#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..

This shows alignment of the LEAP#462 HX108-2 AM kit.

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!