AD9850 HC_SR08 DDS MODULE part #2

Discussion in 'Homebrew and Kit Projects' started by AA7EJ, Nov 8, 2018.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    Please note the tittle and read the following carefully.

    This post is not about AD9850 / AD9851 chip datasheet!

    While researching the specification for actual output level of AD9850 CHIP I found an "adjustment" for output CURRENT of the DAC - AKA the actual physical output of the AD9851 (!) CHIP module.
    (Unofficial schematic from e-bay )

    There seems to be a rumor that one can purchase HC_SR08 (DDS) module with either of the AD985x chips installed. Cannot confirm that since all four of my modules have AD9850 in them.
    That in not my main concern in this post,

    What concerns me - both chips have an adjustment for the DAC current , hence output level via "Rset " signal / pin!
    Such pin (Rset) is accessible on module with AD9851 in it and off DIFFERENT physical appearance than HC_SR08!
    I have not found "bottom view" of the module (with AD9851) where the module part number should be and I do not own one.

    The signal name is "Rset" NOT to be confused , as I did , with "RESET" signal.

    On real HC_SR08 module the "Rset" has a trace - it is connected to the chip (AD9850) thru two "vias" and is connected (internally only , in module) to the R6. That is as far as I got tracing it.

    I have not tried to read / measure the R6.



    Here is part of an article from some obscure "DDS seminar " notes about "adjusting " R6.

    Output Power Considerations
    Combining the two complementary outputs in a transformer does not offer any power gain. In
    the AD985X series of DDS IC’s, the only way to increase output power is to set the output
    current to a higher value by adjusting Rset. Up to 20 ma maximum output current is commonly
    available; however, harmonic distortion of the output may also increase slightly. Use of a
    transformer (Figure 6-4A) does permit more efficient transfer of power to a load by eliminating
    the need for an input termination resistor (Figure 6-4B) which dissipates power that should have
    been transferred to the output termination resistor.
    Output power into a 50-Ohm load for a 20 ma full-scale output sine wave is 2.5 mW or +4 dBm.
    Output power into the same load for a 10 ma full-scale output sine wave is .625 mW or -2 dBm.
    Output power is determined using the equation P = E 2 /R, where E is the RMS voltage developed
    across the load resistance, R. P, of course, is measured in Watts. The units of dBm are arrived at
    using 10 * LOG (P) where P is expressed in milliWatts. As a reminder, the term dBm expresses
    an absolute relationship between the power level of one milliWatt and some other power level.
    The term dB expresses the power level of some arbitrary reference to another power level and
    therefore, it is a relative measurement.

    Sure would be nice to have real schematic for HC_SR08 or means to figure out the DAC current with stock R6 and its value.

    73 Shirley
     
  2. WB3BEL

    WB3BEL Ham Member QRZ Page

    The datasheet for the AD9850 clearly shows how the Rset can be changed to alter the DAC current.

    There is a table on page 2 that shows DAC OUTPUT CHARACTERISTICS
    Full scale output current
    Rset = 3.9 KOhm results in 10.24 mA typical full scale output current
    Rset = 1.95 KOhm results in 20.48 mA typical full scale output current

    The application schematics in the datasheet show a resistor R1 to ground from pin 12 provides this function.

    Many experimenters have copied the application circuits from the datasheet with some modifications. But I am pretty sure they would not modify this area. Maybe they call this Rset resistor by a different reference designator like R6 or something else, but it will still be connected to pin 12 and have the same function.

    I would be inclined to have this at 10.24 mA setting for better spectral purity. If you need higher output power a buffer amplifier might be a better choice. How much output power do you need ?
     
  3. AA7EJ

    AA7EJ Ham Member QRZ Page

    This is embarrassing, but I do appreciate your comments.
    I obviously have a knack for looking for pertinent info in wrong places.

    Now to answer the "output power " question.
    In same table under DAC OUTPUT CHARACTERISTICS there is / are
    Spurious-Free Dynamic Range (SFDR) data.
    It is in dBc.

    So what does the "c" suffix implies?

    Can SFDR in dBc be related to actual "dB" level?
    Perhaps dB can be calculated?

    As far as "power" goes I believe it has to be relative to all of the components used in the circuit.

    Perhaps I could use the actual output level of the AD9850 module, with current default of the R6 resistor ( measured approximately 3.8 k Ohms ) as a starting point.

    Thanks again for your help, appreciate it very much

    73 Shirley


    Addendum
    Here is the scoop on dBc.

    dBc (decibels relative to the carrier) is the power ratio of a signal to a carrier signal, expressed in decibels. For example, phase noise is expressed in dBc/Hz at a given frequency offset from the carrier. dBc can also be used as a measurement of Spurious-Free Dynamic Range (SFDR) between the desired signal and unwanted spurious outputs resulting from the use of signal converters such as a digital-to-analog converter or a frequency mixer.


    As "ratio " dBc saiz nothing about the "carrier signal" level.

    ... the beat goes on ...
     
