DDS VFO

Discussion in 'Homebrew and Kit Projects' started by KC4DYM, Sep 4, 2011.

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  1. KC4DYM

    KC4DYM Ham Member

    Bought a AD9851 moduel off ebay want to use in the dds any good ideas ? Or sites ?

    thanks & 73
    Ray
    KC4DYM
     
  2. W9GB

    W9GB Ham Member

    I would have probably just bought one of the many DDS kits with all the parts (and LCD) !

    eBay seller: kuuco in Guang Zhou, China, provides a schematic diagram with his AD9851 sales

    Analog Devices AD9851: 180 MHz Complete DDS synthesizer - datasheet
    http://www.analog.com/en/rfif-components/direct-digital-synthesis-dds/ad9851/products/product.html

    George Heron's DDS-60 kit / daughterboard
    http://midnightdesignsolutions.com/dds60/index.html

    DDS VFO version 4.0 by CV Niras, VU3CNS
    http://www.hamradioindia.org/circuits/dds.php

    Experimenting with the Analog Devices AD9851 DDS by OH2NLT
    http://www.nikkemedia.fi/hohtola/dc-trcvr/AD9851/ad9851.htm

    AD9851 Mini-kits (Australia)
    http://www.minikits.com.au/kits5.html

    Tech Talk 13 on AD9851 DDS VFO by Jan Skirrow (October 2005)
    http://skirrow.org/Boatanchors/TechTalk13.pdf

    w9gb
     
  3. KL7AJ

    KL7AJ Ham Member

    Does that have both in-phase and quadrature outputs? If so, you can make a killer coherent receiver.

    Eric
     
  4. WD5HHH

    WD5HHH Ham Member

    Quadrature Output

    KL7AJ,

    That requires the AD9854 to get quadrature from one device. Otherwise you must have two DDS chips. You then program one to be 90 degrees lagging phase with respect to the other chip. Most of the Analog Devices DDS chips have this capability. I have been doing some study on this. Want to build a nice super, DDS based VFO but other factors in my life are minimizing my Ham Radio activities.

    Bill
     
  5. AB9LZ

    AB9LZ Ham Member

    If you haven't selected a micro for controlling the thing, Id recommend the PIC, just as easy to program as the audrino, with a *lot* less fru fru. The pickit2 actually comes with some C based sample source code thats a perfect starting point for your custom vfo. If you aren't setup to etch a board, I'd recommend a kit, the narrow profile ad9851 leads are too fine to solder dead big style.

    73 m/4
     
  6. WA6TLP

    WA6TLP Ham Member

    I bought a kit from N3ZI, installed it in my recently restored homebrewed rig and I can truthfully say that it has solved a major problem I had with the rig for 30 years. The kit came complete with everything including a large display and all the parts. My only complaint is (to keep the cost down) a mechanical encoder was included. The encoder has some annoying 'bounce' to it and I will probably replace it with an expensive optical encoder eventually.
    HomebrewSnew.jpg
    Notice the digital readout where the dial once was.
     
  7. AB9LZ

    AB9LZ Ham Member

    Check to make sure the software will support an optical encoder before sticking one in there, a) there could be slight programming differences between the two, especially if the pulse detection is done with interrupts. b) with higher resolution encoders, the pulses come much faster, and there is no guarantee of the start state (see point a) ).

    73 m/4
     
  8. WA6TLP

    WA6TLP Ham Member

    You are right, the unit does need a slight programming change to accommodate an optical encoder, however it has an RS232 interface built into the board and all you have to do is change a couple of parameters in their registers by linking to your PC. I have a USB to RS232 adapter, so that shouldn't be a problem. Evidently, the the debounce algorithm has to be disabled when using an optical encoder.

    Have the radio on tonight and have already had a couple of skips. Probably won't be too long before I replace the encoder. Still, this is such an amazing improvement over the 12AX7 Colpitts oscillator I originally built for the rig.
     
  9. K5UOS

    K5UOS Premium Subscriber

    WA6TLP,

    Awesome rig! I enjoyed the history and the write up linked in your QRZ bio. John, you definitely have the knack!

    73, K5UOS
     
  10. WA6TLP

    WA6TLP Ham Member

    Ha, ha. Thanks very much Don.

