N2PK VNA - 'The Rest of The Story'
The N2PK VNA (Vector Network Analyzer) is a unique device that allows for accurate measurement of transmission and impedance characteristics of a Device Under Test (DUT). In this article, we will explore how the N2PK VNA works and the technology behind its measurements.
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Imagine a simple transmission measurement through a DUT on the block diagram
Let's break down the process of measuring transmission characteristics through a DUT (Device Under Test) using the N2PK VNA. The RF DDS (Direct Digital Synthesis) generates an RF voltage with a reference phase of 0 degrees, which is then applied to the input of the DUT. The output from the DUT to the Detector input is an RF signal with amplitude |VRF| at a phase Theta.
In addition, the RF signal at the Detector input is measured with a 'Through' in place of the DUT. The 'Through' is a short length of transmission line assumed to have unity gain and zero phase. The Detector output is also measured without any intentional RF signal, referred to as the 'Open Detector.' These preliminary measurements are made before the DUT is inserted and measured.
From these three vector measurements at a single frequency, the N2PK VNA can calculate all DUT transmission characteristics such as gain and phase. For group delay measurements, two or more frequencies are required.
Measuring Impedance Characteristics
The N2PK VNA can also measure impedance characteristics of the DUT at each frequency of interest using a standard Wheatstone-type reflection bridge. The DUT's |VRF| and Theta are measured relative to three precision terminations: an open, a short, and one that approximates the system reference impedance (usually 50 ohms). These calibration standards allow for accurate measurements even with high or low DUT impedances.
While measuring |VRF| is relatively straightforward, determining the relative phase information Theta can be done using various methods. Most lab-quality VNAs use a complex and expensive superhet architecture that converts the reference and detected signals to a fixed Intermediate Frequency (IF) for precision amplitude and phase detection.
The N2PK VNA, on the other hand, introduces a simpler architecture using a narrow-band direct-conversion approach. This direct-conversion architecture is less complex than superhet VNAs but provides significantly improved accuracy compared to log-detection devices found in 'VNA on a chip' devices like the Analog Devices AD8302.
The Unique Architecture of the N2PK VNA
Due to the DC conversion of the Detector in the N2PK VNA, the only output available is a DC voltage. This DC voltage depends not only on the magnitude of the RF voltage at the Detector's input but also its phase relative to the LO (Local Oscillator) input.
Highly accurate measurements of this amplitude and phase-dependent DC voltage are achieved using a precision linear analog detector, a 24-bit analog-to-digital converter (ADC), and precise phase control of the LO DDS. The phase information is obtained by making two sequential DC measurements for each frequency and test condition: Through, Open Detector, and DUT.
In each scenario, the first measurement is made with the LO at the reference phase of 0 degrees, and the second measurement is made with the LO phase shifted by 90 degrees. This process provides the quadrature or vector components of each signal at the Detector RF input. As a result, the N2PK VNA boasts a simple yet powerful hardware architecture, made possible by advancements in computer-controlled DDS, high-precision ADCs, and computer control and post-processing capabilities.
Simplifying Home Construction
One of the key advantages of the N2PK VNA is its simplified home construction process. The computer processing eliminates the need for complex setup adjustments and allows for the calibration of many common sources of measurement errors. This simplification opens up the possibility for enthusiasts to construct their own VNAs with ease.
In conclusion, the N2PK VNA offers a unique and accurate approach to measuring transmission and impedance characteristics of a DUT. Its narrow-band direct-conversion architecture and precise measurements make it an excellent choice for both professionals and home constructors alike.
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