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Version:

May 28, 2019:
Revised: v1.0

An AM/FM/CW Scanning RF Signal Generator

This is a low cost readily constructed RF signal generator with a wealth of features which was published in Silicon Chip magazine in June and July 2019. 


Introduction

This signal generator was the result of several years of patient development. Designed around the use of a readily available low cost AD9850 DDS module and controlled by an Atmel ATmega328 processor, it provides a variety of features ideal for many applications.


Figure 1 : The signal generator alive and operating on my workshop bench

During the extensive development process, I built five prototypes, the last two being identical to the version described in the magazine. Of these, the last featured some minor improvements in the PCB layout and shielding. It was built to allow me to carry out some additional detailed testing. Based on all of the builds, I can confidently predict that it should cost less than $US50 to build. Many of my parts came from the usual Chinese providers via the usual websites. Those buying components and materials from local retailers may spend up to about twice this figure.

All of the details are available in the magazines and on the Silicon Chip website.
The June 2019 issue described the circuit and many of the features, and showed results of measurements made on the prototype using a spectrum analyser. The July 2019 issue described the construction of the unit.

Features and Specifications

The specifications were described on the Silicon Chip website in the sample page for the first of the two articles on the signal generator. These are summarised here:

ItemSpecificationComments
Coverage100kHz-50MHz, 70MHz-120MHz Usable up to 150MHz
Tuning Steps10Hz to 1MHz in decade incrementsUser-selected
Accuracy & stability Within 150Hz at 30MHz (typical), 0-40°C, 0-80% humidityCan be enhanced with software calibration
Output level -93dBm to +7dBm (approximate) 50 ohm termination
Attenuation steps 0-80dB in 20dB steps + 0-20dB (variable)
Output socketSMA
Spurious & harmonicsTypically better than -30dBcWithin specified coverage frequency range
AM30% modulation @ 1kHz
FMNB (12.5kHz spacing), 1.75kHz deviation @ 1kHz (60%)
WB (25kHz spacing), 3kHz deviation @ 1kHz (60%)
BC (12.5kHz spacing), 50kHz deviation @ 1kHz (60%) Suitable for standard broadcast FM receivers
ScanningProgrammable start and stop frequencies1kHz resolution
10, 20, 50, 100, 200 or 500 steps/sweepAuto step size calculation
Display16x2 alphanumeric LCD
Power control Soft on/off switch
ControlsTwo knobs and eight switches
Power supply 9-12VDC at 250mA
Dimensions160 x 110 x 25 mm (excluding knobs)
160 x 110 x 45 mm (including knobs)
Weight About 250g


Questions and Correspondence

All of the details are available from the Silicon Chip magazines and their website. Please direct any questions you may have about the design through Silicon Chip and they will be answered as promptly as possible. I do have a 'day job', and the great staff over at Silicon Chip have a magazine to run, so please be patient.

I hope you get as much pleasure from building and using this signal generator as I do. I use it regularly on my test bench for a wide variety of tests and measurements.


Downloads:

Please refer to the Silicon Chip magazine website at www.siliconchip.com.au





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