In a message dated 96-05-03, Duane Marcroft writes: >I'm designing a V.34 on a PCMCIA card for Switzerland. Does anyone have a >source of Metering-Tone or Billing-Tone inductors that are less than 2.5 >mm in height (max height above PCB) and will sustain up to 60 mA of line >current before saturating?
Duane: You will have to watch out for a couple of things in designing a billing tone filter for Switzerland. First, the worst-case feeding condition for the Swiss billing tone test is 50 VDC through 500 ohms. Thus, the maximum loop current is likely to be in the range of 80 mA or so, depending on your DC V-I characteristic. A series inductor must not saturate under these conditions. Second, the Swiss billing tone test is not an immunity test, per se. Rather, they specify the "insertion loss" that parallel connection of your equipment creates for 12 KHz billing tones (see section 4.1.2 of NET 4). For voice equipment, the requirement is 2 dB maximum, and for data-only equipment, the requirement is 4 dB maximum. In essence, they are specifying the AC impedance that the TE presents to 12 KHz billing tones. To meet the 2 dB requirement, you will have to present about 1200 ohms. To meet the 4 dB requirement, you will have to present about 600 ohms. This places some restrictions on the value and Q of the inductor you choose. To meet this requirement, I use a parallel L-C placed in series with either the A or B wire. I have had good success with an axial leaded 3050 uH inductor from Fastron, part number CC/30-302A-01. Philips and Seimens second source this part. The resulting "insertion loss" per the Swiss test is about 1 dB. Unfortunately, the diameter of this part is about 5 mm. Perhaps you can find a smaller, axial part and use two of them. Cutting a hole in the circuit board under the part would allow some more room, too. Please let me know what you come up with. Sooner or later, I expect to face the same problem that you now have with the PCMCIA form factor. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
