This weblog compares a standard Vector Community Analyzer (VNA), the HP 8753, with a NanoVNA to be used by the price range constrained engineer growing a medical system.
A Vector Community Analyzer or VNA injects a identified sign right into a circuit, and determines what modified concerning the sign after it leaves the circuit. VNAs are used in the course of the improvement technique of a tool, to characterise and confirm the efficiency of circuits reminiscent of amplifiers, filters, antennae, cables, and so on. This is quite common, particularly if a given system operates at increased frequencies, the place nonidealities can play a much bigger position and are more durable to foretell.
VNAs exist in lots of sizes, shapes, and worth ranges, all with various options and capabilities. Two examples of VNAs are the NanoVNA-H, and the HP 8753. The NanoVNA-H is an reasonably priced hand-held newbie VNA, and the HP 8753 is a trusty skilled labs workhorse of the 90’s. The market and meant use case of the HP 8753 and NanoVNA-H are basically totally different. The big selection of kit accessible on the market raises many questions, reminiscent of whether or not or not low-cost variations have a spot in knowledgeable engineering setting.
Later on this weblog I’ll measure the frequency response of 4 elementary filter topologies from a pattern PCB on each VNAs.
Preliminary comparability of NanoVNA-H and HP 8753
Upon first take a look at these units aspect by aspect, it will be honest to assume they did vastly various things. Regardless of nearly comical bodily variations, these units are designed to do nearly the identical activity.
|10ppm @ 25C
|50khz to 1.5Ghz
|30Khz to 3Ghz
|70dB (50kHz-300MHz), 50dB (300-900MHz) and 40dB (900-1500MHz)
Evaluating specs of a tool produced by one of many highest finish instrumentation firms for many years with an open-source system may be tough. The specs listed for the Nano VNA-H are probably not utterly correct and I used to be unable to discover a dependable supply for the decision. It’s additionally value noting the frequency vary of the NanoVNA can rise up previous 1.5Ghz with a firmware replace, nonetheless the accuracy and determination goes down dramatically with this modification. Moreover, you may see that the dynamic vary of the NanoVNA-H drops shortly as frequency will increase, attributable to it needing to down convert frequencies better than 300Mhz as its receiver can not instantly measure something above this frequency.
The NanoVNA-H’s accuracy spec is sort of spectacular and seems to have been verified by newbie radio operators however, to my information, has not been verified by way of a NIST chain. Because the NanoVNA and HP 8753 are from totally different eras of design, you will need to focus on the context round why these devices exist. The NanoVNA is an open-source undertaking, first launched in 2019, meant for hobbyist digital circuit design and testing. It’s fairly in style within the newbie radio neighborhood and is a staple on the desk of many HAM operators and tinkerers.
This contrasts the HP 8753, which was developed within the late Nineteen Eighties, meant to be used in skilled settings for use day in and time out by engineers designing varied types of digital units. Lots of the applied sciences and elements used within the design of the NanoVNA have been merely concepts within the Nineteen Eighties. The event of every instrument displays the time it was developed in.
Benefits of NanoVNA
The NanoVNA comes outfitted with many more moderen communication requirements, reminiscent of a microSD card, and a USB-C port for charging and connection to a desktop management setting. This could possibly be helpful for operating automated assessments, gathering massive quantities of knowledge or altering settings with no need to make use of the small contact display. Additionally it is a lot smaller (weight/quantity) making it very interesting for transportable purposes, particularly with the built-in battery.
As it’s open supply, the NanoVNA could possibly be a really interesting launch pad for the event of medical units which have an identical mode of operation, or as a device for an engineer trying to increase their excessive frequency circuit design experience. This makes the approachable price ticket of $50-200 CAD (relying on provider) very interesting for engineers of various ranges of expertise, particularly when engaged on price range delicate purposes. Additionally it is able to being calibrated by the top consumer by way of the SLOT methodology; utilising brief, open, load and termination connectors and cables. That is quite simple and may be carried out in a fashion of minutes. In case your cable and termination plugs’ traits are trusted, you may have some religion that your waveforms are near consultant of what you’re measuring. Lastly, the NanoVNA makes use of SMA connectors, that are very low cost and generally accessible.
