?

B

[ppppenguin]

The stability of dds is exactly equal to that of the xtal that feeds the synthesis. Spectral purity and phase noise depend variously on the number of bits, the dac and filters.

[pwdrive]

Got the 12 volt battery back from the woods as the PP9 battery was starting to get low as these devices draw a fair amount of current, all connected up, the on board regulator gets hotter as one would expect, the output amplitude has increased but not by a great deal, with this module the +ve feed to the buffer stage can be selected from either the battery source (12 volts) or from the on board 5 volt regulator line (5 volts) The amplitude response across the range is flatter using the 12 volt supply for the buffer as opposed to the regulated 5 volt supply, I've terminated the buffer output with 50 ohms, still plenty of output to drive a mixer, the output clips a bit at low frequencies, probably due to the buffer, the actual sine out from the AD9580 chip is sine all the way, so far so good.

Lawrence.