Maker Pro
Maker Pro

Help understanding SNR of a radio reciever

Status
Not open for further replies.

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
So I recorded the output of my guitar with no strings being played and recorded this garbage which was roughly tuned into the medium wave band with the pickup selector switch and more finely tuned with both the volume pot on my guitar and the tone pot on my guitar.




Then I used Audacity to equalize the frequency response, attenuate frequencies that are not important for hearing human voices (e.g.: low pass at 500 Hz and high pass at 2000 Hz, with extra attention to attenuation below 100 Hz to suppress mains hum), then the removed garbage allowed more signal to be amplified without clipping, then this showed clipping which was removed with a hard limiter, and then amplify yet again.

This is all in Audacity. So the computer software is actually part of the radio receiver because proffesional audio gear to do all of these step is not cheap (and Audacity is free software).

Unfortunately Audacity does not have "monitor in real time" due to piracy concerns, so the tuning is a broad frequency sweep on the pots once the medium wave band is established with the pick up selector switch.

This is what it sounds like:

So is my explanation on my video description correct:
dietermoreno said:
I found something interesting when I started to care about SNR values of my shitty guitar recordings that has radio interference bleeding through.

I found that the same EQ, attenuation, and amplification used in Audacity can improve SNR of received demodulated radio signals.

SNR = ((max db level of signal ) - (max db level of noise) ) / ((max db level of noise))

SNR db = 10 log [ ((max db level of signal ) - (max db level of noise) ) / ((max db level of noise))] This equation only works when the SNR is less than 1:1.

In this video:

((-36 db) - (-48 db)) / (-48 db) = ( -36 db + 48 db )/ (-48 db) = 12 db / -48 db
= 0.28 S:N
10log [ 0.28]= 5.5db

Around 5 db SNR is common for AM radio regardless of the receiver used. The only difference with more complicated receiver is less interfering frequencies heard so the signal is more intelligable.

In my previous guitar cover recording videos:

((-2db) - (-48db)) / (-48 db) = +46db / -48 db = 0.95 S:N This is about 4 times greater SNR so about 20 db SNR.

This is standard for FM, but does not take into effect interfering signals.

For comparasion, CD is 97 db SNR which is 70,794 times bigger than the noise, compared to analog radio almost always has less than 1:1 relationship with signal : noise.
So this is why the forumla works for analog radio because must be less than 1:1 because log (0) is undefined and very close to log ( 0) the equation doesn't work.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
I really have no idea what your doing.

I guess you've proved that a guitar can't be picked up by a radio though.

So can we put that one to bed yet?
 

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
guitar feedback regenerative heterodyne receiver

deleted message
 
Last edited:

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
Oh you moved my post.

Here it is in its glory.



Guitar feedback regenerative heterodyne receiver.

Electro132 gave me an idea when he was ranting about sound making radio waves.

I learned in the Wikipedia article about the regenerative receiver that it uses feedback oscillating at the desired RF as an oscillator for tuning, and you tune it with a pot that controls the gain which thus controls the feedback.

I learned in the Wikipedia article about the regenerative heterodyne receiver that it is exactly a guitar amp over driven to feedback and the feedback whine is called a heterodyne tone and when the guitar amp is energized by a morse code RF signal it beeps on and off in morse code and the oscillation of the feedback is also a tuner in addition to generating the beeping.

So I thought I would try this at home. Here are the results.

I was able to tune to a repetitive morse signal (an aviation navigational beacon in the VHF band I think).

The signal is _ ._ _ _ _ _ _ . . _._ _ _ _ _ _ . . _._ _ _ _ _ _ . . _._ _ _ _ _ _ . . _._ _ _ _ _ _ . .

K O B K O B K O B K O B K OB


The repeated word is "OBK".

I look up aviation navigational beacons here at

http://vfrmap.com/?type=vfrc&lat=41.907&lon=-88.248&zoom=10

and I find that "OBK" is the name of a beacon in Northbrook, IL only about 8 miles away from my house. Its frequency is 113.0 MHZ AM.



Pretty cool?


This is my guitar strings pressed against my guitar speaker so the feedback occurs when the guitar amp is energized by a morse signal turning it on and off whining on and off.




Here is the video:


I suppose it would work better if I had an antenna.

Would would be a good place to connect an antenna to the guitar?

