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Old 4th Feb 2009, 7:35 pm   #1
timohaveri
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Default Signal generator problems

I'm in the process of building the RF signal generator described here: http://www.vintage-radio.com/project...generator.html . The circuit is the same except for the FET. I couldn't get a BF244A so I used 2N5457 which is supposed to be an equivalent (?). My varcap is a bit larger (465 pF) so I decided to try the various inductors first individually before I hard wired them to the rotary switch. That way I could find the suitable coils to get the best frequency ranges. I didn't get past trying the first pair of inductors (2.2 mH), though. When the varcap is fully meshed I get a beautiful sine wave in the oscilloscope but when I rotate the varcap the amplitude decreases proportionally and somewhere after halfway the oscillation stops. The frequency increases as it should. Can someone suggest a solution to this problem?

Bytheway, I'm going to add a digital display to the generator if I ever get it to work. I bought a cheap kit from here: http://www.pongrance.com/ddfc.html .

Timo
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Old 6th Feb 2009, 12:40 am   #2
Oldtimer74
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Default Re: Signal generator problems

I presume you are using 2 varicap diodes? They will need to be fairly well matched over their capacity range on the feedback may be insufficient. Why not use a twin gang variable as in the priginal?

Pat G3IKR
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Old 6th Feb 2009, 8:35 am   #3
timohaveri
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Default Re: Signal generator problems

Quote:
Originally Posted by Oldtimer74 View Post
I presume you are using 2 varicap diodes? They will need to be fairly well matched over their capacity range on the feedback may be insufficient. Why not use a twin gang variable as in the priginal?

Pat G3IKR
I am using a variable capacitor. Sorry about the confusing abbreviation "varcap". It is actually a 3 gang device but only 2 of the sections are in use.

Is this caused because of the wrong FET or is there a fault somewhere else? The inductors I'm using are of the kind that look a bit like resistors. Should they be air core coils or something else?

Timo
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Old 6th Feb 2009, 9:04 am   #4
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Default Re: Signal generator problems

It certainly sounds as if there's not enough "gain" in the circuit to keep it oscillating, which could be all sorts of things. It could be the inductors you're using, the ones you describe may be a bit lossy and have a low Q. Air cored coils would be much better.
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Old 6th Feb 2009, 9:19 am   #5
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Default Re: Signal generator problems

According to Mr Towers the equivilents for BF244A are 2N5458 and BFW61....I suspect the 2N5457 is a lower gain device, which probably isn't helping.
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Old 6th Feb 2009, 9:37 am   #6
timohaveri
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Default Re: Signal generator problems

OK. Thanks for the answers. I'll try to get another FET and see if it makes a difference.

Timo
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Old 6th Feb 2009, 10:52 am   #7
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Arrow Re: Signal generator problems

My first thought is that the types of coils you are using are unsuitable. It sounds at though as the freq. is raised, the feedback to maintain osc. is decreasing right up to the point where there is inadequate feedback to maintain oscillation. This is probably caused by low Q of these coils: the amount of feedback is largely determined by the dynamic impedance of the tuned cct. (which is freq. dependant), which itself is a function of the Q of the inductors. Air-spaced and ferrite / dust-iron cored coils are preferable in this type of application; they will have a higher Q.

HTH

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Old 19th Feb 2009, 2:56 pm   #8
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Default Re: Signal generator problems

I'm sad because didnít saw these thread earlier.
I've made the same project from Paul Stennings page a few years ago.

And the first results was like in your case - QUOTE "When the varcap is fully meshed I get a beautiful sine wave in the oscilloscope but when I rotate the varcap the amplitude decreases proportionally and somewhere after halfway the oscillation stops "
I think FET isnít critical, you can use on this position anything suitable (BF244, BF245, BF247 - I've try to use all of them and thing oscillate as good as any of them), careful on pin configuration, and thing must work.
Like "skywave" suggest is, that the types of coils you are using are unsuitable.
I have the same results like's yours - after some point oscillation stops.
Probably you have used coils, which is intended to use in circuits for prevent "wild" oscillations.
Much better results will given by using coils from a scrapped set. Indeed, some of then you can't find, and adder you must wind up by yourself.
I've do it that way, and this part works fine.

