CMF Dual Mode Choke Recommendation

Here is my issue:

  1. I am running a 12ga copper audio line pair for about 5000’ ( in middle of airfield) underground from the source to the load. Source and load impedance is the typical audio 600 ohms.
  2. The source produces a simple 1440hz audio tone at about 10 V/pk-pk on an o-scope at the load end. Current on the audio pair is very low, maybe 10ma if even that.
  3. Also underground (in middle of airfield) are some 120-240VAC power cables that operate airfield lighting. These AC power cables are inducing quite a bit of noise on the audio line pair.
  4. I have already looked into signal/safety/equipment grounding.
  5. I am looking for any CMF Dual Mode Choke part# or specs that I can install on both ends of the audio line pair to reduce the AC induced noise. Appreciate any help.

I’m assuming the noise you’re speaking of is 60 Hz hum? I’m not sure that a common-mode choke would do much for you in that case, but an isolation transformer should help if a ground loop is your problem. {Possibilities in 600:600 ohm are here. It isn’t the cheapest, but the HM3233-ND might be a good option simply from a standpoint of form factor convenience.

If the problem is other, more detail to help me understand the problem & symptoms would be helpful.

Thx for the quick reply Rick. As a matter of fact, I do have some 1:1 600:600 ohm iso xformers on order and will be installing them on both ends of the audio line pair in hopes to reduce the induced noise issue. I am not 100% sure if this is a gnd loop issue at this time. If I remove the 1440 hz tone, and place an o-scope on the load end with the load disconnected, there is hardly no noise on the scope. Once the airfield lights are turned on, there is a large amount of noise (trash) being displayed on the scope (still without the 1440 hz tone). I do not see any sort of 60 hz hum on the scope, just a ton of trash. Also, I forgot to mention, the source is just a 1440 hz tone generator, and the load is a 1440 hz decoder with 600 ohm balance audio xformers on both ends. With no 1440 tone being generated, there is so much AC noise being induced that sometimes the 1440 tone decoder will activate intermittingly. I just want to explore different avenues in case the 1:1 iso xformers don’t help. Thx.

you may need a custom designed narrow bandpass filter centered on 1440 Hz for such a noisy situation.

Do you have any clue what sort of spectral content that trash has? Lots of newer 'scopes have an FFT function, and understanding the character of the interfering signal is often helpful for diagnosing the situation.

How is this cable situated physically, relative to the AC power that’s the apparent source of the interference? Sharing a conduit? Run in a completely separate underground trench on the other side of the runway? Something else? Are these lights LED-based, or still an old incandescent setup? What sort of 'scope were you using and how was it connected–specifically, did it have any connection to the AC mains?

Putting an iso transformer on both ends would basically leave the length of the cable run floating relative to earth ground, allowing any capacitive coupling from a noise source to wave the cable around like a flag. High-frequency stuff can couple capacitively from primary to secondary of an audio iso xmer and end up as a signal on the receiver side, which wouldn’t help things. Is there any chance one of those transformers you ordered is center-tapped? If so, you might try earth grounding the center tap on the
cable side at the receive end, so as to provide a path to ground for any currents that are getting coupled onto the cable. I’d do this at one end only–not both.

1 Like

rick_1976, thanks for explanation.