help me remove interference

Thread Starter

mik3ca

Joined Feb 11, 2007
189
so I was experimenting with my homemade superregen radio yesterday, and it was acting funny.

What I ultimately want to do is run my radio using a wall adapter as a power supply, and connect the output directly into my laptop.

I am only partly successful.

When my laptop is running on batteries, it generally emits a very high pitched sound (that I can hardly hear), but when my radio is connected to my laptop through the microphone socket, I hear the laptop sounds quite well, AND it even overrides the faint stations. In all tests, the laptop is at least 12 inches away from the radio.

another thing I find crazy in another test, is this:

My antenna was about 1 meter long of thin wire connected to a thick antenna stick (about 30 cm long), and that stick was on my bedroom floor. the radio was on one end of my bed. The laptop was at the other end. The AC adapters of both devices were connected to different wall sockets, and no cables were touching each other. Remote radio reception was O.K., but I think there is still some interference on my end that can be removed.

then, I extended the length of my antenna to about 3 meters. I hooked it up to a non-conducting screw on my room light, but the radio itself was in the same location. Reception was somewhat worse, and there was more noise.

In both cases, the antenna was pointing in the same direction.

In another test, I had the antenna pointing in a different direction (away from all sources of noise, and farthest point away from the local station tower). this time, I get no remote station signal.

so I think the problem narrows down to the wire connecting the radio to the laptop.

the output of my receiver is connected to an audio output through a 1500uF coupling capacitor.

I experimented with capacitors and 1500uF is the best for volume.

Please help.
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
a source claims that it is my speaker cable (that connects the radio to my laptop) that is causing the problem, but here's the kicker.

if I make my cable ridiculously short, I would have a problem because my radio is connected directly next to my laptop. If I make it long, I can move it farther away from my laptop.

what do I do here? make it really long, or really short? and why?
 

thingmaker3

Joined May 16, 2005
5,084
then, I extended the length of my antenna to about 3 meters. I hooked it up to a non-conducting screw on my room light, but the radio itself was in the same location. Reception was somewhat worse, and there was more noise.
You were picking up noise from your room lamp.
In another test, I had the antenna pointing in a different direction (away from all sources of noise, and farthest point away from the local station tower). this time, I get no remote station signal.
Antenna is directional, and you were not aimed at sources?


Speaker cables don't pick up much RF noise. If you are concerned about it anyway, use shielded twisted pair for the speakers and ground one end of the shield.
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
Speaker cables don't pick up much RF noise. If you are concerned about it anyway, use shielded twisted pair for the speakers and ground one end of the shield.
but I read somewhere that speaker cables can act as antennas that can cause interference. I want to eliminate it.
 

thingmaker3

Joined May 16, 2005
5,084
The shield will indeed eliminate it, if you ground one end of the shield. The twists help cut it down all by themselves even without the shielding.
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
I was curious, since I live across the street from a hydro-tower, (electricity tower), I was wondering how far away I have to be from it in order to pick up the station. It is blocking the path between me and the station.
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
Research now tells me that it is the power lines across the street that may be causing the problem. now it seems that I need an excellent filter.

Is there anything better than a PI filter that I can make without using complex calculations?
 

Audioguru

Joined Dec 20, 2007
11,251
People use radios, televisions, car radios and cell phones with electricity towers all around and don't have interference nor weak signals, because their circuits have enough tuned circuits for good selectivity to reject interference and have enough amplifier stages to be very sensitive. They also have automatic-gain-control so that strong signals don't overload them.

Doesn't your super-regen radio have a single tuned circuit and a single transistor?
What frequency are you trying to receive?
Is it AM or FM?
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
...have enough tuned circuits for good selectivity to reject interference and have enough amplifier stages to be very sensitive.
then maybe i should add more tuned circuits between the power supply and the radio.

Doesn't your super-regen radio have a single tuned circuit and a single transistor?
Only for the detector portion. Then it gets amplified by a push-pull 3-transistor amplifier.

