sound technology 1700 series cables & IMD - Page 1
Forum section: electronics / testgear

author: Gmanfro
author_rank: Contributor
timestamp: June 02, 2016, 12:32:07 am
content: I have a ST 1700b with options 003 ( automatic set level) and 004 (IMD Analyzer). I've had it for about 5 years and now finally starting to use it as I am starting to rebuild older 70's and 80's power amps. It is functioning properly from the tests per manual. The manual was very confusing at first, but hope I have it figured out now. My questions are as follows which I can not find and answer too,1-I know to use twisted pair/sheilded cable for the input to the analyzer from the DUT, but what type of cables do I use for the outputs from the ST1700B to DUT? coax rg58 50ohm, rg59 75ohm, standard rca cables, or some other type of cable?2-The 2 outputs from the ST1700B to the oscope are bnc connectors with rg59 right now, not sure if that is correct. Question is should I be using rg58 50 ohm cables with 50 ohm min loss pad? I have an analog Iwatsu SS5702 scope and soon to buy a Rigol DS1054Z.3-I can not find out how to use the IMD. Do not see anything in the manual on it and my internet search has not turned up anything. Can anyone lead me to a place where I can read on how to use the IMD or instruct me on how to use it? I think I have the other distortion test figured out and am understanding how to use the ST1700B the more I use it.Any help or direction to threads where I can read up on these questions would be much appreciated.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 03, 2016, 07:20:11 am
content: (1)It is quite hard to see the output connector layout on the various images on Google,but most audio sources used in distortion analysis (at least in Broadcasting) are balanced,& normally use twisted pair cables.The output impedance of the audio generator was,historically,600 Ohms,but more modern practice is for the generator output impedance to be 40 Ohms or even less.If your device generator does have an unbalanced output capability,you can use shielded twisted pair audio cable with one side earthed,or coax if you like.Cable impedance matching is not a big deal at audio frequencies.(2)As above,it is fairly immaterial if you use 75 or 50 Ohm coax for the outputs to the 'scope.(3) To measure IMD it is normal to input two fixed tones to the DUT,so you may be able to find out the control function which does this.

author: Gmanfro
author_rank: Contributor
timestamp: June 03, 2016, 10:54:45 am
content: Thanks vk6zgo,That clears up a lot of questions I had especially if it would contribute to giving me an incorrect reading. The analyzer was moded by QSC Audio products with an unbalanced output before I bought it and duly noted in the manual on what they did. I will note the 600 ohm output in the manual so as not to forget. I do have the shielded cable hooked up as you mentioned and am using the unbalanced output for amp tests.I am still a bit lost on the IMD, but you lead me in the direction I need to know to figure this out. I will do some more reading and testing and will ask more questions if I stumble.

author: commongrounder
author_rank: Frequent Contributor
timestamp: June 05, 2016, 07:37:15 pm
content: As pertains to InterModulaton Distortion measurement. As vk6zgo said, this measurement is made using a generated two-tone source. There are several standards that define which two frequencies and relative output ratios to use. Look up SMPTE IMD for one standard (i.e. 60hz and 7khz at a 4-to-1 ratio). There are also several european standards (like 11/12khz or 19/20khz). The service manuals for the DUT usually defined what standard was used to set the spec for the unit. Your 1700b has a second low distortion oscillator in addition to the main oscillator to allow the creation of this two-tone signal. The analyzer section will automatically null the two tones and the residual is your IMD measurement (more or less). I don't own a 1700b myself (mine is a Tektronix AA501 dual-SG505 setup) but the principle is the same.Best of luck!