Weird oscillation while on battery
I tried changing the 100uF capacitor at the input of the PSU (see near the switch) to 2200uF and now the oscilaltion happens at a bit greater audio levels.
I have also tried a 100uF near the AF amplifier chip as datasheet shows but no luck.
Any ideas why this is happening with batteries?
Note the TRX is not complete yet.
50,60,100,120 Hz are audio frequencies too, and if you measure these values then it is suspicious your measuring is not correct.
Be specific please where you measured the signal, how, how much is the frequency, amplitude.
If the batteries are 5 x 1.5V AA cells then I suspect that you are drawing too much current which is dropping the voltage which turns off the circuit which raises the voltage allowing the circuit to turn on and draw to much current....
Your capacitor across the battery is after the switch(so it will start off discharged) so it may be adding to the problem by trying to charge (along with powering the circuit) which will draw more current from the battery.
I've not looked up the current draw of any of your components but 'start up currents' can be quite high until the circuit settles down.
Just a wild guess but a trap I fell into as a young engineer.
Susan
As Susan says - you may have built a relaxation oscillator when running off batteries, try putting a 4700uF in parallel with the batt's and see if the osc stops - there's your answer ...
If you add another AA battery in series with existing ones, most probably the problem will disappear.
I suspect that is the problem. I have added a 2200uF accross the batteries instead of the 100uF shown in the schematic near the switch. Do you think it needs more? The situation gets better as oscillation now happens at higher volume level. However, when the volume setting gets too high, oscillation begins again. To stop this oscillation I have to turn the volume pot back to minimum. Then I can increase it again up to a point without oscillation occurring.
Today I noticed that with the 2200uF in place and by removing the 5V regulator, oscillation happens ar a rate of a few Hz/sec. The speaker really goes in and out at that rate. If I replace the 2200uF with a 100uF the oscillation frequency goes higher. I do not know if this is relevant but I point that here.
Vfone, I will try this and let you know. Although this will be a problem, since the device will be handheld and an extra battery adds to the weight and size.
I measure the non-loading current to be 300mA max at max volume. Most of this is consumed by the AF amplifier chip. The rest of the RX circuits draw a total of 70mA and most of it (~40mA) is drawn by the LO buffers.
So a total of 300mA won't be able to cause a voltage drop to the batteries I guess. I have measured the loaded voltage and it is not less than 5v, it is closely to the unloaded voltage of the batteries. These batteries have a 1.3v and more when charged and the circuit is designed to work with 5V and less.
So something else might be going on there and I wonder why this is not happening when connected to a PSU.
I am still suspicious your measuring method has an issue here. Maybe the ground connection of your PSU was good, and now your batteries are not grounded to your scope well. Still it is not clear to me where you measure this oscillation, how much is it, etc.
I do not measure the oscillation, I hear it on the speaker loudly.
I managed to remove the oscillation by using a 2200uF and by lowering down the 470k to the BJT preamplifier to 39k to lower down the AF gain. I am not satisfied by this solution as there is a very big electrolytic (in size as well) and the AF gain is lower. I still do not know what is happenning. Any ideas?
Disconnect separately each blocks from the PSU and connect to the battery if you can. Just an idea.