RF Trap With Attenuators

Description

  The RF Trap With Attenuators is intended for use in harmonic measurement. It is particularly useful when the available spectrum analyser's dynamic range is inadequate to directly measure the very small harmonics required in modern 'digital' communication transmitters. The ready built circuit attenuates the fundamental of the transmit frequency by more than 50 dB and the harmonics from 16 dB to about 22 dB (depending on the capacitor selected), thus reducing the required dynamic range of the spectrum analyser by approximately 30 db. Input and output attenuators are included to prevent the RF Trap With Attenuators from causing a serious mismatch with either the circuit under test or the spectrum analyser.

Measurement Technique

  The maximum input level to the ready built version RF Trap With Attenuators should not be greater than +22 dBm (your design may differ from this, and the following may have to be re-calculated). If the output of the circuit under test is greater than +22 dBm, a power attenuator should be connected to the input of the RF Trap With Attenuators that will ensure that the +22 dBm input level is not exceeded. The output level of the RF Trap With Attenuators should then be =< 6 dBm (16 dB to 22 dB insertion loss!). This will ensure that the spectrum analyser input is not overloaded. Attenuators should be selected within the spectrum analyser to give less than -10 dBm at the input to the spectrum analyser input mixer stage when making the measurement. The insertion loss off the combined attenuators is first measured at the required fundamental and harmonic frequencies using a known signal source. This is normally a signal generator that has been calibrated to a power meter. The circuit under test is now connected to the combined attenuators and the fundamental and harmonics measured. Either the combined attenuator insertion loss is used to calculate the actual level at the output of the circuit under test or the output is measured using the spectrum analyser offset facility.

Circuit and Simulation Components with typical values

Circuit Description

  The resistors R1, R2, R3 and R4 form a pi attenuator. The centre of this pi attenuator is grounded via an RF Trap, formed by the series l/c circuit (L1, C1). The l/c component values are selected for resonance at the fundamental frequency of the test signal.

  In the circuit diagram, the simulated input to the RF Trap With Attenuators is provided by V2 and R6. This represents a voltage source with an impedance of 50 Ohms at the COAXIAL INPUT. The COAXIAL OUTPUT is terminated by the 50 Ohm resistor R5 and monitored by a voltage level probe. The frequency is swept using the above circuit simulation and gives a frequency response as seen below. This simulation is an over simplification and ignores the effects of the PCB and component strays. 

  You can build the circuit in a FLEXI - BOX, using values selected with the Calculator For RF Trap With Attenuators. This Excel spreadsheet gives you the flexibility in selecting frequencies, power handling, response shape and attenuation. A suitable escutcheon can be printed from the escutcheon for RF Trap With Attenuators, and you can modify it as required.

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