Cable to connect scope to microphones - Page 1
Forum section: electronics / beginners

author: ksat44
author_rank: Contributor
timestamp: September 18, 2023, 06:53:33 pm
content: Was hoping to get a recommendation on a proper cable type to use to connect my oscilloscope up to 2 microphones- one for each channel. The mics are piezo or possibly dynamic-type, I believe, and are able to generate a measurable voltage without the use of an external power source. Anyway, the cable needs to have the traditional female BNC on the scope end and a female RCA on the microphone end and the length needs to be around 6 feet each (if that's pertinent). I don't need pinpoint accuracy here, but I would just like to keep voltage losses to a minimum, since the mics don't generate a whole lot of voltage on their own to begin with.I notice there are 75ohm cables for sale that already have the BNC & RCA connectors I need for this setup, but am not sure if I'd be better off with 50ohm BNC and just solder on my own RCA instead.Any thoughts?

author: TimFox
author_rank: Super Contributor
timestamp: September 18, 2023, 07:40:54 pm
content: Either a 50 ohm or 75 ohm six-foot cable between a standard BNC plug and a standard RCA plug will work here.At these frequencies and length, the cable (if RG-58/U 50 ohm) will represent a 25 pF/ft or 150 pF total capacitance across the 1 M\$\Omega\$ and (probably) 15 pF input capacitance of the 'scope (plus the small capacitances of the two connectors).RG-59/U 75 ohm coax has less capacitance, but the miniature cables may have more.What is the microphone's impedance? There are no series losses in this circuit, but the total load capacitance forms a low-pass filter with the output impedance of the microphone.If a 200-ohm dynamic microphone, then the roll-off is -3 dB at over 4 MHz, but a piezo microphone has a capacitive output impedance, and the cable capacitance will load it (capacitive voltage divider).A dynamic microphone with an internal step-up transformer (for a high-Z input) might have an output impedance of, say, 50 k\$\Omega\$, and the roll-off would be -3 dB at around 17 kHz.

author: ksat44
author_rank: Contributor
timestamp: September 19, 2023, 01:30:00 am
content: Thank you for the very detailed reply. The microphones are mounted to alligator clamps are are meant to be used with a device called a ChassisEAR. The impedance information is, unfortunately, not disclosed by the manufacturer. I can try to take a resistance measurement of the mic, but I'm not sure if that would be helpful.Due to the small size of the clamps and the fact the ChassisEAR device is meant to allow the user to listen in on structural vibration vs. vibration in air, I would assume the mics are a contact/piezo-type.

author: TimFox
author_rank: Super Contributor
timestamp: September 19, 2023, 03:43:31 am
content: Unfortunately, the ohmmeter will not give an accurate measurement of a microphone's impedance.I would suggest you just try the cables discussed here--no damage can occur, and if it doesn't work you will need something more complicated with an amplifier near the microphone.

author: ksat44
author_rank: Contributor
timestamp: September 19, 2023, 03:45:57 am
content: Understood. Thank you, again.

author: liaifat85
author_rank: Regular Contributor
timestamp: September 19, 2023, 08:13:38 am
content: You may get some more suggestion from here:https://forum.allaboutcircuits.com/threads/wiring-a-microphone-to-use-in-an-oscilloscope-2.178515/

author: EPAIII
author_rank: Super Contributor
timestamp: September 19, 2023, 09:04:34 am
content: Since your microphones already have a cable, perhaps all you need is a RCA to BNC adapter.https://www.mouser.com/ProductDetail/AIM-Cambridge-Cinch-Connectivity-Solutions/25-7510?qs=u6EbdIq5mXlzSkIFFGtgDg%3D%3D

author: 807
author_rank: Frequent Contributor
timestamp: September 19, 2023, 09:47:37 am
content: Quote from: ksat44 on September 18, 2023, 06:53:33 pmIf you order a pre-made lead, you need to specify a BNC male on one end, not female.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 19, 2023, 09:59:05 am
content: Quote from: EPAIII on September 19, 2023, 09:04:34 amIn the very first post the OP clearly stated that a cable at least 6 feet long is required.

author: EPAIII
author_rank: Super Contributor
timestamp: September 20, 2023, 08:07:29 am
content: Well, his "high quality" microphones are already equipped with cables of unknown length and RCA connectors so why not just go with that and the RCA to BNC adapter I suggested. We're not talking rocket science here.There are thousands of inexpensive, RCA extension (male to female) cables available at reasonable prices and many of them are specifically made for microphone use. Just be sure to get the male-female version.https://www.amazon.com/s?k=RCA+extension+cable&ref=nb_sb_nossThat is just one quick search. I am sure Mouser also has such cables if he wants to keep it to a single supplier. Or get it all from Amazon. Just one result of a quick search.https://www.amazon.com/Adapter-Connector-Coaxial-Coupler-Selection/dp/B09TT3X8F6/ref=sr_1_1_sspa?crid=35GNV5IH3YWCA&keywords=rca+to+bnc+adapter&qid=1695196985&sprefix=RCA+to+BNC%2Caps%2C170&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1Quote from: wasedadoc on September 19, 2023, 09:59:05 am

author: wasedadoc
author_rank: Super Contributor
timestamp: September 20, 2023, 09:09:03 am
content: Quote from: EPAIII on September 20, 2023, 08:07:29 amThe OP already has the microphones and knows the length of their non-detachable cables. That is why the minimum length of the extension and conversion to cover the route between microphones and 'scope could be stipulated.