Disadvantages of NanoVNA
The most important drawback to its open-source nature is many illegitimate clones exist on the market with varied ranges of high quality and functionality. Moreover, there’s not lots of data accessible for the accuracy/repeatability over its frequency vary, which could be a main detractor for delicate purposes. Whereas this system may be calibrated by the top consumer, altering the frequency span of the show requires recalibration, which could possibly be a problem if performed repeatedly. Moreover, the NanoVNA is barely able to instantly measuring S11, S21. All different parameters are calculated from these two measured values, which may introduce some inaccuracy.
Benefits of HP 8753
The most important benefit of the HP 8753 is its frequency vary, from 30khz to 3ghz. It has quite a few options the NanoVNA doesn’t, reminiscent of 4 port measurement. There are two features that the HP 8753 wins out over any open supply VNA: upkeep and calibration. Whereas the HP 8753 was initially manufactured by HP within the very late 80s, Keysight (a spin-off of HP’s instrument division) nonetheless affords restore/upkeep for it, which could be a huge profit for firms with out in-house restore capabilities. That is evidently not the case for the nanoVNA, as you’re by yourself if one thing is broken.
Moreover, the HP 8753 can nonetheless be professionally calibrated. This can be a key benefit, particularly within the medical system subject as ISO 13485 requires gear to be calibrated when used for verification and validation. Whereas the HP 8753 just isn’t open supply, schematics exist within the service manuals for your entire system and may be discovered on-line. That is additionally nice for studying about excessive frequency schematic design, albeit from a really totally different period of circuit design.
Disadvantages of HP 8753
Unsurprisingly, many of the detractors for the HP 8753 are attributable to its age and price. It’s cumbersome, weighing in at 34 kilos, making it lower than preferrred for transportable purposes. Because of its age, the display is dim, making studying it a problem. It lacks extra fashionable communication interfaces, with our mannequin solely being outfitted with a floppy drive and HPIB. This may be irritating when you find yourself attempting to get a number of high-quality plots off the instrument for studies, shows and so on. Moreover, the HP 8753 makes use of type-N coaxial RF connectors, which may be fairly costly in comparison with BNC connectors or the SMA connection current on the NanoVNA.
To get a way of how these devices examine in real-life, I measured 4 filters on a pattern PCBA: A band go, band cease, excessive and low go filter. These filters have been chosen attributable to their ubiquity, nonetheless there are numerous different superior circuits the VNA can analyze.
After producing and viewing these graphs, I used to be fairly happy to see that the info coming off each the HP 8753 and NanoVNA was very comparable. Some variations could possibly be defined by elements reminiscent of time since calibration for the HP, and connector/cable nonidealities for each devices. Basically, I believe the info popping out of each units could possibly be used for particular factors within the improvement course of with none concern.
Unsurprisingly, the reply to the query I posed in the beginning of this weblog just isn’t as simple because it may appear. In the event you spend most of your time at a desk or bench performing verification of digital circuits, a trusted system just like the HP is an interesting choice. If you’re an engineer at a start-up the place funds for costly devices may not be accessible, or you’re doing design work that can later be verified by higher-end devices, I might significantly take into account an open-source system like NanoVNA. Regardless of which choice you select to make use of, an individual new to high-frequency design will profit from utilizing these units to grasp excessive frequency circuit behaviour that isn’t apparent in on a regular basis circuits, and from perusing the schematics and move charts made accessible by each producers.
Photos: StarFish Medical
Mike Ganzert is an Electronics Engineer at Starfish Medical. He obtained his Bachelor’s of Utilized Science in Electrical Engineering from UBC Okanagan in 2021, with a spotlight in medical units.