The methodology employed by the 930A Communications Test Set to measure Return Loss on 2-wire circuits. Return Loss is a test that measures the accuracy of an external AC termination.
The formula used is:
Equation 1 : Return Loss = 20*LOG ((Zref + Zx)/(Zref - Zx))
When in any 2-wire trunk type, the 930A measures Return Loss by using a Return Loss Bridge. Figure 1, below, provides a simplified circuit diagram of the bridge used by the 930A.
930A Return Loss Bridge Configuration for 2-wire Return Loss Measurements
* L1 and *L2 are connected only on 2-wire Loop & Ground Start Trunk Types.
The bridge converts Equation 1, which is expressed in impedance, to an equation expressed as the LOG of a voltage ratio (see Equation 2, below).
Equation 2: Return Loss = 20*LOG ((0.5 * Vexc)/(Vbridge))
Where: Vexc is the bridge excitation voltage, and Vbridge is the voltage measured across the bridge.
The range of Return Loss Measurement is -60 dB to +60 dB.
When the receive signal equals the send signal, the Return Loss value is 0 dB. This should be the reading when the signal is transmitted into an AC open or short termination.
In 2-wire Return Loss, the AC pick-off circuit is connected to measure across the Return Loss Bridge. For accuracy, no filtering other than 60 Hz and broadcast RFI filters are used.
When in 2-wire Loop or Ground Start modes, the hold/feed coil L1 is in place across the Tip (T) and Ring (R) leads, and is compensated for on the reference arm of the bridge (L2).
The Model 930A Communications Test Set supports 2-wire Return Loss Measurements for 600 and 900 ohm trunk terminating impedances. Units equipped with Purchased Option 40 (Wideband) extend 2-wire Return Loss Measurements to 135 and 1200 ohm terminating impedances.
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