Test Equipment set up and impedances - Page 1
Forum section: electronics / projects

author: valvedoctor
author_rank: Contributor
timestamp: January 18, 2015, 10:04:48 pm
content: Hi everyone,I build and test vacuum tube amplifiers and the such, and have a few simple questions.I have a HP 654a Test oscillator as my sine source, and I also have a HP 8903b distortion analyzer which also has it's own oscillator. I have a HP 5315a counter monitoring each source, and an HP 3561a spectrum analyzer also monitoring the output of the device under test.Question 1:The 654A has options for the output impedance. Most of the things I build have 600 ohm input impedance. On the 654A, there are 50, 75, and 600 ohm selections that I can choose from. If I choose 600 ohms, would I need a 600 ohm feedthru termination to go into the 600 ohm input of the device under test, or would they be matched and not need anything?Question 2:The device under test wants to see a 600 ohm load. All of my test equipment is 50 ohm for the most part. How should I terminate the output of the device under test for my measurements to be accurate? Would a 600 ohm load in parallel on the output suffice?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 18, 2015, 10:11:29 pm
content: What does "vacuum tube amplifiers" mean? Are you talking about audio equipment or RF equipment?It is remarkable to find new audio (or RF, for that matter) circuits that actually have 600 ohm input impedance.We would expect a "bridging" (high-impedance) input for audio circuits, and a 50 (or 75) ohm impedance for RF.Saying that your amplifiers have 600 ohm output impedance also seems unusual in modern times.And saying that your test gear has 50 ohm input impedance implies that you are using RF gear.So, without a better description of what you are doing, your test gear and the circuits under test seem mis-matched.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 18, 2015, 10:16:28 pm
content: The output stages usually have feedback from output to control gain but more importantly to ensure a stable low output impedance ( notionally zero ohms).In order to alter output impedance that is present at the source connector, a suitable build out resistor is switched in between the output connector and the output from the amplifier.So if You select 600 ohm output impedance ... no need for any feed throughs.

author: valvedoctor
author_rank: Contributor
timestamp: January 18, 2015, 10:27:12 pm
content: I work with audio stuff. 600 ohms is an industry standard for recording and broadcasting.My device under test has an input impedance of 600 ohms and an output impedance of 600 ohms.The 8903B and the 654A both has 600 ohm outputs. So my first question is do you need an terminations if going directly into 600 ohm input of the device under test?EDIT, the 3561a dynamic signal analyzer has a 100K input impedance. So how should this be connected to a 600 ohm output impedance device under test?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 18, 2015, 11:24:52 pm
content: Quote from: valvedoctor on January 18, 2015, 10:27:12 pm600 ohms was a common input and output impedance half a century ago."Matching/terminating" impedances was a concept that was inherited from the telephone industry 100 years ago.Modern audio gear typically uses rather low (100 or even lower) impedance, and quite high (5-10K) "bridging" input impedances.Terminating transmission-line impedances were fiddly to use, and the technique was essentially abandoned in the solid-state era.QuoteYes it would appear appropriate to directly couple a 600 ohm source/output impedance to a 600 ohm terminating/input impedance.QuoteThe amplifier requires a 600 ohm load to operate at its design parameters. So it is appropriate to simply put a 600 ohm load resistor across the 600 ohm output. The input to the test equipment would be considered a high-impedance "bridging" load.

author: valvedoctor
author_rank: Contributor
timestamp: January 19, 2015, 12:58:46 am
content: So the exact value of the input impedance of the test equipment doesn't alter the instruments readings as long as it's significantly greater than the output impedance of the device under test?So to sum up, we should match the oscillators output impedance to the input impedance of the device under test by inserting a feed thru resistor, unless they are equal (600 ohms into 600 ohms)?And for the output impedance, we should provide the device under test's specified output impedance by placing a resistor across the output, then directly couple the signal into the various test equipment, as long as the input impedance of said test equipment is significantly higher?I think I might be getting it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 19, 2015, 01:20:08 am
content: To be precise, 604 ohms in parallel with 100,000 = 600.37 ohms Although a few ohms high or low won't make any significant difference.You can use this calculator to estimate parallel resistance values:http://www.1728.org/resistrs.htmIf the output impedance of your signal source equipment (oscillators or distortion analyzer, or whatever) is 600 ohms, then you should connect it DIRECTLY to the 600 ohm input of the Device Under Test (DUT). That is assuming that the DUT truly has a TERMINATING input impedance. Often, gear would have high-impedance ("bridging") input impedance so that you could connect a signal to several destinations and then terminate the connection at a 600 ohm resistor at the end of the line.Dunno what a "feed thru resistor" is? Do you perhaps mean an impedance-matching pad? But a moot point in any case if the source impedance is 600 ohms and the terminating/destination/input impedance is also 600 ohms.

author: valvedoctor
author_rank: Contributor
timestamp: January 19, 2015, 01:35:34 am
content: Yes, the device under test uses a 600 ohm to 600 ohm input transformer for isolation. The output would also go to a 600 ohm load.Check out this schematic for instance:http://www.geocities.jp/dave_group_japan/eqp-1a.gif