But in this project - there are much more troubles then your first trouble is.

Next trouble you will find in interaction between coils.
That's good described in Paul's text. There is all manner of interaction inside RF devices.
I've solve this trouble with an elegant trick.
Instead using 1x12 position switch, I've used 2x12 position switch.
Do it that way, you can to effect that unused coils are in the air - there are not connected anywhere.
You can saw picture like I've sort elements and coils.
My unit cover a frequency range of 150 kHz to 30 MHz over 6 ranges.




But worst trouble which you would reached is mistake into Paul's schematic.
I've never solve that part to the ends, my thought is that Paul due an error when was redraw a circuit diagram. If he read this, maybe he can suggest something.
As you can read on introduction page, the output is not a pure sine wave, so the unit may not be suited for more exacting electronic development work.
Using unit for aligning old radios this won't be a trouble.
But stability will be very important thing, otherwise the unit will not be useful at all.
In my case, TR2 cause instability, and something around them isnít good.
When you turn attenuator (VR1-level pot. into diagram), to any side, like you changing the level, oscillator frequency changing too.
My first thought was that level changing, cause change to the oscillator voltage, and that could be reason for frequency instability - but did not.
At finally, I've reached some improvements changing schematics this way.
(above is an original, lines marked red color are changed)
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Last edited by Bero; 19th Feb 2009 at 3:18 pm.
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Old 19th Feb 2009, 6:12 pm   #9
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Default Re: Signal generator problems

PM me your address and I'll send you a couple of BF244s,
Jim
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Old 20th Feb 2009, 9:22 pm   #10
timohaveri
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Default Re: Signal generator problems

Quote:
Originally Posted by jim_jobe View Post
PM me your address and I'll send you a couple of BF244s,
Jim
Thanks for the offer. After some experimenting I am beginning to believe that the problem is not with the FET but indeed the coils, as I was told earlier. So for the time being I'm sticking with the 2N5458 that I currently have installed (as suggested by davidv). I really appreciate the offer, though.


Bero, thanks for the very useful tips. Inspired by your reply I fired up the oscilloscope earlier today and experimented with some home-made coils as I don't have any from scrapped sets (the main reason being that don't have the heart to gut any set). Anyway, I tried air core coils as well as coils wound on a piece of ferrite stick. I was still experiencing problems with non-linear behaviour during the travel of the variable capacitor. The best results were with a ferrite core coil (some 70 turns giving a frequency of 500 kHz and up). I got 9 volts peak-to-peak and it went to 7 Vp-p with the varcap at minimum capacitance. To get these results the coils had to be next to each other (parallel) such that the direction of the windings were opposite to each other. Still not ideal results. Does your device keep a constant amplitude when the frequency is changed, Bero?

I also tried with very small coils with sometimes quite weird effects seen on the scope. I was somewhere at 10 MHz at maximum and still got adequate amplitude so the FET seems to work at least. Every time I tested a new coil it worked with the varcap at max capacitance and started to act funny when the capacitance decreased no matter what the starting frequency.

Also I tested the modulation part the first time today but it didn't oscillate at all.

I'll do some more experimenting. Maybe I can find a solution.

Thanks for the interest in this topic.

Timo
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Old 20th Feb 2009, 9:51 pm   #11
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Default Re: Signal generator problems

Hello Timo,

This is probably a daft question but are you sure the capacitor isn't shorting?

Regards Ant
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Old 20th Feb 2009, 10:01 pm   #12
timohaveri
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Default Re: Signal generator problems

Quote:
Originally Posted by Antlong View Post
Hello Timo,

This is probably a daft question but are you sure the capacitor isn't shorting?

Regards Ant
Not daft at all. Unfortunately, the solution is not that simple this time. The capacitor seems to be in perfect order (and it better be, since it's new).

