27Mhz: AM RX/TX Experiment

@7/6 2012

transceiver is amazing project, i always wondering how does it work. I decide to make a simple transmitter and a receiver separately, so i can experimental the walkie talkie from ground up. and i also try to keep it when complete, put it to box, then a interesting toy for son was done.

philosophy of the experiment:

*exploit what i have, this means I must change transistor/coil/transformer to utilize the circuit from web.
*test, even a simplest test would dig into it.
*Do what i can/know to improve it.
*start from the simple things.

i have some experience to play with VXO/CXO, refer to CXO: experiment CXO for TX, so it's the time to get a transmitter work. What i want to do is:

*A 100mW ~ 500mW output power from transmitter, with Vcc=3.6V-4.2V
*a simplest super-regen receiver setup to test the transmitter.
*A crystal oscillator work in overtone mode, but not Pierce oscillator, it's different choice compare to toy car RC transmitter.
*AM signal @27.095Mhz, implement the AM in a separate stage
*A class C PA, this is a chance to play around RF amplifier.

PA: Experience of PA Design CXO: experiment CXO for TX

Let me show you the cute picture of them. the left is the finished transmitter. The right one is the simple receiver.

Fig.1 cute image for the transmitter and the receiver

The transmitter

Fig.2 The transmitter

The following note was writing after playing the transmitter/receiver few days.

Transmitter choice and the trade off

1. overtone mode is hard to shift frequency,

so VXO is given up. another choice is make the 27.095 crystal work in fundamental mode, then multiply the VXO up to 27mHz, but much more complexity.

2. To implement Amplitude Modulate,

few choice: the Modulation transformer is give up, i don't have it. use A LM386 as Collect supply is attractive, but i make an more simple way, the T3, a voltage follower. left the amplifier the audio work to be done in the future. This way is common on google. the good experiment make by Alan's Lab the AM Walkie-Talkie Experiments. differential amplifier modulator is attractive too.

3. The PA stage:

i don't have any transistor suitable for RF power amplifier, i want a cheap one. The SS8050 seems like be able to endure 1A current . and Ft is 100Mhz, the available Gain in top HF band is very poor, but so what, i going to use it. let's see what happens. [class C PA really not suitable for AM, use it here just because here need a PA , have fun with it @2014/6/27]

4. The output filter

The output filter C5/L4/C6 copy from a toy car transmitter which use Pierce oscillator. but the toy transmitter not use it as a filter, it be used as a tank for overtone mode. this is a big misunderstand to use it here, not suitable , whatever try it.

5. stage couple

stage couple is not take serous design. just couple together with roughly "match".

Transmitter adjustment

the most hard part is PA stage. because the output filter is actually a resonant tank instead of low-pass filter. the Class C PA stage is now a tuned PA stage. L3&L4 must be carefully adjust to achieve a perfect sine wave output. The big pitfall is the RFC, L3 is a dominate factor of output power, because the couple capacitor C8 is too small, L3,C8,C5 almost resonant @28Mhz too. this is strange arrangement i made, cause lack of understanding the circuit. more PA experiment recorded in another article:PA: Experience of PA Design.

modulation stage have another weakness, there is no audio amplifier, so the modulation depth is very limited, when it work with 4V supply, the modulation depth is acceptable with a 200 mVpp input. but with 12V supply, modulation is so weak. here is a modulated wave capture from my oscilloscope.

The simple receiver setup for testing

Fig.3 The receiver

It's super-regen type receiver, and be mass built for toy car receiver. this receiver is not be fine designed, it just work. the sensitive is also not very good.

( quick fix C5: 0.1uF, C1:1nF)

L1,C3 form the tank tuned to 27Mhz, Antenna is coupled through a tuneable inductor L4, which cancel off short whip antenna small capacitance. R1 with C4, C6 make a audio low pass filter, at same time has some gain. C9 couple the signal to Lm386.

This super re-gen receiver is lack of sensitivity, seems the re-gen does not boost the signal. anyway, this set-up is just for testing the transmitter.

Anyway it works

this is a really 'experimental' purpose junk project, but start from here, you have some sense with AM/PA etc.

In door testing

only send signal 5 meter away, seems the receiver is responsible for this bad performance.

outdoor testing

get very small volume of audio from 30 Meters away, cause bad receiver performance and low AM modulation.

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