#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.
Here’s a quick demo..
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.
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).
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.
I initially tested the circuit on a breadboard with a 5V power supply.
The following traces capture the audio oscillator signal.
With 47nF capacitors / 410Hz:
With 22nF capacitors / 1.0kHz:
The resulting modulated output (captured with peak function to demonstrate the modulation):
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.
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!