AM Mediumwave 50mW reference oscillator with buffer
RE-TX1MW05
inside mw transmitter oscillator
By Guy, de ON6MU

About the 50mW MW AM oscillator

In this project, you will make a simple 2-stage low-power broadcast-type circuit, using a crystal oscillator integrated circuit and an a collector modulated AM oscillator with buffer/amplifier.
You'll see that you can receive the signal through the air with almost any AM MW radio receiver.
Remember that transmitting in the MW band is prohibited in most countries! This project is ONLY used for educational purposes and for use a an reference oscillator of 1MHz with a buffer transistor.

A wide range of different circuits have been used for AM, but one of the simplest circuits uses collector modulation applied using solid-state electronics as I applied here (Q1).

The oscillator is a build-in type with Xtal oven and gives extreme high precision and stability. HF-output of the oscillator is approx. 5 mW and the supplied voltage may not exceed 5 volts (in this case approx 3.3volts is set). To prevent low impedance overload of the oscillator we use a buffer (Q2). The signal is amplified by Q2(2N3904) and brings the power up to approx. 50mW with 100% modulation and a 50 Ohm load.
The cristal oscillator IC output is a square wave hence we need to get a smooth sinus. The output is guided via an L-filter and low-pass PII filter circuit cleaning up the signal pretty good and ensuring spectral sinusoidal purity (C7 C9 L1 C10).
You can leave out the buffer(amplifier) section all together if no higher output is needed and no low impedance load is applied.

The amplificated signal of Q2 is further cleaned up and adjusted for 50 Ohms by L2/L3,C11,C13,C14

The project can be fed with any voltage between 8 up to 15 volts, BUT, you need to set P2 so no more than 50mW (unmodulated is measured) which gives a total current around 40mA. Much more power would distort the modulated signal and probaly burn out T2.

 

AM

Amplitude Modulation (AM) is a process in which the amplitude of a radio frequency current is made to vary and modify by impressing an audio frequency current on it.
This was the first type of modulation used for communicating signals from one point to another and is still the simplest to understand.
A radio frequency current has a constant amplitude in absence of modulation and this constant amplitude RF carries no information, i.e. no audio intelligence and is of no use to radio telephone (voice communication), but has application in morse code communication.
In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Thus, most of the power output by an AM transmitter is effectively wasted: half the power is concentrated at the carrier frequency, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, only one of which is needed.

Filter
2 filters are used. One to get a clean sinusoidal signal and the second stage which is coupled to the load through an impedance matching circuitry (L2/L3,C11,C13,C14). Care is taken at this stage so that no harmonic frequency is generated which will cause interference in adjacent band (splatter) on other bands. This 3-element II-type narrow lowpass filter circuit for the desired frequency cleans out any remaining harmonic signals very efficiently. Remember: at this stage their are still harmonics that could be useful for testing. The second harmonic will be at 2Mhz, third 3Mhz etc.
To prevent any other harmonic and leave only the fundamental frequency, further filtering is required.


Modulator
Is done by Q1 (PNP BC328, 2N3906...). Audio information is impressed upon the carrier frequency at this stage.
Collector modulation is applied here. Modulation depth can be controled by changing P2 (be carefull not to exceed 50mW unmodulated).

Housing/shielding
The whole circuit needs to be mounted in an all-metal/aluminum case. If you're unable to obtain an all-metal case, then use a roll of self-sticking aluminum tape (available from your hardware store) or PVC box painted with graphite paint or any conductive paint.

MW oscillator in a box shielded with conductive paint inside

Specifications

Schematic AM reference oscillator with modulator and rf amp/buffer: fig1

schematic for a 1MHz AM modulated oscillator transmitter

Parts list 1MHz AM oscillator/transmitter

 



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