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AD8302 VNA part #2

Discussion in 'Homebrew and Kit Projects' started by AA7EJ, Nov 5, 2018.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    So to add to confusion I switched forum!
    This post is basically "electronics 101 " type of questions.
    Here they are , in no particular order.
    After re-reading the AD8302 app note I know little more about the AD8302 MODULE.
    Basically a copies from app note.
    1. The Screenshot from 2018-11-05 08-52-19.png Screenshot from 2018-11-05 09-05-27.png



    1. There are two "basic" schematic, which I attempt to include here. One shows the value of the attenuation
    (dB) of source . Source is in my case AD9851 which has ADJUSTABLE output.
    The $64 question is - why do I need attenuation if I can adjust the level of single coming from the source?
    But to be honest - at this point I do not know the actual signal level coming from ONE output of the AD9851 MODULE.

    2. Either attenuator should present 50 Ohms load" , BUT R1 / R2 are already 51 Ohms. Does that present "problem " with paralleling attenuator "load" / last resistor with R1 /R2?

    3. All the schematic I have looked at use "limiting didoes" at the input to the AD8302 AKA parallel to R1/R2.
    Since this is "low level" testing tool , not a SWR detector - no even by an accident !, are these diodes technically necessary? Or if used should they be at the input to the attenuators?

    Thanks for reading
    73 Shirley
     
  2. AA3EE

    AA3EE Ham Member QRZ Page

    The attenuators are there to taylor the input from whatever you're testing to a level that's compatible with the AD8302. Since you know the output of the signal source, you can taylor the directional coupler and attenuators to keep the level within limits.

    In a high power transmitter, you use an off the shelf directional coupler (30, 40, 50 dB etc), and then add attenuators to reduce the level to what's safe for your detector. When dealing with many KW of power, the attenuators must deal with a few watts of power, so sometimes a high power 10 dB attenuator is first, then a smaller power level attenuator to narrow down the levels.

    Any attenuator should always match the impedance of the transmission line which in turn matches the transmitter (ie everything is 50 Ohms). The exception is a "minimum loss attenuator" which is commonly used to match 50 to 75 Ohms or vice versa.

    If you plan properly, limiting diodes are not necessary.

    In the second image, the two paths have different value attenuators because the reflected reading is usually much smaller than the forward (10% reflected is 2:1 VSWR).

    Dave
     
  3. KW4TI

    KW4TI Ham Member QRZ Page

    Not to make your life more difficult, but could I make a suggestion?

    I have considered using the AD8302 as a phase detector for a VNA. It is quite a nice chip, and it can detect phase over a wide range of frequencies and signal power, which is a great strength.

    One of its deficiencies, however, is that it is using a simple XOR-like mixer to produce a phase signal. This means it can not measure the sign of the phase (whether one signal is lagging or leading the other) and its phase resolution can deteriorate significantly near 0 and 180 degrees shift.

    However, if you use two AD9851 generators, you can generate two signals that are off by a fixed frequency (say 10 kHz intermediate frequency) and then measure the signal of one frequency relative to the other, heterodyning the two signals. Then you can use the phase detector of the AD8302 to produce a signal at the intermediate frequency and by measuring the phase of the IF signal, you can get the full phase as well.

    I am working on a VNA based on the EU1KY analyzer that uses the NE612 rather than the AD8302 as the phase detector / mixer. I use a 74HC14 schmitt trigger so I can exactly synchronize data acquisition on a microcontroller to the intermediate frequency.

    https://github.com/profdc9/VNA

    Generally a pair of 1N4148s is a good idea before the AD8302 to make sure the signal voltage does not rise about 0.7 V or about 7 dBm, as long as the frequency is sufficiently low that the capacitance of the pair doesn't affect the signal too much. However they are only to protect from damage from other voltage sources and not strictly necessary. I think the 1N4148 is on the order of 1 pF junction capacitance per diode. At 180 MHz the impedance of 2 pF capacitance is about 442 ohms which would shunt the signal input to the mixer.

    The AD8302 can take an input signal up to 0 dBm which is quite high, and so if you want to use the full 60 dB dynamic range you will need to put out 0.6 V peak-to-peak from the AD9851.

    Dan

     
  4. AA7EJ

    AA7EJ Ham Member QRZ Page

    As far as attenuators - I settled on 21dB on incident wave and on 10db on reflected.
    In theory , if I set the DDS-60 amplifier correctly I really do not need the diodes protection.
    But I will make room on the PCB to add them if I feel ambitious. or just as a prevention during initial setup.

    I have a small challenge - it took off the mini coax connectors which came with the AD8302 module. The reason was pretty stupid - it was too big to fit into original layout.
    Now I have to solder the coax directly to the module and adding diodes at the solder points would be a pain. So I will make room for them after the attenuators where I have to solder the coax to the PCB.

