This works good with an older Omnitron MFD Weight Chek. It puts out a strong signal I can hear with a VLF receiver but unlike the cheap ebay crap it's clean. I was never able to measure the frequencies with a frequency counter. i was hoping to do a hack job on it and put in a variable channel. I took it apart and hooked up the counter to the output and I got nothing. I have an app for an audio spectrum analyzer so I set it near the headphones and was able to see the frequency. So the secret is the listed frequencies are only nominal.
www.imagelocators.com