Custom Value Terminator Using a 75 Ohm Terminator Case - Page 1
Forum section: electronics / beginners

author: Smoky
author_rank: Frequent Contributor
timestamp: October 07, 2020, 05:20:13 am
content: I own an HP 35660A Dynamic Signal Analyzer that I plan to use to test vintage audio equipment that I fix. It states that the output impedance of the device under test needs to be matched at the input of the DSA.I have a handful of these 75 Ohm terminators that were used for video use. I also happen to have a pair of spanner wrenches from an old Rockwell woodworking router that fit the flats of the body of the terminator too, so I cracked one openInside of it is a CGW RN60D 75 Ohm resistor with one end crimped to the BNC center pin and the other is soldered to the screw-on cup that has a hole in it.One piece of audio equipment that I'm working on states in its manual that its output impedance is 150 Ohms.Don't tell me it's as simple as replacing this resistor with a 150 Ohm metal film resistor?I was thinking I would need to buy a project box but, maybe, not now

author: magic
author_rank: Super Contributor
timestamp: October 07, 2020, 05:41:44 am
content: For frequencies below 100MHz yes, it's that simple.But you should also use 150Ω coax then, if such animals even existWhich brings us to another point - are you sure you need impedance matching with short cables at frequencies below 100kHz? Is your audio device even rated to drive 150Ω load?The usual way to do audio interconnects is simply: relatively low source output impedance, reasonably low capacitance cable (to avoid HF rolloff due to RC) and high impedance input at the end (at least 10kΩ typically). Transmission line effects don't degrade the signal unless you are testing a kilometers long telephone line or something like that.

author: Smoky
author_rank: Frequent Contributor
timestamp: October 07, 2020, 05:56:01 am
content: Thank you MagicI'm working on a stereo equalizer and on the back of the manual is where I read what the output impedance is.I made 36" RG-400 BNC to RCA patch cables over the weekend for the analyzer.I really don't know what the heck I'm doing but I love doing it

author: magic
author_rank: Super Contributor
timestamp: October 07, 2020, 06:06:49 am
content: No, I'm not sure if an EQ will even drive 150Ω load properly. Set the analyzer inputs to high impedance (if there is any choice), connect the cables and have fun. Same way as if you were feeding its output to an amp.

author: radiolistener
author_rank: Super Contributor
timestamp: October 07, 2020, 06:25:15 am
content: the resistor needs to have low inductance. Also resistor needs to be precise.You can use SMD resistors, but they have too small max power.If you will use wires or resistor leads for soldering, max frequency will be very limited.You can expect about 10 MHz for SWR 1.0 or 100 MHz for SWR 1.5-2

author: radiolistener
author_rank: Super Contributor
timestamp: October 07, 2020, 06:30:08 am
content: Quote from: Smoky on October 07, 2020, 05:56:01 amfor audio frequencies it will be much easier, you can use usual resistors and it's leads and soldering doesn't matter for such frequency.

author: Smoky
author_rank: Frequent Contributor
timestamp: October 07, 2020, 06:53:40 am
content: Thank you RadioListenerI scrolled through the menus and found where the input impedance can be typed-in.Just for the sake of it, I typed 150 Ohms and it put it on the top of the screen. I also typed in 200 KOhms and it accepted that value too.I guess any value can be programmed into the analyzer?But this flies in the face of what the diagram shows of the cable connections to the device under test to and from the HP 35660A in the user's manual when performing a frequency response test so I thought that an external terminator was required at the input (last picture).

author: Smoky
author_rank: Frequent Contributor
timestamp: October 07, 2020, 07:00:47 am
content: What I also need to watch out for is the maximum voltage at the input of this analyzer. It reads +/- 4 volts peak max.I owned this analyzer for two years and I'm just trying to get familiar with it. I've asked around since then trying to find someone who has used this analyzer in this manner but I haven't tripped over anyone as yet.It was made in 1986