    Last edited: Nov 8, 2018
  4. WB3BEL

    WB3BEL Ham Member QRZ Page

    As you said dBc is dB relative to the carrier. You can understand these measurements by looking at the datasheet page 6.
    It shows a group of spectrum analyzer plots which are measuring the SFDR.
    The first plot titled TPC1 shows the Typical Performance Characteristic SFDR at 1 MHz output frequency. The carrier on the spectrum analyzer is the vertical line near the y axis about -8.6 dBm It has a small dark triangle with the point facing up . The delta marker is just to the right of the carrier and has a clear triangle with the point facing down. The delta marker on the plot shows the difference in level between the carrier and the spurious it's 76.642 dB.

    If you go back to page 2 and look at the SFDR wideband (Nyquist bandwidth) for 1 MHz analog output you see typical is 72 dBc in the table. So the TPC 1 plot shows this unit they measured was slightly better than their datasheet typical number.

    As a second example, plot TPC2 on page 6 is for 41 MHz output frequency. The carrier output level is about -10 dBm and the highest spurious is now to the left of the carrier and is -54.8 dB below. The table on page 2 has a spec for SFDR wideband (Nyquist bandwidth) 40 MHz which is close to this test case and it has a typical spec of 54 dBc. So it is similar to the TPC2 test plot.

    That is how to interpret the plots and you can also get an idea that the output signal is about -10 dBm with the DAC current set to 10.24mA fullscale with the analog devices test configuration. This test configuration is probably with a 50 Ohm resistive termination + spectrum analyzer load like the figure 15 on page 15.
     
  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    Problem solved !
    While rummaging for some more parts OF found DDS-60 acquired years back and used just once for "proof (sic) of (software) concept " !
    DDS-60 uses AD9851 (!) and has real amplified RF output adjustment for up to whopping +15dBm!
    Hence I have a starting level of approximately +15dBm to "scale " everything else in the project to.

    73 Shirley
     
  6. KX4OM

    KX4OM Ham Member QRZ Page

    Shirley,
    There is a lot of very useful info on the AD9850/51 modules on PHSNA@groups.io

    A recent post to the group contains homebrew PC board images by Terry, VK5TM. That board is similar to the footprint of the "skinny" AD9850/51 modules sold on eBay, but with no on-board filter, and the output is through a Coilcraft transformer, which Terry says "gives a better output signal both level wise and harmonic suppression wise." He has designed some other AD9850/51 VFOs sig gens as well. FAR has one of the circuits that he designed that uses an 8-pin PIC. I bought one of those.

    Back on topic, there are some files in the PHSNA group that discuss the 200 ohm output. Look under the folder "K1RF" for information on the advantages of using the transformer balanced output rather than an unbalanced output. Also, I recall one of your posts addressing the output low pass filter on the DDS modules. Both the typical eBay 9850 and 9851 modules have a cutoff of 70 MHz. The PHSNA files describe modifying the AD9850 boards to 30 MHz, which I did for mine.

    VK5TM posted on Sept 28, 2018 and Steve, K1RF commented the next day on the latest info. Steve also lists several links to AD9850/51 modules and datasheets for 5v and 3.3v. Steve's group files contains testing that demonstrates the comparisons between balanced and unbalanced outputs.

    Ted, KX4OM
     
  7. KX4OM

    KX4OM Ham Member QRZ Page

    Shirley,
    I just noticed your post above on the DDS-60. While the boards for the for the PHSNA project that use the AD9850/51 modules include an amplifier (ERA MMIC or 2N5109), the base design does not use output leveling. The leveling is done in the measurement software for the project using 5th order polynomial curve fitting. An advantage of the DDS-60 version of the PHSNA provided by George, N2APB provides leveling. George is the designer of the DDS-60 and DDS-30. See the N2APB-N2CX folder in the PHSNA files.

    Ted, KX4OM
     
  8. KX4OM

    KX4OM Ham Member QRZ Page

    Posted by me:
    "A recent post to the group contains homebrew PC board images by Terry, VK5TM. That board is similar to the footprint of the "skinny" AD9850/51 modules sold on eBay, but with no on-board filter, and the output is through a Coilcraft transformer, which Terry says "gives a better output signal both level wise and harmonic suppression wise." He has designed some other AD9850/51 VFOs sig gens as well. FAR has one of the circuits that he designed that uses an 8-pin PIC. I bought one of those."

    "DDS SIGNAL GENERATOR BY VK5TM USING AD8950 MODULE"

    From Terry's website blog, the design was copied without his permission. See http://www.vk5tm.com/index.php

    Terry makes PCBs available for sale of several of his designs, and schematics, board design photos and software are available on his web site for amateurs' use (non-commercial)

    Ted, KX4OM
     
    Last edited: Nov 10, 2018 at 8:48 PM
  9. AA7EJ

    AA7EJ Ham Member QRZ Page

    Ted, thanks for the comments.

    I did look at the "balanced versus unbalanced " output from DAC.
    Unfortunately I am not in a position to monkey with the module(s) itself.
    I keep reminding myself about the spec of my project - limited to HF , up to 30 MHz.

    I think I "talked" to Terry on here. Senior moment - OF cannot always remember.

    I am not going to discuss using somebody else intellectual property especially when they make a living with it.
     

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