New ideas in an old fashioned AM tuner

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Re: New ideas in an old fashioned AM tuner

Postby LW1DSE » Thu Apr 28, 2016 8:25 am

In this post I want to add some miscellanies about the project.

For those interested in repeat my line of thinking, this is an easy and simple program made for MS DOS QBASIC, which helped me to obtain a better values for the circuit elements of the oscillator and RF stages, but I suppose than can also be adapted to another programming language, surely more modern that this.

Here is the code used:

/code:

CLS

LET Tc = 16 { Value in Farad of the trimmer capacitor }
LET P = 552 { Value in Farad of the padder capacitor }
LET Tl = 38.4 { Value in Farad of the fixed capacitance in the oscillator coil }
LET Lo = .000089 { Value in Henry of the oscillator inductor }
LET Fi = 465000 { Value in Hertz of the intermediate frequency amplifier }
LET Ls = .0001885 { Value in Henry of the antenna and RF tuned inductor }
LET Ct = 37.4 { Value in Farad of the antenna trimmer capacitor plus fixed and starys }

PRINT "Fsint", "Cosc", "Fosc", "FI conv.", "Error"

FOR i = 1 TO 21

READ G { Step Gang capacitance read from data statements }

LET Cs = Ct + G { Total capacity in the RF section: gang (Read from data statements), trimmer and stray }

LET Fs = 1 / (6.28 * SQR(Ls * (Cs * 1E-12))) { Incoming signal frequency }

LET Co = Tl + (((Tc + G) * P) / (Tc + G + P)) { Total capacity in the oscillator section, gang (Read from data statements), trimmer and stray }

LET Fo = 1 / (6.28 * SQR(Lo * (Co * 1E-12))) { Oscillating frequency }

LET Fic = Fo – Fs { Intermediate frequency obtained }

LET DFi = Fi – Fic { Intermediate frequency error respect to the wanted }

PRINT Fs, Co, Fo, Fic, DFi

NEXT i

DATA 12,25,50,75,100,125,150,175,200,225,250
DATA 275,300,325,350,375,400,425,450,475,500


/end code.

The first sentence clears the screen between tests. Then, in the following, I load the fixed values of some constants that will not be changed in the program. Following it, a simple titles for the columns of results from the computations below them.

A for – next loop for the total points of the gang capacity for which is wanted to estimate the tracking error. The capacity values to be read are stored in the data sentences at the end of the program. With this value, simple frequency calculations are made for oscillator and incoming signal paths, and checked (substracted) to obtain the final error.

Bibliography:

 F. Langford Smith “Radio Designer’s Handbook”, 1997.
 George E. Valley and Henry Wallman “Vacuum Tube Amplifiers”, 1948.
 Samuel Seely “Electron-Tube Circuits” 1950.
 ARRL “Radio Amateur’s Handbook”, 1949 (Version in Spanish).
 Héctor Algarra y Alberto Rodríguez “La Técnica del Receptor de Televisión” (In Spanish).
 Some patents in PDF format at Google Patents, Advanced Search.
 Tubes data sheets found in the Internet from Google Search.

Software:

 MS DOS 7.10 edit.com
 Microcap IV (For DOS) (Simulation program for DOS’ers)
 Microsoft Word, Office 97
 LT Spice (From http://ltspice.linear.com/software/LTspiceIV.exe)
 Microsoft Paint.
 Express PCB and Schematic capture at http://www.expresspcb.com
 Adobe Acrobat Reader from http://www.adobe.com

Thanks to Luciano Alvarez LW1EL for support my large comments and crazy ideas almost every Sunday nights at the QAP frequency.
If anything can go wrong, it will.
If anything just cannot go wrong, it will anyway.
If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong.
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Location: Barrio Garay, Almirante Brown, Buenos AIres, Argentina

Re: New ideas in an old fashioned AM tuner

Postby LW1DSE » Thu Jun 16, 2016 6:01 am

Here there are two short videos (less than 10sec each) showing the receiver's signal at the follower's cathode, with and without the auxiliary diode. The second channel of the oscilloscope, is grounded, and sited at the center of the AM signal, say, when the modulation index would become -1 or -100%, no instantaneous carrier amplitude.

This video (https://youtu.be/wxk8QYT9yCg) shows the signal with the second diode still wired in the circuit. Note the high negative amplitude and the presence of high audio components in the signal. (Unfortunately, my camera didn't take good photos at very near target respect to the len, so the focus is poor, but sufficient to see the details).

This other (https://youtu.be/hEnDtoqUy3c) looks the same signal, but with the diode un-wired.

The oscilloscope setting stayed untouched between films, which can also be checked in the read out in the bottom of the CRT screen. Also, no other control of the tuner has been altered. Note audio signal in Spanish, as it has been taken from a local AM station, Radio Mitre AM 790 in Buenos Aires.

You can also see, that the entire signals displaces downward with the diode removed. It's cause is because the coupling capacitor can't pass DC, then at the grid side the DC becomes almost zero (a small contact potential exists there), then, the higher the negative hemicycle amplitude, the more negative the mean DC voltage it has, and vice versa, which also checks the enhancement in the rectification of the signal.
If anything can go wrong, it will.
If anything just cannot go wrong, it will anyway.
If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong.
User avatar
LW1DSE
 
Posts: 37
Joined: Wed Aug 06, 2014 9:49 am
Location: Barrio Garay, Almirante Brown, Buenos AIres, Argentina

Re: New ideas in an old fashioned AM tuner

Postby LW1DSE » Wed Jun 22, 2016 6:32 am

OK, guys. Here are two more videos showing the performance of the receiver. It is relative, because I have no measurement instrumental, so all test are made over the air, receiving a radio station, one of the most powerful that I can receive. There is another most stronger, but it has too high audio distortion that it is almost impossible to support listening it, plus its politic content that is insufferable.

So, this first video (
https://youtu.be/h7vR9dMuyHE) shows the antenna shorted out by a crocodile clip, a black one at the left of the rod. Yellow voltmeter shows the AGC line voltage, in its minimum base bias in this case. The bench one looks the screen regulated power supply (See the main circuit in the first post, around the triodes in the EABC80 and the ECC82). The oscilloscope is punched at the cathode of the audio filter. Unfortunately, the sweep reacts with camera and shows a too slow point moving in the CRT face, but it is clear that no audio at all.

The second one (https://youtu.be/LCGnks1e27I), is with the antenna in normal operation, without the shorting. See that with about only 2 or 3 volts at the grids circuits achieve the full control, as the screen voltages practically does't move from its adjusted value. This is clear that if a dropping resistor would be used, the screen would move more, and the grid needs hard effort to control the audio levels. All this, was provided in the description of the regulator and the AGC subsystem, several post below.

The "temperature" instrument, is in fact, a 1mA taut thread milliammeter rescued from an old PID control, and will be calibrated in relative strength signals, and no much care by the moment.
If anything can go wrong, it will.
If anything just cannot go wrong, it will anyway.
If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong.
User avatar
LW1DSE
 
Posts: 37
Joined: Wed Aug 06, 2014 9:49 am
Location: Barrio Garay, Almirante Brown, Buenos AIres, Argentina

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