The ADCs are sampling the final IF with The acquisition and control of the PLLs, switches, attenuator etc.Each receiver consists of two down-convert mixers.It also avoid potential isolation issues that could arise if the receive paths would merge into a single mixer/ADC. This increases BOM cost but allows measuring two parameters at once (S11 and S21 or S22 and S12). Both ports have completely separated receive paths.Instead of directional couplers, resistive return-loss-bridges are used (easier to implement for wide bandwidth).In each signal path, two RF switches are used in series to achieve higher isolation between the ports. The stronger part of the signal can be routed to either port.After the amplifier(TRF37A73) the signal is split and the weaker part of it fed into the reference receiver.The stimulus signal power can be adjusted between approximately -42 and -10dbm with a digital attenuator (F1958).Its output signal is slightly filtered to reduce the amount of harmonics. The stimulus source for frequencies above 25MHz is a MAX2871.Its own reference clock is either a 26MHz crystal or an external 10MHz signal. It also serves as the stimulus source for frequencies below 25MHz. The main clock source is an Si5351C, providing all the required clocks for the different blocks.Learn more about the challenges in RF designs.A project like this takes several month and its a one-off device, slightly higher than absolutely necessary part costs are not an issue. BOM cost reduction is not the primary goal.To be usable, the display would need to be reasonable large and high resolution, also GUI development is faster/easier on a PC than on a ♜. Generally, I prefer standalone devices but it seemed not worth the effort here.
No display, instead the data is send via USB to a PC.Both limits are essentially set by the specs of affordable RF ICs. Especially the first link also explains some general concepts of a VNA, what it is used for, how it works and some compromises when choosing a hardware architecture. There already are some other DIY VNA projects online, especially these two served as inspiration. Ports are not 50 Ohm terminated, see also discussion here.Low sweep speed: Especially with averaging enabled, a sweep can take a few minutes.High thermal drift: you have to leave it running for 30-45 minutes before using it.I already own a pocketVNA, however I found some issues with it and didn't use it very often: There is a new and improved version of this project under developmentĪ VNA can be very helpful for RF experiments so I decided to build one for my next project.
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