2. Operation
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2.1 Theory of operation
TC-5062C Pneumatic 6 GHz TEM Cell is made to work well beyond the typical TEM Cell
operating frequency range limited by cell resonance. A typical TEM Cell is a 2-port
symmetrical device; RF voltage is applied to one port while the other port is terminated
at 50 ohm while maintaining 50 ohm characteristic impedance along the cell. Due to the
expansion and contraction parts of the cell, the wave propagation beyond a certain
frequency is no more propagated by TEM mode alone and creates resonance. To
eliminate the resonance problem, half of the cell is replaced by the wave absorbing
material. One commercial implementation is G-TEM Cell. The size of the G-TEM design
is too large for typical small device applications due to the type of absorber used.
Tescom borrowed the concept of G-TEM, but changed the termination implementation
scheme, and designed a very compact broad band TEM Cell that can be used on a
desktop.
The operation principle is essentially the same as with the TEM Cell. The E-H field
inside the test volume is proportional to the input voltage and inversely proportional to
the cell height. If a radiating object is inserted inside the cell, the radiated wave toward
the input port is guided by the transmission line and picked up at the input with a
receiver such as a spectrum analyzer. With this method, RFI from a radiating Device
can be measured quantitatively. Since this apparatus is very broadband, it has many
applications in the area of EMI, EMS, receiver sensitivity test, etc.
2.1.1 Standard E-field Generation (Ref. 1, Figure 3)
Consider a case when a 50 output impedance Signal generator puts out V (Volt) of RF
signal into the input connector of a TEM Cell. Then the E-field, E (Volt/meter), developed
inside the TEM cell is found by the following formula.
E (Volt/meter) = V (Volt) /
L (meter) ………………… (1).
Here, L is the distance between the septum and the top wall. In TC-5062C, L is 22 (cm).
Since TEM Cell produces a TEM wave, an orthogonal H-field (ampere/meter)
proportional to E-field exists. They are related by the free sp
H (Ampere/meter) = E (Volt/meter) / 377 (Ohm) ....... (2)
Field distribution inside the cell is shown in Figure 3.
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