I saw that inside my guitar there is a wire that is stripped at the end that makes contact with the inside of the guitar. This is the ground wire, right?

You connect an antenna to the ground wire, right?

I tried holding some wire to that loose wire as an antenna and all that accomplished was "beeping" on and off in cadence with my body shaking while holding the wire and the beeping stopped as soon as I wound the two wires together instead of holding them together.


Would this feedback loop be better to accomplish with electronics rather than with acoustics?

A problem I had is I have to get the gain just right to tune to a station right before my amp starts feedbacking in a continuous loop, so often I mess up and I have to turn the gain down to start over because mechanical resonance builds in my guitar speaker.

Would using electronics instead of mechanical resonance achieve the same feedback loop and eliminate the high decay time of mechanical resonance?

I mean by high decay time I mean as soon as a few beeps happen then the speaker goes crazy with continuous feedback.


by using electronics I mean I would use an audio mixer as the detector, the mixer output is connected to the line in of the guitar amp, the headphone line out of the guitar amp is connected to a Y splitter, one part of the Y goes to headphones, and the other part of the Y goes to the mixer forming the feedback loop.

anten. --> ground wire of guitar --> guitar output jack --> detector-mixer --> amp --> line out

^ V
^ V
^ V
^
^ Y splitter
^ V
^ V
^ V
^ V V
^ V V
^ V V
<<<<<<<<<< V
V
headphones


You can see the correct formating of my diagram if you quote my post.


Or perhaps would it be more effective to leave the feedback loop to the mixer only and connect the guitar amp to where the headphones currently are in the diagram (after the guitar amp has been removed from its current position in the diagram)?


anten. --> ground wire of guitar --> guitar output jack --> detector-mixer --> >>>>

^ V
^ V
^ V
^
^ Y splitter
^ V
^ V
^ V
^ V V
^ V V
^ V V
<<<<<<<<<< V
V
audio amp

You can see the formating of the diagram correctly if you quote my post.
 

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
dietermoreno said:
You connect the ground wire to the antenna right?

Where do I connect an antenna wire then?



I really have no idea what your doing.

I guess you've proved that a guitar can't be picked up by a radio though.

So can we put that one to bed yet?

No we can't put it to bed yet.

Wait did you just say we've proved that a guitar can not pick up radio waves?

Then that would crush my dreams and lead me to think that those voices and morse code beepings are a sign of me losing my sanity.




So this is the ground wire I am talking aout in these images I found on Google.


wd2hh3l21_00.jpg




So after looking at his image I found on Google of a regenerative receiver, I see that that is not the place to connect the antenna, and the antenna should be connected to a piece of aluminum foil layed across the pickup contacts and the aluminum foil should be connected to the ground wire.

regenerative-detector-schematic-fig1.gif



So then after looking at that image of the regenerative receiver, I see that it can receive AM not just morse code when it is not over driven to oscillation whining.

So then for the best results a separate audio gain stage should indeed be used in order to recieve more than morse code (right now I can only hear morse code). The separate audio gain stage is needed to amplify the audio output of the regenerative detector to be loud enough so the regenerative detector gain does not have to be over driven to oscillation whining.



antenna wire
V
ground wire V
V V
V V -----------V------------V
V V V
Vground-----------V ------------------V-----V
V V V V
V foil V foil
V V V V
ground ----------pickups 1 pickups 2
V V V
V V V
V V V
V pickup selector switch (rough tuning)
V V
V V
V V
ground-------------log pot (fine tune 1)
V V
V V
V V
ground------------linear pot (fine tune 2)
V
V
V
guitar output jack ---> >>>>>>
V
V
RF signal
V
V

ground------------detector-mixer --> >>pot
mains-------^ ^ V
^ ---<-
feedback signal V
^ V
^ V
^
^ Y splitter
^ V
^ V
^ V
^ V V
^ V V
^ V aud sig
<<<<<<<<<< V
V
audio amp

You can see it correctly if you quote me.
 
Last edited:

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
Wait did you just say we've proved that a guitar can not pick up radio waves?

No, we've just shown you cant transmit from a guitar to a radio.

Anything can be a crappy antenna if your wiring is shoddy enough, even a guitar.

What you're getting is NOISE. It's something we try to reduce or even eliminate.

