bandwidth

[mstechca]

I just found out something interesting.

I played with my superregen, mainly with the base bias resistor and the pull-down capacitor that connects to it.

I find that when R is between 82K and 140K, and C is 150pF, I am able to pick up my local TV station fairly well with an antenna :wink: , even though reception in my room sucks :cry: .

and when I was calculating the approximate frequency using R and C, using 1/(R*C), one of the values was actually 75Khz, which is the bandwidth of the TV audio signal (according to what one of you have said).

so I think my circuit and my calculator answered my question.

 

[audioguru]

The squegging frequency of your super-regen doesn't matter much because its audio frequency response is poor and it can't hear beat frequencies between the transmitted stereo subcarrier, transmitted SCA or Second Audio Program and its squegging oscillator. Since it oscillates or nearly oscillates, its bandwidth is too narrow for proper reception of 75kHz FM radio and TV and its bandwidth isn't used to receive FM anyway since its AM detector slope-detects on the side of the carrier.

Instead of playing with the squegging frequency, you should concentrate on making the bias of the transistor at the operating point where it is just barely oscillating, for its highest sensitivity. As all the other super-regen artilcles show, it is best to do the squegging frequency setting at the transistor's emitter, and do the sensitivity adjustment at the base.

 

[mstechca]

Until I saw your answer, I took the first stage of my amplifier circuit and changed the collector resistor (in the common-emitter amplifier) to 120K from 18K. I noticed alot more gain, and the quality seemed somewhat better. But after a minute or two, the quality begins distorting (must be because of the coupling capacitor. So I added a 62K resistor from collector to ground, and everything seems OK.

Now the station is in pretty clear, but I still get background hum in the speaker, no matter what decoupling capacitor I use across te battery.

I also tried adding a 10K resistor from the collector of my detector (shown way above) to ground, and the selectivity has improved dramatically.

HOWEVER, I still want to somehow or another get C.T.V. in (air channel 9). But it is 60km away from me, while CH (air channel 11) is less than 10.

For some odd reason, picking the perfect resistor between base and ground allows me to partially receive C.T.V.
I do realize that the resistor between base and ground is the resistor that controls sensitivity, but if I make it too low, I won't receive anything.

Is there anyway to determine the optimal resistor value?

 

[audioguru]

Now the station is in pretty clear, but I still get background hum in the speaker, no matter what decoupling capacitor I use across te battery.

The picture's carrier is pretty strong and is AM modulated with a fully-modulated 60Hz sync pulse that is 4.5MHz away from the sound carrier. Since your super-regen doesn't have the many tuned circuits of a TV for excellent selectivity, it picks-up both the sound and picture carriers.

For some odd reason, picking the perfect resistor between base and ground allows me to partially receive C.T.V.
I do realize that the resistor between base and ground is the resistor that controls sensitivity, but if I make it too low, I won't receive anything.

Is there anyway to determine the optimal resistor value?

Every transistor has a different amount of gain. Each will need to be adjusted separately since your circuit doesn't have any negative feedback to make things the same.

 

[audioguru]

Channel 9 is CFTO in Toronto and has 316kW of power. You should be able to pick it up pretty well straight across the lake.

 

[mstechca]

Here's the circuit I have been playing with. The output port is connected to Harry's (from harry's homebrew homepages) power amplifier.

The components in yellow make one filter, and the components highlighted in green make another one. I change the values of the capacitor in each one so the frequency can change, and the filter won't cut the signal off.

I am still playing with the items highlighted in cyan (light green-blue), because it seems to adjust the bandwidth the receiver can tolerate. When I have the capacitor set to 150pF, I get an oscillation on some frequencies, but the local station comes in somewhat clearer. At 300pF, I don't get an oscillation.

The components in red (except for the capacitor) appear to change gain for some odd reason. When I changed the resistor 10K to 120K, and used the 62K pull-down resistor, volume went up, gain went up, and so did distortion.

But the capacitor in red was the most noticeable. As soon as I made the value above 2uF, The frequency is chopped (in other words the unit turns on and off at a set rate).

So it makes me think that I have to play with the red components or even the 470K bias resistor.

Are there any equations out there I could use?

I hear that the higher the coupling capacitor, the better the signal can go through. Not in my case.