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Homebrew Directional Coupler for VNA
Written by Administrator   
Sunday, 03 August 2008 19:20

My directional RF-coupler for 0.1-60Mhz ready to put into the housingFor RF-measurements at discrete components (resistors, inductors, capacitors or even transistors etc.) a vector network analyser must analyse the RF-response from the device under test (DUT). For that reason a directional coupler is necessary to distinguish the sent generator signal from theand reflected DUT-response. I build my own with a transformer I bought from Mini Circuits.
Absolute directivity of the coupler is not even that important, as the VNA is anyway OSL-calibrated using calibration standards of open, short en well defined 50Ohm load.? But the better the bridge derectivity the more sensitive the measurement will be - important for especially if the response from the DUT is very small in magnitude.
As I couldn't find any smaller RF-enclosure in stock, the directional coupler finally became almost as big as the whole main VNA. In fact the coupler could be made much smaller, but I simply didn't want to buy another smaller enclosure than the one used - the enclosure is almost empty...
I followed the strategy to use SMA connectors for precision, easy availability and consistency with the main VNA unit.

Last Updated on Monday, 24 November 2008 16:17
Homebrew Vector Network Analyser (VNA)
Written by Administrator   
Wednesday, 18 June 2008 19:24

My home-brew VNA, based on N2PK's design. I use SMA conncetors for performance and easy availability. On the right you see the home-brew directional coupler for reflection measurementsBeing an active contester, I was always keen to optimize my radio station as much as possible. It is essential, that antenna's work as they should and therefore I needed an instrument to verify RF performance of electronic components, antenna's and equipment.
Some time ago I bought an MFJ-259B RF analyser, but I was not happy with it for various reasons. Very soon I decided to build my own network analyser, to match real measurement lab-quality, but without the bulky size of the usual commercial equipment such as Agilent, Rohde&Schwarz or others...
At that time Paul, N2PK came up with his wonderful concept of a small unit which completly matches my desires.
So I decided to build the unit as N2PK suggested and documented. In the meantime many people have found out the exceptional usefulness of this design and even commercial (blank) PCB boards are available from Ivan Makarov nowadays. Because Ivan's board was not available when I started to build the unit, I had to source my PCB somewhere else based on the original suggestion by N2PK.

The two DDS-chips responsible for generating the RF signals at various phase shifts. These SSOP-chip components to solder require some experience with SMT soldering. Both DDS-chips refer to a ultra low phase noise master oscillator (Valpey Fisher) running on 148.388MHzThe whole project was based on modern SMT-technology. Some parts really need some skill to solder properly as SSOP-type of chips had to be soldered and components of 402-size are really small. Get strong glasses...:-)

Basically the nice thing of Pauls design is, that the final measurements are made on DC level as the vector network analyser works as a double DDS driven direct to DC-conversion unit. Four DC measurements are made sequentially exactly at the same frequency: The first two at 0/90 degree phase difference are evaluated against 180/270 phase shift to finght any drift issues during the measurement circles.
The unit is operated and driven by the PC, which also collects the data through software.
The result is a very easy to use graphical user interface (GUI) and also easy documentation of the results as all data can be exported as standard .csv files to be used for presentations or incorporated into databases, if necessay.
More information and comprehensive documentation of the VNA can be found on Pauls website

altAs an example I measured the noisefloor with open detector input, which is extremly low ! Note that the generator level is +4dBm. The chart was taken without any software average nor with CDS sampling technology (taking not only 0/90degree samples but also 180/270degree for fighting drift issues during each measurement sampling circle)
With CDS sampling the noisefloor further drops below -130dbm - really enough for detailed stopband analysis of modern filters.

Recently I re-adjusted my 4square antenna at my 40m contest station - obviously the sensitive but accurate vector network analyser pin-pionts still some performance lacks...
Howver the antenna performance after the measurement and alignmet session was already better than ever before and I achieved a very directional pattern with front/back ratio's in access of 20db

Last Updated on Monday, 24 November 2008 16:16

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