It's like throwing mud at a car and saying "Look how wonderful, my car gets dirty if I throw mud at it!"
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
So after looking at his image I found on Google of a regenerative receiver, I see that that is not the place to connect the antenna, and the antenna should be connected to a piece of aluminum foil layed across the pickup contacts and the aluminum foil should be connected to the ground wire.

After the first 26 words (in bold) the rest is all wrong.
 

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
Round 2.


In the last video for guitar regenerative heterodyne reciever I used the guitar amp as the regenerative detector and since my guitar amp doesn't have multiple line ins I had to use acoustic feedback loop of the guitar strings right next to the speaker cabinet, but in this video I achieved the feedback loop with electronics instead of acoustics for better results.

Also the last video had no antenna and the strings were subject to picking up background noise but in this video an antenna is added and the strings are muted with cloth towels since electronics instead of acoustics are used.

I made the feedback loop with one of those short 8 inch long cables used to connect guitar effect pedals together connected to an audio mixer.

The mixer line in 1 is connected to the guitar. The mixer line in 2 is connected to one part of a Y splitter. The audio line out to the audio amp is connected to the other part of the Y splitter. The input of the Y splitter is connected to the 8 inch long cable. The 8 inch long cable is connected to the line out of the mixer. Forming the feedback loop of the regenerative detector.

The antenna is coupled to the pickup contacts. Tuning is accomplished with the pickup selector switch, which can only tune to 2 settings. When both pickup sets are selected (more coils = lower frequency) it has too much mains hum, so I only use selecting one pickup set (less coils = higher frequency).

More tuning is accomplished with the channel 1 and channel 2 gain pots on the regenerative detector.

Gain on the regenerative detector below oscillating feedback squeel is AM and very close to oscillation is heterodyne CW morse code.

Currently mostly only mains hum is heard on AM mode so we are only interested in CW mode.

The SNR of CW mode is increased by decreasing the band width to the audio frequencies that CW uses. This is accomplished with the tone controls on the guitar amp. Eliminating bass (not used by CW) attenuates mains hum and attenuates booming radio voices that are unintelligeble due to mains hum and lack of frequency selectability.

The frequency selectability over the last video is improved. Now 2 CW stations are heard at the same time and each one can be attenuated by adjusting the gain of the regenerative detector.

The CW station heard in the last video is still the loudest station received in this video, so that means that it is the closest.

Adding a vari cap made out of toliet paper rolls, aluminum foil, and some wire could improve the frequency selectablity if the vari cap was coupled in series with the pickups and antenna.

There is no vari cap here, but I will try that in the next video.

The slightly higher heterodyne frequency sounding beeping (and quiter) is

_ _ _ . _ . _ . .

O R D

O'hare beacon.


The slightly lower heterodyne frequency sounding beeping (and louder) is

_ _ _ _ _ . . _ . _

O B K

Northbrook beacon.


Google says the frequencies of the beacons are:
ORD: 113.90 MHZ
OBK: 113.00 MHZ

So they are very close in frequency so that is why they are bleeding through into each other and they both start with _ _ _ so it makes it confusing.
 
Last edited:

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
There is no way you were getting RF feedback from your guitar speaker to the strings.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
I will repeat... "Anything can be a crappy antenna if your wiring is shoddy enough, even a guitar."
 

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
There is no way you were getting RF feedback from your guitar speaker to the strings.

Bob

No, it was audio feedback. That's why it only worked for morse code and not voices.

And I realized that that was a silly idea and I stopped using feedback from guitar speaker to strings and instead used a mixer and some cables to form the feedback loop.

So I shouldn't have gotten any ideas from Electro132 that sound waves can tune to radio stations.

Well it is sort of true but not quite true. It is audio signals, but the audio signals are sinusoidal electric current, not sound waves, that are providing the feedback in the regenerative detector.

More likely my "regenerative detector" when I was using speaker feedback to the strings wasn't a tuned circuit at all and by pure chance happened to pick up a powerful CW beacon signal a few miles away from my house without having to be tuned at all.

but now I think in the second video when i use electronics instead of acoustics now I think it is my first real tuned circuit, well sort of.


So for better results I think I go buy that 10 m wire for my Grundig short wave receiver and use that as an antenna so I get more stations on my guitar radio experiment and I think I can make a vari-cap out of 2 toliet paper roles and aluminum foil connected in series with the pickups and the antenna wire that will improve the selectability.
 
