QRP Labs
Spectrum analyser input attenuator PDF Print E-mail
Written by Hans Summers   
Wednesday, 16 September 2009 22:23

Spectrum Analyser theory
Construction Techniques
Power Supply
Sweep Generator
Logarithmic Amplifier
145 MHz IF Filter
1st Mixer
2nd Mixer
8 MHz IF Filter
Low-pass Filter
Input Attenuator
10MHz Crystal Calibrator
Alignment and Operation

The simple man's Spectrum Analyser

Input Attenuator

The input module consists of an input amplifier and step attenuator. The purpose of the input amplifier is to allow the choice of high impedance or 50-ohm input. Both can be very useful at times. Note the additional box stuck to the back of the input module. It contains the 10MHz Crystal Calibrator. A piece of coaxial cable passes through holes in the module walls.

A Maxim MAX4178 unity gain 300MHz buffer amplifier is used as in the 145MHz IF Filter module. A double-pole-double-throw (DPDT) switch bypasses the amplifier in the 50-ohm input or passes the input signal through the amplifier in the high impedance mode.

The input attenuator allows accurate measurements of the relative strengths of spectrum peaks. When strong input signals cause too many responses in the analyser and obscure observations, the input attenuator can also be used to eliminate the weaker frequency components. It consists of 6 T-network sections designed for 50-ohm input and output impedances and attenuations of 1, 2, 4, 8, 16 and 32dB. Each attenuation section may be bypassed by a DPDT switch, such that in combination any attenuation from 0 to 63dB in 1dB increments may be selected by switching in appropriate sections. For example, 26dB attenuation is obtained by selecting 16, 8 and 2 dB switches.

The circuit diagram of the input module is shown below.

The theoretical resistor values required to get the desired attenuation with 50-ohm input/output impedances are not obtainable. To solve this problem, practical-value resistors are paralleled to get the required theoretical values. The necessary parallel resistances were calculated using a spreadsheet. They are indicated on the diagram as a single resistor with multiple value labels. For example in the 4dB section the vertical resistor is made from a 120-ohm, 1K and 4.7K resistor in parallel. The calculated values are such that very precise dB attenuations are obtained.

The DPDT switches have 2 fixing nuts. They are bolted into the screened box (constructed from PCB stock as usual) using one of the nuts, and the whole assemply fixed into the front panel with the other nut. The BNC RF-input socket bolts through the front panel, a washer taking up the nut-thickness between the front panel and the input module box. (In fact, the washer is no more than another piece of inch-square PCB stock with a hole drilled to take the BNC socket thread). In this way, coaxial integrity of the signal path in the analyser is preserved right the way from the input socket to the output.

Last Updated on Tuesday, 19 April 2016 06:54
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