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Old 1st Sep 2014, 6:45 pm   #1
Miguel Lopez
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Default Doubts about IF coils

Hello everybody

I intend to build a 455kHz oscillator for future radio tests. I think to use a JFET based oscillator and my idea is to use an LC tank on the anode of the output transistor.

I have several IF (I guess) coils from an old Soviet radio. (See the attachment.)

My doubts are as follow.

Are those really IF coils?

May I use one of them, with the respective capacitor in the anode on the output JFET to increase the frequency stability?

May I use the entire IF chain for this purpose?

Thank in advance.
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Old 1st Sep 2014, 7:09 pm   #2
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Default Re: Doubts about IF coils

The capacitors look about right for an IF, for a simple oscillator with a simple coil I would (and have, many times) use a colpitts circuit, such as http://www.play-hookey.com/oscillato...scillator.html nothing is critical to make it work and you get a nice low impedance output from the source of the FET.
 
Old 1st Sep 2014, 8:17 pm   #3
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Default Re: Doubts about IF coils

That isn't a conventional IF with magnetically-coupled tuned circuits in pairs in cans... one can between amplifier stages.

It looks to be a coupled-resonator filter where a group of tuned circuits each screened from the others are coupled together by small capacitors at their 'hot' ends.

One resonator, with the coupling capacitors going to grounded resonators on each side, will resonate on the centre frequency of the filter.

David
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Old 1st Sep 2014, 8:48 pm   #4
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Default Re: Doubts about IF coils

Soviet had never been conventional. Here some other examples of that configuration on the VEF206 and SelenaB210 radios
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Old 1st Sep 2014, 9:21 pm   #5
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Default Re: Doubts about IF coils

Those are interesting snippets you post there, Miguel, to me they suggest a thoroughness of intent by the designer. When I was a (lot!) younger, one of my favourite radios was a jumble-sale Astrad Solar, it had large (for a '70's transistor set anyway) double-tuned IFTs for both AM and FM that had side-by-side coil formers under the rectangular screening can. The AM cans were plain internally, the FM ones had a dividing screen with an obviously carefully dimensioned aperture in it. Of course, I had to try swapping them over (they just clipped in place)- AM signal was greatly diminished, FM selectivity became wide-open, evidently now somewhat over-coupled. Wish I had hung onto it, with its tuned AM RF stage, good SW bandspread and plentiful facilities.

It was customary in the West to denigrate anything associated with the Eastern Bloc but the occasional bits of their stuff I came across made me think that they must have had some pretty good theoreticians/designers, they were just often let down by indifferent/outdated components/manufacturing technology. Would that be your take on things, having seen so much more of it?,

Colin.
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Old 2nd Sep 2014, 12:17 am   #6
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Default Re: Doubts about IF coils

I always think that Soviet era radios are a bizarre combination of overengineering and poor construction.
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Old 2nd Sep 2014, 7:42 am   #7
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Default Re: Doubts about IF coils

They look like examples of fairly sophisticated filter design, a step beyond simple synchronously tuned coupled pairs. These are good structures for designing filters where the bandwidths are less than 10% of the centre frequency.

They'd been used a little in the West but were not generally known about until the publication of "Handbook of filter synthesis" by Anatol Zverev in 1967. He worked at Westinghouse and wrote the seminal book which drew most of filter technology together and made their design into a regular process of deciding ona shape template, scaling it and then scaling a standard solution. Look in any newer filter book and you're sure to see Zverev cited as a primary reference. If anyone wants to collect a classic radio/electronics book, this is one of the biggies. Some people find it terrifying!

The other point about the circuits Miguel has posted is that they represent a shift to doing all the selectivity in one block rather than having it distributed down the length of anIF strip as is more commonly done with IF transformers. This foreshadows modern trends forced on us by the availability of crystal and ceramic block filters, but the distributed filter-gain-filter-gain-filter structure with IFTs has advantages in dynamic range.

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Old 2nd Sep 2014, 1:10 pm   #8
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Default Re: Doubts about IF coils

Quote:
Originally Posted by merlinmaxwell View Post
The capacitors look about right for an IF, for a simple oscillator with a simple coil I would (and have, many times) use a colpitts circuit, such as http://www.play-hookey.com/oscillato...scillator.html nothing is critical to make it work and you get a nice low impedance output from the source of the FET.
I lifted one of the coils of post#1 (L2) and use it for the schematic shown in the above link. It did not oscillate. Is there any treak to make it oscillate?

