AN3250| Application Note
Maxim > App Notes > WIRELESS, RF, AND CABLE Keywords: mlp, min loss pad, minimum loss pad, 75, 75ohm, 75 ohm
May 27, 2004
APPLICATION NOTE 3250
Characterizing the S-Parameters of 75!& Circuits using 50!& Lab Equipment
Abstract: RF engineers working with cable, terrestrial, or satellite TV applications are frequently required to make S-parameter measurements. Using a minimum loss pad to transform the conventional 50 test port impedance to the 75 device provides a cheap, easy way to get reasonable measurements. For most general lab applications below 1GHz, a PCB-mounted minimum loss pad built from 1% 0402 or similar resistors offers a quick and easy means to test a 75 circuit with 50 lab equipment. In most cases, the only correction factor required is the insertion loss of the MLP 5.7dB plus any addition connectors. Difficult calculations or even Smith Chart work is often not required to make basic S-Parameter measurements. RF engineers working with cable, terrestrial, or satellite TV applications are frequently required to make SParameter measurements on these circuits. The first time the uninitiated engineer uses a Vector Network Analyzer to verify that a TV tuner input offers the return loss they expect, the problem be
comes obvious: How DUT on a 50 VNA? If the situation warrants the cost, the answer is do I measure [S]-parameters of my 75 to buy lab equipment designed specifically for measuring 75 circuits (75 source and load impedance test ports). Otherwise, using a minimum loss pad to transform the conventional 50 test port impedance to the 75 DUT provides a cheap, easy way to get reasonable measurements. When an IC manufacturer specifies the input return loss (|S11|) of new cable TV LNA, the measurement is necessarily referred to 75 . That is to say, if |S11| = -30dB (reflected power is only one part in a thousand essentially a perfect match), the idea is that when driven with a 75 source impedance, the device input will allow virtually all of the power to be transferred to the LNA. The same tuner input will NOT offer good return loss when driven from a 50 source impedance. Directly connecting this perfectly-matched tuner input to a 50 VNA will yield an |S11| measurement something close to -14dB - with reflected power
now one part in 25! So with this same 50 VNA, how can we verify that the TV tuner input is as good as we say it is? A matching circuit is required; it should have flat frequency response and the lowest insertion loss possible. The industry-standard answer to this is the "minimum loss pad" (often seen "MLP") -- the simple resistive network of Figure 1. The key feature of this network is that it transforms a 75 DUT load impedance into 50 for the source impedance of the instrument to the native 75 measurement instrument, and transforms the 50 impedance of the DUT. In this way, reflections are removed, the response is flat, and the loss of the network is easily backed out of the measurement to get to the DUT. These "minimum loss pads" are widely available from test equipment vendors, and the network can be constructed very quickly on the bench when needed. The term "minimum loss" refers to the fact that this network offers the lowest insertion loss of the possible configurations of resistive networks that offer
the same transformation.
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