What frequency are you trying to receive?
103.5 Mhz in Toronto


Is it AM or FM?
FM.
 

Audioguru

Joined Dec 20, 2007
11,251
then maybe i should add more tuned circuits between the power supply and the radio.
No.
Tuned circuits should be at the inputs and outputs of RF amplifier transistors.
A cheap ordinary FM radio tuner has a tuned LC circuit at the antenna that feeds an RF amplifier transistor that has automatic-gain-control. The output of the RF amplifier transistor is also tuned. In a super-heterodyne radio, there is a high frequency oscillator that is also tuned and this oscillator's output and the amplified RF are mixed together in a mixer transistor stage to produce a lower IF frequency. The IF (Intermediate Frequency) is tuned with a few LC circuits and/or crystal filter circuits and amplified with a very high gain multi-stage amplifier. Then the IF amplifier feeds the AGC for the RF amplifier and feeds a demodulator.

A good FM radio circuit does not pickup interference.

103.5 Mhz in Toronto ... FM.
Wikipedia lists Z103.5 with a studio in Toronto. The transmitter power is 30,700W (it was 100kW then 50kW and is now a little less) from their transmitter in Orangeville, Ontario. Their antenna is slightly higher than the CN Tower in Toronto and the station is heard all over southern Ontario. Most FM radios in your city will receive it perfectly. The $1.00 scanning FM "radio" from The Dollar Store might not.

Your super-regen circuit has an AM demodulator. It might receive an FM station if it is tuned to one side and uses slope detection. An FM radio is tuned to the center of the radio signal for good selectivity and good sensitivity.
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
Tuned circuits...
oops, I forgot you were talking about tuned circuits.

A cheap ordinary FM radio tuner has a tuned LC circuit at the antenna that feeds an RF amplifier transistor that has automatic-gain-control.
Mine does the same, but there really isn't any AGC involved.

The output of the RF amplifier transistor is also tuned.
Mine is set at a fixed frequency, since my receiver is a superregen.

...and amplified with a very high gain multi-stage amplifier.
mine is amplified with a transistor push-pull amplifier.

Then the IF amplifier feeds the AGC for the RF amplifier and feeds a demodulator.
I won't enter AGC until I can tune into the remote station.

It might receive an FM station if it is tuned to one side and uses slope detection.
My radio receives many FM stations. I just have a problem with Z103.5
 

Audioguru

Joined Dec 20, 2007
11,251
An FM radio has a very high gain radio-frequency amplifier before the signal is demodulated, not a high gain audio amplifier. Then it is very sensitive. AGC allows it to avoid being overloaded by strong local signals.

It has many tuned circuits for good selectivity. Its IF amplifier provides most of its selectivity and sensitivity. Your radio doesn't have an IF amplifier.

A weak FM station has the same volume as a strong local station because their frequency deviation is the same.
 

Thread Starter

mik3ca

Joined Feb 11, 2007
189
An FM radio has a very high gain radio-frequency amplifier before the signal is demodulated...
My first amplifier uses a PN3563 transistor which is meant for RF applications.
This helps demodulate the signal.

, not a high gain audio amplifier.
my high gain amplifier follows my RF amplifier.

Then it is very sensitive.
this is the part I need to work on. High-end sensitivity.

AGC allows it to avoid being overloaded by strong local signals.
I managed to make it so that there is no signal between 102.9 and 104.5, but that isn't what I want. I can't see AGC helping me yet.

It has many tuned circuits for good selectivity.
I don't want to have to play with multiple capacitors.

Its IF amplifier provides most of its selectivity and sensitivity. Your radio doesn't have an IF amplifier.
If there is a simple version of this IF amplifier that I can tie in, then I could probably bind it in.
 

Audioguru

Joined Dec 20, 2007
11,251
An IF amplifier has a few fixed tuned circuits that are tuned to a single frequency. The oscillator's capacitor and the capacitors at the input tuned circuit and at the output of the RF amplifier are tuned to change stations.

Then the radio is a super-heterodyne design, not a super-regenerative design.
 
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