HP8757D Scalar network Analyzer

[@jluke6861]

Thank you for you great videos and the time you put in.

[@danishnative9555]

I used a Wiltron 561 in the past with much satisfaction. Never been privileged enough to own a scalar or vector though. Quite a few for sale these days, but never have the detectors, auto-tester or open/short cal hardware. Pretty much a dead weight without the accessory goodies. Nice find TG. Cheers!

[@tgsoapbox]

Thanks - That was exactly it, being able to get detectors of known goodness meant being able to acquire the rest of it without worry - Even the directional coupler wasn't that much of a risk as I have 3 detectors and a set of known good stand-alone couplers.

[@mohsenamini8875]

Thank you

[@TheDefpom]

Cool another video, wives can be very patient, mine certainly is.

[@tgsoapbox]

Completely agree - Looking forward to your vid on the new microscope.

[@TheDefpom]

tgsoapbox - lol, you saw that eh🤓

[@tgsoapbox]

Last comment at the time I watched the video - I have to say, if it wasn't for limited bench space here I'd get one - BTW Congrats on the cracking pace for the videos, there was even a thread on your DefCAD use on the forum.

[@TheDefpom]

tgsoapbox - I just have so many queued up that I wanted to try and catch up a bit. I didn’t see the Defcad thread, I know Dave just put his Davecad notepad on Github though.

[@davidv1289]

Interesting bit of gear, thanks for the demonstration. I'm curious - how are the sweep generator and the SNA linked together? Is it thru the GPIB or do they have a dedicated interconnect?

[@tgsoapbox]

It's actually gpib but on a specific system control connection. You plug a gpib cable from the 8757 system interface into the sweeper's gpib port and it controls it from there. I believe that it uses a non standard way of driving the lines but it can work in some cases on a normal bus (in fact that was what I was doing in the video). HP does say though that it should be completely private and if you need to control the sweeper then you use the pass through commands.

[@tgsoapbox]

I should have added that the 8757 has both a normal gpib port and this special gpib based system control port on the back.

[@davidv1289]

Thank you for the info, I've used a lot of HP test equipment and never seen that control port before.

[@tgsoapbox]

NP - I cut some images from the manual to help show it: Rear panel - You can see the HPIB & System interfaces here: i.imgur.com/Ik0M8ea.png Setup diagram - You can see how they're connected: i.imgur.com/ARV0klG.png

[@smokyatgroups]

Thanks for you clip there's not many like it... I'm trying to use the older 8757A with HP8350... it seems more confusing than your D model especially with forward reflected measurements . It still wants to see a separate input on the reference channel rather than taking 100% reflected on direct. bridge as the reference.

[@tgsoapbox]

Hmm, it shouldn't be - When you say Forward Reflection do you mean the reflection from the device after transmission? Like an S22 measurement?

[@smokyatgroups]

Thanks kindly for your reply: I meant return loss dB or equiv. SWR. Using the 8757A /85027A bridge after short/open Calibrate. for Meas - Mem function, for a normalised trace return loss measurement, it seems not possible without an additional thru (S12) signal on reference channel? (Ratio Meas only msg). Sorry not good at my clarity.

[@tgsoapbox]

@@smokyatgroups That doesn't seem right - When you're using the 85027A it should be just one connection to the A port. I don't have the 8757D anymore as I acquired an 8720C so I gave the unit to my Dad. Are you sure that the unit is correctly detecting the 85027A? CAL->CONFIG SYSTEM will show if that is the case I think. If it is, what steps are you doing to get the return loss? Also do you have the detector connected when you turn the 8757A on? Quick look at the manual seems to imply you need to either have that or configure the inputs again when you change it in.

[@smokyatgroups]

@@tgsoapbox Yes the system recognises the bridge and detectors ok. It had the LCD display kit fitted and CAL done about 18 months back. When following "abbreviated meas sequence" from manual. After open/short cal on channel 1 with bridge, getting an error msg: 'ratio measurements only' when pressing (Display) then (Meas - Mem). Does this mean I have to use 'A/R' function and thus an additional detector Ref channel and splitter in a separate reference loop? It seems superfluous. If true, then manual isn't correct which is unusual for HP. Appreciate your thoughts, Gerry

[@tgsoapbox]

@@smokyatgroups Unfortunately I don't have the device to try out the commands but you don't need either channel B or R to do an SWR measurement just on reflection. From the manual downloadable from Keysight, the setup is in Figure 26 on PDF page 80. There is a boxed section on page 81 of the PDF which shows the steps for a typical SWR measurement - Try these ones (make sure not to use [A/R] as you can also do this in ratio mode). Also sadly errors in HP manuals aren't exactly rare but they do tend to be uncommon in the operating sections so I agree there.

[@davidr4210]

hello can you help me understand the difference between the 8757 and the 8756 SNAs? I heard the 8756 are a pain to system cal while the 8757 are easy to cal. Are the 8756 hard to use or less accurate?

[@tgsoapbox]

Hey David - Would love to help you but I've never used the 8756 so I can't really comment on it - That said, there is a good article in the HP Journal on the 8757A (www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1986-02.pdf) which does refer back to the 8756 so I think giving it a read might provide some insight. Calibration of the 8757 though is really easy, there are something like 2 - 4 performance tests that you need to do on the unit. The reality is that most of the calibration work is on the signal generator side and in the sensor heads (which I don't think are user-adjustable but I could be wrong on that). I did take a look at the 8756A service manual and there are a lot more tests so I would expect it to be a bit harder but as I said I haven't used one so I can't really comment - Sorry I couldn't help more.

[@davidr4210]

@@tgsoapbox thanks for the reply I will take a look at the article. Recently found your channel and it’s very interesting.

[@dgaborus]

Is it possible to use a scalar network analyser as a spectrum analyser by only displaying one of the inputs?

[@tgsoapbox]

While the SNA (8757D in this case) is similar to a SA (e.g 8563E) with a tracking generator it isn't designed to perform the broad sweep of the SA. The SNA uses an external signal generator and a modulated signal to determine the information about the DUT at the measurement point rather than taking the input from the DUT and mixing it with an internal LO (technically a lot of SAs can also use external LOs but for this purpose I'm ignoring that) to get the signal parameters. Having both an SNA (or VNA) and a SA in the lab is useful for a full range of measurements but a lot of them can be done somewhat similarly with just an SA and tracking gen. Does that answer your question?

[@dgaborus]

@@tgsoapbox Absoutely. Thank you