AmateurRadio.com

If you’re a workbench ham, you like to measure things. Some like to just get a rough cut while others like to be “nuts” about it. There are groups, like Volt-Nuts and Time-Nuts, who focus more on what’s formally called Metrology. Me? I’m somewhere in the middle but tending toward more precise and reliable measurements, although I do get the Cal Lab magazine each month. Perhaps it’s because I taught classical measurement theory to PhD students for over 30 years and understand true-score theory (e.g., every observed measurement is determined by the “true score” plus some error). Adding multiple observed scores and modeling the errors is something I taught and used for a few decades. Reading Bob Witte’s books from HP as well as Joe Carr’s various texts helped me transfer over the statistical base into electronic measurement issues although clearly I have a lot to learn in this realm.

Building out a good test workbench has been one of my priorities over the past few years. I’ve been inspired by many others, mostly on Youtube, but the IMSAI Guy channel is one of my favorites.

The Soldersmoke Blog says his identity is Michael Cassidy W6UAB in Oakland California. He doesn’t exactly say on his Youtube Channel. Whether this is his real name or not, he is very educational in his videos. I have learned a lot but he’s cost me money! He is a clear workbench Elmer to me. I’ve acquired a couple of professional lab-grade test pieces on the used market for far, far less than what they cost new because he first went through them thoroughly on his Youtube Channel. The first was the HP 4735a LCR meter. Here’s my latest one.

The HP 8714ET VNA is a two port VNA. Yes, I have a few NanoVNAs. In addition, the SDR-KITS VNWA 3SE Automatic 2 Port Model with their MagiCal calibration device. Plus, I picked up a LibreVNA with their LibreCAL device. I recommend R&L Electronics for LibreVNA products. Why would I need another VNA? If you are seriously asking this question, my friend the Smoking Ape has some Cat Videos for you to watch. Because I wanted it is the simple answer, LOL.

Shown above is the 8714ET connected to the eight-port test set with an old Barker & Williamson Model 425 Low Pass Filter as the DUT. I purchased it at a hamfest for $5 to use the case but have not yet.The larger monitor on the right serves dually as a second screen to my PC (HDMI port) and as the VGA output with customizable color from the VNA. This is the conversion loss measurement shown. Note the USB floppy drive under the monitor.

Plus, it’s a device with a lot of features that raise the bar for bench testing. It is monochrome green in the internal display but customizable colorized in the VGA output. It’s remotely reachable by HP-IB (I use an HPIB-to-USB adapter), LAN (internal web server), and via a 3.5″ floppy disk. I purchased an inexpensive USB-based floppy drive for the Dell Precision 1700 PC that runs my workbench. And, yes, the 8714ET will indeed format a 3.5″ floppy to DOS. I suspect that I will retrieve screen images, the underlying data, and Touchstone files via the web interface but, heck, I’m kinda digging having a floppy disk in my Lab. In time, I will use some of the many IBASIC programs written for this line of VNAs. The PS/2 keyboard that I just ordered plugs into the rear of the VNA, making editing or writing IBASIC scripts much easier as well as entry for some features on the VNA.

I’ll let IMSAI Guy’s video playlist on his earlier model (HP 8711C upgraded to an 8712) give you the fuller run down on all the things it will do, especially with IBASIC programs that automatic a lot of testing, storage of calibration data, and so forth. But I added something to mine, the Agilent multiport test set, 8-port, 50-ohm model. I found it for $200 on eBay and it is pristine, almost as much as the 8714ET that I found there from a “junk shop” vendor. Very lucky it seems on this set of purchases.

An engineer friend (N5WDG) gave me some high quality test cables when he built a new house and cleared out some extras from his workbench. Good ones can cost almost as much as the used test gear scored online so I’m grateful for the crate of “stuff” that Thomas N5WDG, a WAN Engineer for AT&T, handed to me a couple of years ago. The connections between the VNA and the multiport test set require a standard DB25 male parallel port cable plus two specific firm metal connector jumpers. I found these new-in-box on eBay for $25 each. So with the $1500 price of the 8714ET VNA added to the multiport test set and cables I purchased, I have about $1,750 in this “like new” lab grade VNA from the 1990s.

One thing that the multiport test set provides is a much easier way to tune filters like repeater duplexers and such. The reference materials for the device lay out the wiring diagrams and how to tell the multiport device to configure the ports remotely from the VNA itself. The 100 db dynamic range will help a lot for tuning the standard 2M and 70cm duplexers that are in common use. Covering 300 khz to 3 Ghz, the 8714ET has up to 1600 data points which allows a more thorough sweep for a given frequency span. Nice.

There are always multiple documents to study on new “old” test gear. Each one has a design theory to the things it claims to measure. There are always quirks that support communities (e.g., HP Test Equipment Groups.io; EEVBlog Forum; Youtube) can education you on to more fully utilize gear like this. I also read the archived HP Journal for articles published announcing the release of gear like this. It’s very instructive to have one or more of the design engineers to outline the theory of the device, detailing how they approached the critical aspects of implementing the instrument. Lots of reading and study ahead!