Daiwa CN-901V N issues - directional coupler modification - Page 1
Forum section: electronics / rf-microwave

author: IK0BGG
author_rank: Newbie
timestamp: January 20, 2021, 09:48:47 pm
content: Daiwa CN-901V N directional coupler modificationIK0BGG Jan. 2021After having discovered that the CN-901V’s directional coupler has a very poor directivity - especially on 430 MHz band that make it useless as a VSWR meter - I decided to modify the module in order to reach an acceptable instrument functionality.For more information on the Daiwa CN-901V’s performance, please refer to the following documents:Daiwa CN-901VN Test Reporthttps://mega.nz/file/wQBGBTbA#X9GrGHBuGjgGsNvhj0puQhMBgNAWllb53QaKlAI3aXEDaiwa CN-901N Technical Analysishttps://mega.nz/file/ZdAyTLRT#22kHNuPdl8u0IlAONlb6kNsf-wvHPeA0Cjz3Y8qhy9cJust to remind the original coupler’s design , here is a picture of the module:My target was to keep the modification effort to a minimum by reusing the steel box and PCB as they are. With such a constraints, the power metering performance of the instrument as a wattmeter is guaranteed only on the two amateur bands of 144 MHz and 432 MHz, just like the original design did.This is the modified schematic diagram.I have found that the right termination impedance of the coupler’s arm - very important for the directivity performance - is neither 91 nor 100 ohm but 60 ohm; in order to have a 60 ohm 0.5W resistance two 120 ohm 0.25W resistors have been paralleled.The frequency response compensation is performed by a capacitive trimmer, I have used a high Q, 6 turns, precision air capacitor mod. 8053 made by Johanson or Airtronic; of course other type of trimmer can be used but with the above one the regulation is really smooth and accurate.The resistance in series of the 1K60 diode has been set at 150 ohm (instead of 100 and 150 ohm) for both arms, for symmetry reasons.All the components’ leads must be kept to a minimum length, the trimmers are soldered to the former adjustment slot and tilted in order to minimize the connection’s length to the microstrip.The coupler’s calibration is made by feeding the unit with a signal generator, the other port is terminated, and the output voltage is read on a microammeter or a voltmeter. The trimmer must be adjusted in order to read the same value on the two frequencies of 145 MHz and 435 MHz, after a few cycles one will reach the equalization point; I used an HP8642B with its power output set at +20 dBm and a 20µA microammeter.Once the directional coupler has been calibrated, it is necessary to mount it in the instrument, connect it to the PCB and calibrate the 200W and 20W readings of the forward power and the 40W and 4W of the reflected power by acting on the printed circuit trimmers. It will be necessary to use a dummy load of adequate power, to exchange TX and ANT port for the reflected power, and a 145 or 432 MHz transmitter.The CN-901V N must be connected in cascade with a calibrated wattmeter with which it can be compared, I used a precision power attenuator to terminate the instrument and measured the power with a Marconi 6960A RF Power Meter equipped with a 6910 power sensor.Since to make a proper calibration it is necessary to be able to generate power levels not lower than half of each full scale reading, so a power amplifier could help.I have tested the behavior of the VSWR meter with a 25 ohm load (VSWR=2.0) and with the complex impedances my 144/432 MHz collinear antenna gives in and out the specified frequencies of operation having as a reference instrument the HP8753ES. Since the readings were in good accordance the result of the modification is satisfactory.Another modification could regard the main transmission line that has a return loss of only about -24dB@435MHz and that is too low in impedance, as the HP8753ES shows:By examining the main line one can figure out that, in order to improve the Return Loss figure, a couple of corrective actions could be taken .1.The line impedance is too low and one could rise it by milling a bit, as per the green rectangle2.The connection point of the connectors is too deep, this constitutes a parasitic capacitance and one could mill the pcb track as per the red rectangles.Since the overall performance is satisfactory, I did not implement the main line modification yet.CONCLUSIONSThe original directional coupler of the Daiwa CN-901V N directional wattmeter has been modified in order to gain a proper level of directivity up to the 430 MHz band, the power calibration is guaranteed on both 144 and 432 MHz bands but not on the entire specified frequency range of 140-525 MHz.Calibration procedure of the modified directional coupler is simple and based on two precision capacitive trimmers.A further improvement could regard the main transmission line because its impedance is different from 50 ohm; a modification of the line could further improve the accuracy of VSWR readings.Daiwa CN-901V N has a good quality cross needle meter, a nice steel case with bumper protections; with the modified coupler it becomes a quality amateur grade power and VSWR meter for the 144 and 432 MHz bands.I would suggest Daiwa to improve both the design and the manufacturing of this product.A pdf copy of this post can be downloaded from here:https://mega.nz/file/dBw0zIBQ#x76cmJCQbbE8dupT_yFpnD3Cy_XKVAL6qPI6Bfh4fckFor any further information I am also available here: ik0bgg(at)gmail(dot)comBest 73 de IK0BGG Angelo

author: Kartika
author_rank: Regular Contributor
timestamp: April 01, 2022, 05:59:25 pm
content: Quote from: IK0BGG on January 20, 2021, 09:48:47 pmhi how to reach this ? ?

author: Hamelec
author_rank: Regular Contributor
timestamp: April 04, 2022, 11:37:26 am
content: Quote from: Kartika on April 01, 2022, 05:59:25 pmhttps://www.eevblog.com/forum/rf-microwave/daiwa-cn901n-issues-technical-analysis/otherwise give Angelo a mail:ik0bgg(at)gmail(dot)com