Timo
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Old 21st Feb 2009, 12:57 am   #13
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Default Re: Signal generator problems

I have seen caps LOOK good, but a tiny piece of lint or flake of metal can be shorting things.
once had a oxide layer built up on the copper brushes of one of my capacitors, this caused very odd behaviour, on a small section of travel it went HIGHER in frequency when INCREASING capacitance (overlapping the plates more)
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Old 21st Feb 2009, 4:31 pm   #14
Bero
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Default Re: Signal generator problems

I’m just interesting, is it text I’m writing understandable?
I’m asking this, because acutely I don’t speak English.
All I know I learned watching movies so if something sounds weird in this text its not intended, all those words be written by spell check program.
You are from Finland, I’m from Croatia, and we communicate in English language on forum hosted 1000 miles from us. I hope that forum owner won’t get out us from there, when he read this.
About capacitor:
I’ve used an air capacitor from scrapped 1970-ies grundig transistor.
It has originally mounted 1:3 gear, and that’s beautiful for achieving good precision.
In my experience, most difficult will be wind up coils for lowest range.
So I decide to get them from a scrapped set. I’ve used input antenna coils from old set those, which purposes are to prevent IF frequency to goes into valve radio.
In my case, they given lowest range 150-340 kHz.
Another coils useful from scrapped sets are those used in oscillator part of shortwave range in a valve radio.
Trouble is because you need to have a pair of each. I have luck that fined two the same scrapped sets.
You need to go to a garbage yard, and find a scrapped 1970-ties TV. There are a lot of useful parts inside an old TV, useful for this project.
Wounding coils for adder ranges won’t be such trouble. Inside old TV you can find a lot of coils, or useful lacquer thin copper wire.

“Timohaveri wrote:
I was still experiencing problems with non-linear behaviour during the travel of the variable capacitor.“
I think you don’t need to consider too much with linearity of scale pointer.
I’ve seen a lot of good valve sets with non-linear scale.

“Timohaveri wrote:
Does your device keep a constant amplitude when the frequency is changed, Bero? “
I think you don’t need to waste too much time with this.
When you dealing with low-frequency device, you will reach this.
But, in case of RF device, I think this is impossible. The same thing confuses me at first, and then I’ve tried to see what’s happen when I hook a factory made device (RF sig-gen) to the oscilloscope. The same thing – amplitude goes up or down (and this means an output level) like frequency is changed up or down.
I think this is because resistance in AC circuitry depends on frequency. (In case of very high frequencies).

“Timohaveri wrote:
The best results were with a ferrite core coil (some 70 turns giving a frequency of 500 kHz and up). I got 9 volts peak-to-peak and it went to 7 Vp-p with the varcap at minimum capacitance. “
This is an important part of the story. Like I’ve mentioned before, stability will be very important thing. This means, when you adjusting some circuit in a radio, and need to move to the front end of a radio, than you need to adjust input level from sig-gen.
It is important that when you turn level pot, frequency stay the same.
It is not important if some stage were aligned to the slightly wrong frequency, until all stages are aligned to the same “wrong” frequency.
But if frequency is changed when you turning output level, such instrument won’t be much useful to the owner.
So, if you have oscillator who oscillates to strong, he will probably be very sensitive to any strange effect, because part of his energy will easily “escaped” somewhere if some low resistance path appear in “neighborhood”. This will affect to change oscillator frequency.
Opposite, if he do not have enough gain, at some point oscillation stops (like in your case). Optimum is probably somewhere in the middle.

I didn’t know almost anything about these RF devices, I was constructing only audio amplifier before, and that’s not the same.
Seems to we can build almost the same thing, and in one case will work good and adder won’t.
If you interesting, I can send you schematics, and picture, like I’ve upgrade this project with a tuner, to get range 88-108 MHz.
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Old 23rd Feb 2009, 3:09 pm   #15
Bero
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Default Re: Signal generator problems

Here is a variable cap from my unit.
It has 2 more section inside them, used for FM.
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Old 23rd Feb 2009, 3:13 pm   #16
Bero
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Default Re: Signal generator problems

And here you can see FM tuner, I've remove cover so you can looking inside.
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