    Either way - any suggestion how to solder mini coax and NOT to melt the braid to the center conductor?

    I am thinking to wrap few turns of wire around the braid, solder it - check for short - and then solder the ground pig tail thru the PCB hole.


    21 dB is just a resulting attenuation using the resistors I got , nothing special there.

    Yes, from beginning of this project I wondered about how to detect the phase sign.
    I actually have another project , in very early stage of design , which will utilize the AD985x ability to change phase.

    Here is a silly idea - could I measure one reflection then change frequency or source phase and do another measurement? Probably won't do since I need relative value...

    On the other hand - full load (antenna) scan should give an idea which side of the graph shows what phase ( short or long) "polarity".

    I'll have plenty of opportunity to play with software when I have the hardware buttoned up.

    Thanks for all comments, appreciate that.

    73 Shirley
     
  5. KW4TI

    KW4TI Ham Member QRZ Page

    I solder to the cheap RG-174 coax all the time. Strip off the outer conductor, pull it back and away from the center, twist it into a thick wire, and then strip off the center conductor, twist that wire, tin the outer braid and inner conductor make them more durable and wetted, and then solder them on. Sometimes if the coax is flimsy I will twist a thin solid copper wire around the braid before I tin it to add some durability and an easier point to solder to.

    Dan
     
  6. AA7EJ

    AA7EJ Ham Member QRZ Page

    "... want to use the full 60 dB dynamic range..."
    This is where the math gets little out of hand....

    Let's make an assumption that I do not care to "measure" SWR beyond 1:5 .
    That is "return loss " in dB around 3 and less.

    Now AD8302 input range is -60 dBm to 0 dBm.
    deciBell ratio in reference to 1 mW at 50 Ohms , hence POWER ratio.

    Now the input and output in datasheet is specified as "Vx" , assuming voltage.

    My DDS-60 can put out +15dBm and I have an attenuation of 21 dB so in theory my actual source - input to AD8302 would be roughly -5dBm.

    I am not sure how to correlate "reflection coefficient " to "return loss" , besides using math formula of -20x log whatever.

    I guess I need to go back to AD8302 datasheet, again, to figure out what exactly - assume return loss (ratio) in dB or just "voltage " to be able to calculate actual return loss is the output of it.

    PS
    Hardware is coming along nicely. Waiting for A/D converter module.
     
  7. AA7EJ

    AA7EJ Ham Member QRZ Page

    Pretty much what I am going to do.
    Actually this is little overkill - I need to connect the attenuators from coupler to AD8302 module about 3 inches apart !
    Perhaps twisted pair would work same.
    But coax is more fun to solder!
     
    VK4FFAB likes this.
  8. AA7EJ

    AA7EJ Ham Member QRZ Page

    Here it is - ready to be wired.
    Please note at lest $10 worth of 4 and lesser size screws.
    Putting nuts on those buggers has been a challenge.
     

    Attached Files:

  9. AA7EJ

    AA7EJ Ham Member QRZ Page

    Nitpicking silly question / remark.
    Back to "protection diodes".
    The source generates "single ended" signal - sinusoidal wave from close to 0 Volts to +15 dBm. (I know is should not be mixing Volts and dBm)
    The coupler passes AC varying form some minimal level to some max level.
    It is still "single ended (?) " in reference to 0 (GND).
    So why some designs using TWO diodes - both connected to GND - when only positive signal over .7 V (silly not German diode) will be passed thru to ground ?

    And will resulting distortion of the sine wave screw things up for the AD8302?
     
  10. WB3BEL

    WB3BEL Ham Member QRZ Page

    It might make sense to draw this out as a block diagram like this example rather than trying to describe it all in words.
    You can tell what the levels are between the component modules. You can adjust them as you see fit. You can understand what each piece does and how changes you make to it will impact the system.

    VSABlockDiagram.jpg

    It is not clear if you want to only measure VSWR higher than 1.5:1 or less than that. But 1.5:1 is a return loss of close to 14 dB.
    So if you only care about return loss to this level you need directivity in your coupler of perhaps 20 db or greater.

    The directivity is not the coupling value. It is a figure of merit of how much reflected power would appear on the coupler's reflected port if the device under test had a perfect termination. This example uses a device under test with a return loss of only 10 dB or a VSWR of 1.92:1 .

    On your question about the protection diodes, they are really there to keep you from damaging a circuit if you make a silly mistake in the lab. If your system level design is such that damaging levels are never possible. Then the protection diodes do not do anything. When you are making measurements you do not want the protection diodes conducting. Choose your levels so that you are always significantly below that diode threshold when you are taking measurements. Some designs have back to back diodes, because they can be used in AC coupled circuits where the voltage can be both above and below zero even when powered with a single supply voltage polarity.
     
    Last edited: Nov 14, 2018 at 7:54 PM

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