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Project Notes

#484 Crystal-Locked 455 kHz AM Oscillator

A crystal-locked 455 kHz carrier oscillator with fixed-frequency audio tone amplitude modulation (AM) for IF alignment.

Build

Here’s a quick demo..

clip

Notes

Transformers in AM radios are generally aligned to an intermediate frequency of 455kHz, meaning that band-pass filters attempt to isolate the intermediate frequency carrier before extracting the signal.

A test signal at 455kHz modulated by an audio tone is therefore a useful bit of test equipment for audibly checking the alignment of the filters.

This project is a build of a very common modulated 455kHz design, and it works like a charm! One of the many sources for the design is a Silicon Chip article from Jan 2008 - The Minispot 455kHz Modulated Oscillator.

Circuit Design

This is a simple circuit in two basic parts:

  • a 455 kHz crystal-locked oscillator providing the carrier
  • an RC astable multivibrator running at an audible frequency that is used to modulate the carrier

The circuit works across a wide voltage range (from about 3V and up).

Schematic

The frequency of the audio signal is determined by the values of R1, R2, C1 and C2 (making R1==R2, and C1==C2 maintains ~50% duty cycle). With R1 = R2 = 33kHz, possible values of C1/C2 include:

C1, C2 Predicted Frequency Measured Frequency Note
47nF 465Hz 410Hz As used in the original and many derivative designs
22nF 994Hz 1.0kHz What I decided to use

The predicted frequency is calculated by:

f = 1/(ln(2) * ( R1 * C1 + R2 * C2))

See LEAP#049 for more on the RC oscillator design.

Breadboard Build

I initially tested the circuit on a breadboard with a 5V power supply.

CrystalLocked455kHzModulated_bb

CrystalLocked455kHzModulated_bb_build

The following traces capture the audio oscillator signal.

With 47nF capacitors / 410Hz:

signal_47nF

With 22nF capacitors / 1.0kHz:

signal_22nF

The resulting modulated output (captured with peak function to demonstrate the modulation):

signal_modulated

Ugly Tubular Build!

I have (lots) of old solder tubes that I’ve been saving for a rainy day. This seemed to be a good opportunity to try and put them to use.

First step was to build the “business end” ugly style in a for that would fit in the solder tube. One thing I got a little off - I’d make the probe end longer next time, in order to get deep inside the guts of a radio.

CrystalLocked455kHzModulated_build_business_end

One tube for the electronics, and two tubes to hold 4 x AAA (they just fit nicely)

All joined together, the thing is a bit of a beast, but it works just fine. The discoloration of the tube (and components inside) appears to be an unexpected CSI super-glue experiment! I used super glue to join the tubes, and the fumes appear to have attached themselves to any specs of dust or oil - no latent fingerprints tho!

Build

Credits and References

About LEAP#484 RadioOscillators
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.