    Read your bio page and man oh man, you are guy after my own heart -- tubes and all. I used a triode/pentode for a receiver mixer just to avoid the noise you mentioned, but really didn't notice it was any quieter than a pentagrid such as you used. Sure would like to QSO with you sometime, but it appears you are a CW man and I'm not. I nearly scrubbed out of Radioman 'A' school because I just couldn't pass the 16WPM code requirement. Finally squeaked by after sweating bullets, then the next year they dropped the code altogether. Just barely got my general/advanced because of the code. Always had a disconnect in my brain for sound patterns, most likely.

    Perhaps you are a more careful worker and are able to build robust and drift free VFO's but all my attempts were failures and my rig would drift especially while transmitting when the final would cause the chassis to heat up. If you are having any problem with drift and/or you want to bring your rig up to date with a digital readout, I highly recommend using some kind of DDS like mine, but don't mess around with a mechanical encoder, go for a optical one for more money.

    73's.
     
  11. K5UOS

    K5UOS Premium Subscriber

    Ham radio is just a reason to build something. All the better if it is made with tubes. Looking at your rig it reminds me how much black crinkle say “vintage homebrew”.

    I still get a thrill building analog VFO’s or VXO’s for receivers, transmitters or transceivers. Building a stable oscillator is still a challenge for me but a lot of the fun, too. But I did look at the same DDS you are using a while back. I am building a 15M CW transceiver and needed a 30.3MHz local oscillator. My IF is 9.216MHz and the high side LO must cover 30.216 to 30.366 for 21 to 21.150 MHz coverage. The DDS would have been perfect but I ended up building a heterodyne oscillator using a 28.248MHz crystal and a 1.967 to 2.177 LC VFO. The transceiver is a work in process because I wanted to finish another 40M transceiver for the 4SQRP Wed and Friday CW nets @ 7.122MHz. But a stable LO is 80% of the work. Like you say drift is hard to handle.

    I appreciate the workmanship that went into your rig. Don’t feel bad about not working CW. I can’t work voice modes either.

    Thanks for showing your rig.

    73

    K5UOS
     
  12. AB9LZ

    AB9LZ Ham Member

    John's rig is amazing, I can only imagine the tremendous amount of work that went into that thing, but as he's finding out, now the fun begins! I usually spend more time debugging and optimizing "finished" projects than I do building them in the first place. That thing will provide a lifetime of tinkering ;) It's sad to see that beautiful analog dial go tho.

    Heh, I have that same problem ;)

    After my last experience, I'm sold on using DDS's for three reasons...

    1) Frequency agility, large frequency hops are almost instantaneous, implementing a RX/TX IF shift allows you to completely eliminate all of the transmit mixer components (lo, mixer, BPF's) from your project... makes building multiband rigs far easier. RIT and unlimited splits are a simple matter of programming.

    2) Stability...

    3) Having a powerful computer/display at your disposal to do other stuff, like co-ordinate bandswitching relays, S, SWR and voltage metering (the PIC's have built in AD converters) etc.

    73 m/4
     
  13. K5UOS

    K5UOS Premium Subscriber

    Mark, I think John's vintage rig is one of the best I've seen in a while. The analog dial did give it a beautiful vintage face but if the mods make it more stable I understand why he is pleased with the change.

    I agree on all the above particularly 1). Your point about eliminating the transmit mixing circuit is a big selling point.
    I just can't bring myself into the 21st century. I think I am hopelessly analog!

    I hope the OP is finding what he needs.

    BTW, tomorrow the Classic Exchange starts for CW.

    http://www.classicexchange.org/sep11/sept11ann.html

    73, K5UOS
     
  14. G3TXQ

    G3TXQ Ham Member

  15. AB9LZ

    AB9LZ Ham Member

    Which microcontroller did you use, and did you find source code somewhere, or start from scratch?

    73 m/4
     
  16. G3TXQ

    G3TXQ Ham Member

    I used a PIC 16F716. I wanted something simple, but with an A/D input to handle the Corsair II offset tuning voltage.

    I wrote the code from scratch in assembly language.

    73,
    Steve G3TXQ
     
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