VNA Details


External power comes from a regulated 12 VDC, 2.5 A modular supply. I verified that this switching supply is pretty quiet with only about 2 mV rms ripple at 150 kHz, plus harmonics. There are two stages of passive LC filtering before the 12 V heads for the VNA board. 5 V is provided by an LM7805 bolted to the back of the chassis. 


Power Supplies



I also added a -5 V, 3 W switching supply for use with wideband opamps that I generally use for buffers. That supply (TDK CC3-1205SF-E) has a high switching frequency (700 kHz), is well-filtered, and resides in a soldered-up box with feedthru capacitors. Residual noise could not be detected on my spectrum analyzer without a low-noise preamp, with the only detectible peaks being less than 1 µV at 7 MHz. Now that’s quiet!

The Elrasoft USB interface board plugs directly into a wirewrapped fanout board that I made on perfboard for distribution of digital signals. I prefer double-row 0.1 inch headers since those are usable with mass-terminated cables. This board also has the driver transistors for the RF relays and attenuators that are part of a full S-parameter test set.

Below is an overview of the completed chassis, without the test set modules which are still in development. Interconnects are SMA jumpers that I got on Ebay--a really good deal--but watch out because a few of them had not been crimped!

USB Interface and Digital I/O