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Daiwa CN-901V N issues - technical analysis

Discussion in 'Radio Circuits, Repair & Performance' started by IK0BGG, Jan 15, 2021.

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  1. IK0BGG

    IK0BGG Ham Member QRZ Page

    Daiwa CN901V N directional wattmeter
    technical analysis

    IK0BGG Jan. 2021

    After having detected bad performances in terms of poor directivity of the CN901V N wattmeter, serial number 2007, I decided to disassemble it in order to understand how it has been designed and built.

    [​IMG]

    The hearth of the instrument is the directional coupler I took out from the box:

    [​IMG]

    The unit is hosted in a tinned steel box with a red seal label covering a long slot in the upper wall of the box. I left in place the label and removed the cover after having taken away the solder seal put on its right side.

    [​IMG]


    Here is what the directional coupler looks like once the cover is removed:
    · a suspended microstrip main transmission line links together the two N input and output connectors.
    · two other microstrip transmission lines realize the directional couplers of the forward power (left) and the reflected power (right)


    One may immediately notice the realization characterized by solder paste residues, excess tin, tiny little tin balls walking throughout the box.
    At a first look one is impressed by the two couplers - that should be designed to operate up to over 500MHz - manufactured like an audio frequency circuit: the components are left with long leads.
    There are long leads in order to make possible the calibration of the unit by changing the components’ position by acting with a dielectric tool from the outside through the alignment slot. By moving the components one can obtain a power calibration but the consequence of the distorted coupler’s topology and the lead’s parasitic parameters is a very poor directivity, especially on the 430MHz band, as a result VSWR measurements are erroneous.


    Here is the schematic diagram of the unit.

    [​IMG]

    Let's see into detail of how the forward power coupler is built.


    [​IMG]

    The 100 ohm 0.5W resistor has long leads in order to be moved during the alignment process, the left lead is also connected to a weird place: it is soldered to the center of the microstrip instead of being connected to its land which, of course, has been left free.
    The 150 ohm resistor which is in series with the diode has an excessively long lead connected to the microstrip and it has been clearly moved when calibrating.


    As for the reflected power coupler, the situation is as follows:


    [​IMG]

    The 91 ohm 0.5W resistor (the blue one on the left side) has its right lead soldered to the right land (unlike the forward coupler one) but it is excessively long in order to move it while calibrating. The 100 ohm resistor (on the right and in series with the diode) has the lead connected to the microstrip that are very long and wandering. The welding of the pin of the coaxial antenna connector to the passing microstrip line is made with an excess of tin and with the solder paste and dirties that has not been removed; solder residues are interposed between the main line and the reflected power coupler‘s line.

    By carefully watching the coupler’s detail one can deduce that it seems to have been designed by hand drawing!

    In fact the coupler’s line width is not constant and also the other traces have not been accurately drawn.

    [​IMG]


    The coupler performances have been measured by feeding the unit with a signal generator and reading the output voltage, in relative units, as a function of the frequency; here are the forward and reflected power characteristics that have the same amplitude only in two points: 145 MHz and 435 MHz amateur bands.

    [​IMG]

    Since the power is proportional to the square of the voltage, the accuracy of the power readings outside the above two amateur radio bands is completely out of specs.
    The directivity is very poor but measurements are not shown in this report.


    CONCLUSIONS

    The directional coupler, the heart and the most critical component of the Daiwa directional wattmeter, has inadequate performance for a device that must operate up to over 500 MHz.
    The components are left with long leads in order to change their position during calibration to achieve the desired frequency response of the wattmeter, the power calibration is reached only on the two ham radio bands and not on the entire specified frequency range.
    A consequence of the distorted coupler’s topology and parasitic parameters is a very poor directivity, especially on the 430MHz band, and so erroneous VSWR measurements.
    A careful examination of the PCB traces shows that it seems to have been designed by hand drawing! In fact the coupler’s line width is not constant and also the other traces have not been accurately drawn.
    Due to this design and manufacturing of the directional coupler, the performances of the Daiwa CN901V N, s. n. 2007, are really poor and far from what is stated in the product’s technical specifications.
    It is under study a modification of the directional coupler in order to make the instrument work properly.


    The pdf version of this analisys can be downloaded from here:
    https://mega.nz/file/ZdAyTLRT#22kHNuPdl8u0IlAONlb6kNsf-wvHPeA0Cjz3Y8qhy9c


    A test report of the Daiwa CN901V N can be downloaded from here:
    https://mega.nz/file/wQBGBTbA#X9GrGHBuGjgGsNvhj0puQhMBgNAWllb53QaKlAI3aXE


    For any further information I am also available here: ik0bgg(at)gmail(dot)com

    Best 73 de IK0BGG Angelo
     
    AI3V likes this.
  2. G4COE

    G4COE Ham Member QRZ Page

    I've not read all the post so may well have been mentioned.

    What looks like a 'thru link' on the left hand side of the broad middle track, it looks a little precurious... is it supposed to be a pin that the solder hasn't adhered to or is it a reflection.

    Maybe thats why they stuck a DO NOT OPEN label, so no one can see the quality and stop them tempering with those long leads.... a poor do with the looks of things, to save some preset capacitors perhaps?

    Dave
     
    Last edited: Jan 15, 2021
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Silly design.

    I've never found a "Daiwa" SWR/power meter that was properly designed or manufactured, and I've opened a lot of them.

    I have here an old CN-620B which was rated for 2kW and also rated to 150 MHz.

    So, for fun I put it in line and ran a kilowatt through it on 144 MHz. Smoke emerged from the meter.

    Of course I performed a postmortem on the meter (it was cheap enough that if I had to toss it in the garbage, that would not bother me, but I "wanted to see"). The microstrip between the input and output connectors had two right-angles in the strip and the corners were not beveled 45 degrees as they should be to maintain impedance. Right at one of the "corners" in the microstrip it got so hot it burned the PC board, and that's what caused the smoke.

    I did repair this, and did use a razor blade to bevel the microstrip corners, peeling off the original "corners" by heating with a soldering iron and then using the tip of the blade to peel away the corners that should not have been there. There were other layout problems as well, and I did my best to fix all of them.

    I still have that meter after 30 years, and it still works but I wouldn't put a kilowatt through it on VHF. I have it in-line with a little Xiegu G90 (20W HF transceiver).

    I've also had "Daiwa" coaxial switches fail, but that's another subject.:p
     
  4. VK4GAP

    VK4GAP Ham Member QRZ Page

    Different Models same Brand , an CN-101L 1.8-150MHz and a CN-103L 2M-70CM both by Daiwa , got them relatively cheap i thought at the time secondhand .
    The HF one was showing 20w low on HF so i decided to take a peek , and was i stunned with both .
    So it turned out the 1st is only any good for HF now (trust-able) , the other 2/70 really belongs in the Bin but i still have it mainly as a emergency backup for showing a go/no go situation .
    pic is the HF one and that is supposed to be accurate to 150MHz ! .

    1.jpg

    SENSOR.png
     
    Last edited: Jan 16, 2021
  5. W1VT

    W1VT Ham Member QRZ Page

    A microstrip coupler is one of those great ideas that is much harder than it looks. Most published designs have poor directivity though I did come up with a low power design with 33 dB of directivity. But, the spacing was just 20 mils so it wouldn't be useful for most wattmeter needs.

    Zak W1VT
     

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