Last edited:

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
A regernerative receiver uses RF feedback, not audio.

Bob
 

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
A regernerative receiver uses RF feedback, not audio.

Bob

That doesn't make any sense, because the regenerative detector that forms the feedback loop in the regenerative receiver is a triode tube or a simple PNP transistor that demodulates radio waves into audio at the same time as amplifying the audio thousands of times.


Do I really need the stupid "tickler coil" when I could just directly connect the connections to make the feedback loop instead of using induction?

And wouldn't the "tickler coil" make the receiver vulnerable to untuned radio interference bleeding in in the gap between the coils?
 
Last edited:

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
No, it is multiplying the RF, not the audio.

Bob
 

dietermoreno

Dec 30, 2012
238
Joined
Dec 30, 2012
Messages
238
Wikipedia said:
Simple regenerative receivers lack an RF amplifier between the antenna and the regenerative detectors, so any change with the antenna swaying in the wind, etc. can change the frequency of the detector.

Okay, so the RF amp is not needed.


Wikipedia said:
Regenerative receivers require fewer components than other types of receiver circuit. The circuit's original attraction was that it got more amplification (gain) out of the expensive vacuum tubes of early receivers, thus requiring fewer stages of amplification. Early vacuum tubes had low gain at radio frequencies (RF). Therefore the TRF receivers used before regenerative receivers often required 5 or 6 tubes, each stage requiring tuned circuits that had to be tuned in tandem to bring in stations, making the receiver cumbersome, power hungry, and hard to adjust. Regenerative receivers, by contrast, could often get adequate gain with one tube. In the 1930s the regenerative receiver was replaced by the superheterodyne circuit in commercial receivers due to its superior performance and the falling cost of tubes. Transistors, either bipolar or JFETs are used in regenerative receivers today. In recent years the regenerative circuit has seen a modest comeback in receivers for low cost digital radio applications such as garage door openers, keyless locks, RFID readers, some cell phone receivers.

So I understand this part correctly as I talked about it before.



Wikipedia said:
Regeneration can increase the gain of an amplifier by a factor of 15,000 or more.

Cool. That's why I'm trying to build a regenerative receiver so I can use simple electronics components with only one gain stage before an audio amp, instead of having to amplify several stages in Audacity like I did to hear audible AM stations previously, so I can build a real radio instead of cheating using Audacity to amplify and clean up the garbage picked up by my guitar.



Wikipedia said:
A disadvantage of this receiver is that the regeneration (feedback) level must be adjusted when it is tuned to a new station. This is because the regenerative detector has less gain with stronger signals, and because the stronger signals cause the tube or transistor to operate on a different section of its amplification curve (i.e. grid V vs. plate V for tubes; gate V vs drain V for FET's, and base current vs. collector current for BJT's).

Yes, I have already discovered how difficult it is to tune to a station without having the mixer-detector oscillating infintely in a whine that hurts my ears.


When the regenerative detector is used in the self-oscillating mode, i.e. without a separate oscillator, it is known as an "autodyne".

So is this what I have built is not actually a regenerative receiver, but is actually an autodyne receiver that can only receive CW and cannot receive AM?

So is the correct location for the vari-cap to be inserted in the circuit do I want to put it actually in the feedback loop before demodulation instead of imediately after the antenna?

but how do I put a vari-cap in the feedback loop, when this would be completely useless for tuning since the mixer-detector demodulates so the vari-cap would only be acting as a tone control for audio demodulated?

So do I actually have the wrong type of transistor (audio mixer), and I should buy the correct transistor and then let the audio amp do the demodulating?
 
Last edited:

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
If you want to build a radio receiver you should leave your guitar and amp out of it. Look for circuits for radio receivers. If you get one that works and produces audio, you can run it through your guitar amp to get louder audio.

Bob
 

davenn

Moderator
Sep 5, 2009
14,077
Joined
Sep 5, 2009
Messages
14,077
If you want to build a radio receiver you should leave your guitar and amp out of it. Look for circuits for radio receivers. If you get one that works and produces audio, you can run it through your guitar amp to get louder audio.

Bob

Agreed
most of this thread has been totally pointless and also has absolutely nothing to do with the thread title

Dave
 
Status
Not open for further replies.
Top