Why can’t I use the 2-cord test method?

Why is this method preferred?

Because the 1-cord method provides the lowest uncertainty measurement and measures both connectors and the fiber of the cabling under test.

But since you’re making measurements with two cords, wouldn’t it make more sense to set the reference with two cords as well? Sometimes, to understand a measurement with a light source and power meter it is easier to explain it by using Watts or dBm (actual power) instead of dB (a relative measurement).

Let’s examine the two methods but first let’s assign some values and make some assumptions that will be common to both methods:

• • Testing single-mode fiber
• • RGT = reference grade termination
• • SGT = standard grade termination
• • Reference to reference grade termination = 0.2 dB
• • Reference to standard grade termination = 0.5 dB

Let’s first figure out the actual loss of the link.  We just sum up the loss of each component and the result is 1.50 dB.

For the 1-cord method, assume the light source is emitting -10.00 dBm as measured with a large area detector on the power meter that introduces no loss. After connecting to the cabling under test as shown, the -10.00 dBm sees a 0.5 dB loss at the first connector, 0.5 dB loss for the fiber under test, and another 0.5 dB loss for the second connector. The power meter will measure -11.50 dB (or 1.50 dB of loss).

Now, for the 2-cord method, -10.00 dBm is still emitted at the far end of the launch cord but when connected to the receive cord, the power meter measures -10.20 dBm instead of -10.00 dBm. After connecting to the cabling under test as shown, the power meter will measure the same as the 1-cord method: -11.50 dBm. However, the reference is -10.20 dBm, so the calculated loss will be -11.50 dBm – 10.20 dBm = -1.3 dBm, which is optimistic, as the link’s loss is clearly 1.50 dB. But, that is not what you want – you want a true loss.