How these circuits work? (AM transmitter and receiver)

demodulation of am transmitter

Hi, All,
I am testing these 2 circuits for 125khz.

**broken link removed**

**broken link removed**

Basically I patched two boards according to the provided schematics, one transmitter and one receiver. The variable capacitors for both boards are set at around 10nF so that the resonant frequency is around 120khz. (not exact)
I connected a function generator to the input of the transmitter and vary the square wave frequency pumping into the input. A osilloscope is used to monitor the waveform of the receiver output. Two boards are placed around 10cm apart.

I found that only when input square wave is at 120khz, I could see a similar sinewave on the scope. At other frequencies, the waveform distorted.

Is it an expected result for these 2 circuits? I am expecting them to work for other frequencies.

Please advise.

Wanchope

working am transmitter

What sort of distortion are you seeing?

transmitter 125khz

Hi, throwaway18,
I measured the output of the transmitter (at the antenna feed) and the output of the receiver respectively. With the input square wave at the resonant frequency, I saw two nice sinewaves at both terminals. When the input frequency starts to vary, the output of the transmitter changed into a few orders of sinewave harmonics with the amplitude reduced. And the output of the receiver followed the o/p of the transmitter quite closely, and it seems never match with the input square wave.
Hope it will clarify a bit. I know that is hard to understand.

Wanchope

125khz receiver

The right-hand transistor in the oscillator circuit is supposed to be an oscillator. It is meant to make a
carrier signal by itself.

The transmitter input is a microphone. It is intended for audio frequency signals around 300Hz to 5KHz.

An AM radio link can only carry signals at much less than the carrier frequency.

An AM transmitter at 125KHz carrying a 3KHz audio tone will occupy 122-128KHz.

Use your scope to check the transmitter output with no input to the transmitter. Are you getting a carrier signal out of the transmitter?

The receiver ciruit is inteneded to only pick up signals around the frequency it is tuned to.

120 khz transmitter and receiver

the receiver is a regenerative type

am transmitter function generator

Hi, throwaway18,
When I disconnect the function gen from the transmitter, i could not locate a carrier signal output from the transmitter. Only when some input applied, I could see the modulated waveform out from the transmitter.
In the text mentioned in www.zen22142.zen.co.uk/Circuits/rf/amrec.html It says the last transistor will perform the demodulation as well as amplification. But I found not much demodulation has been done by this transistor. The o/p waveform of the receiver is very similar to the modulated waveform out of the transmitter only the amplitude smaller.
So how should i demodulate the waveform? Using an envelop detector or a LPF will do?
Thanks.

Also attached the waveforms for input signal and output waveform.
(sorry for my poor hand drawing)

Wanchope

am transmitter oscilator

in fm to demodulate any carrier takes carrier perception

so ... if this technique isnt used

and a signal at a receive point method is used
to center your known singnaling frequency

you wont see the carrier

just the modulation hence your result

to see the carrier the reciever must be aware of it first ... carrier sync...

so you always sync from a dat known first

method structure to transmit data with handshakes

carrier = ok
rising edge = ok
lock = ok

receive+ = more signal at a given frequency
receive- = less signal at a given frequency
((+)+(-))= ~

null or the carrier

any !!! signal beat against 1hz will equal its self

or a mirror of the actual carrier

anyway the devices you use arent fast enough for data

you need to use FET's or better is bilateral mosfets

please refer to communication systems by simon haykin

i could not locate a carrier signal output from the transmitter

Click to expand...

The transistor oscillator in your transmitter is not working.
I'm not surprised, I'v found single transistor RF oscillators of this type are
usually reluctant to work.

The first thing to note is that using a power supply instead of a PP3 battery is
likely to result in it not working. "The 1nF capacitor C7, couples signals from the base to the top of L1, and C2". It's actually relying on there being an oscillation on the 9V supply. If you want to use a power supply you could try 500ohm in series with the input.

The gain of transistors varies from one transistor to another.
The bc109c datasheet says the current gain (Hfe) can be anywhere between 420 and
800 at 2mA collector current.

Check the voltage across R2. With the oscillator not oscillating it should be
around 3-6V, it will depend on the Hfe of the transistor, 4V is probably about right

The voltage between the base of Q2 and the ground connection should be 0.6-0.7v higher than the voltage across R2.

Since this circuit is intended for 500KHz to 1300KHz and you are using it at a lower frequency you may need to increase the value of c2 and c7. You could try making the values 3-5 times larger.

So how should i demodulate the waveform?

Click to expand...

Personally I'd go look for an old radio receiver that covers the longwave band
to start off.

in fm to demodulate any carrier takes carrier perception

Click to expand...

We are discussing an AM system.

you wont see the carrier

Click to expand...

Here should see a carrier on the output of his transmitter with his oscilloscope.

[qoute]
Basically I patched two boards according to the provided schematics, one transmitter and one receiver. The variable capacitors for both boards are set at around 10nF so that the resonant frequency is around 120khz. (not exact)
[/qoute]

If we assume that original TX oscillator did work reliably :
as schema is with common base based oscillators , you should change the variable capacitor to the value when its reactive capacitance will be equal to reactive capacitance of the L1 at the middle of old working frequency - 500kHz to 1600KHz ~ 1 MHz .
Increasing it much will decrease Q1 amplification in oscillator and oscillator will fail to start .
When you find the capacitor value adjust L1 accordingly for 125 kHz to have resonance at 125 kHz .

Then start to adjust (increase capacity) C2 to achieve oscillation with connected antenna.

Thanks, throwaway18 & artem,

I did use power supply for my testing. Therefore I tried putting a 470 ohm resistor in series with the power supply. But i could not find the carrier waveform at the output. Instead i found a 10Hz oscillating waveform. Then i checked using a 9 volt PP3 battey. The same happened, no carrier either. Voltages across R2 is measured at 3.8 volt.

When the function gen connected with 3khz square pulses, the output for the cases with and without 470 ohms are different. (as attached) Does the output waveform mean anything at this point of time? What waveform should I be expecting from the output of the transmitter? Is it like one of them inside my drawing?

And C2 and C7 also changed accordingly to 470pF and 4.7nF, not much difference anyway.

artem,
Please correct me if i am wrong. Did you mean i can just increase the inductor value and keep the capacitor at 500pf in order to oscillate the circuit?

Increasing it much will decrease Q1 amplification in oscillator and oscillator will fail to start .

Click to expand...

So with a larger inductor, there will not be any reduction in the Q1 amplication while the increasing capacitance will?

Thanks
Wanchope