The FET that I used is 2N3823.

The resistor : I used 1K, 3K3, 4K7, 10K and 33K.

The capacitors: I used combinations of 500+1000pF 1000+1000pF 1000+2000pF and 2000+2000pF

Any help welcomed
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Old 2nd Sep 2014, 2:00 pm   #9
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Default Re: Doubts about IF coils

I can't help, I have never had one not oscillate so never had the need to fault find.
 
Old 2nd Sep 2014, 2:35 pm   #10
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Default Re: Doubts about IF coils

Quote:
Originally Posted by paulsherwin View Post
I always think that Soviet era radios are a bizarre combination of overengineering and poor construction.
I know what you mean Paul, even if I'd have phrased it a little more kindly! The Astrad Solar mentioned is a good case- plenty of design ambition, a very credible rival for, say, a Hacker Super Sovereign but let down by primitive plastics and crude metal-work. I liked the turreted VEFs, too, but also same drawbacks- many of the machined parts have tolerances from a different era.
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Old 3rd Sep 2014, 11:22 am   #11
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Arrow Re: Doubts about IF coils

Quote:
Originally Posted by Miguel Lopez View Post
I lifted one of the coils of post#1 (L2) and use it for the schematic shown in the above link. It did not oscillate. Is there any trick to make it oscillate?
The capacitors: I used combinations of 500+1000pF 1000+1000pF 1000+2000pF and 2000+2000pF
In post #1, L2 is tuned by 1300 pF. Therefore, the net capacitance of the two capacitors in series in your oscillator need to be about that value. Your largest selection is 2000 pF and 2000 pF: net capacitance = 1000 pF: too small.

Al.
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Old 3rd Sep 2014, 2:13 pm   #12
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Default Re: Doubts about IF coils

It should oscillate at another frequency, I guess.

I will try a Hartley oscillator with another coil.
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Old 3rd Sep 2014, 6:26 pm   #13
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Default Re: Doubts about IF coils

The coil L2 of Post#1 has a ferrite tube around the coil, and bellow the aluminium can. So, I guess its inductande should be relatively high.

On the other hand, what is the purpose of diode 1N4148 on the following circuit?

http://mysite.du.edu/~etuttle/electron/circ220.gif
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Old 4th Sep 2014, 1:25 pm   #14
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Default Re: Doubts about IF coils

Quote:
Originally Posted by Miguel Lopez View Post
The coil L2 of Post#1 has a ferrite tube around the coil, and below the aluminium can. So, I guess its inductance should be relatively high.
The coil L2 of Post#1 is tuned by 1300 pF. If the resonant frequency is 455 kHz, the inductance of the coil will be in the range 90 → 100 µH

Quote:
Originally Posted by Miguel Lopez View Post
On the other hand, what is the purpose of diode 1N4148 on the following circuit?

http://mysite.du.edu/~etuttle/electron/circ220.gif
The F.E.T. is N-channel. Making the gate +ve with respect to the source will increase the drain-source current. An excessive +ve voltage, gate-source, will produce an excessive drain-source current, probably damaging the FET. Since this F.E.T is N-channel, what is required is a range of d.c. voltages at the gate which go from slightly +ve to much more -ve, thus causing the drain-source current to vary accordingly. The diode is thus acting as a rectifier for the approx. sinewave occurring across the L/C tuned cct.

Al.

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Old 4th Sep 2014, 1:38 pm   #15
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Default Re: Doubts about IF coils

Thanks Skywave

I tested that circuit yesterday and it worked fine. I got a sine wave up to 180kHz with little distortion. I will wind another coil with less inductance to increase frequency. Still I want to use the coil from the openning post for an LC tank to tune the output.

The most interesting part of the circuit is that its amplitude did not change with the supply voltage nor with the change in both L and C. It remained constant at 1,2Vrms (+/-) from 5Vcc to 24Vcc. And changing L or C from a frequency of 50kHz up to 180kHz, the amplitude changed almost nothing.

Without the 1N4148 diode, the amplitude was much higher but the waveform was distorted. I also replaced the 100K gate resistor for 470K to reduce distortion.
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Old 4th Sep 2014, 1:51 pm   #16
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Default Re: Doubts about IF coils

The 1N4148 diode rectifies the RF at the gate of the FET, and makes DC to bias the gate more negatively, reducing the DC bias current of the FET, and reducing the gain of the FET.

This acts to stabilise the amplitude of oscillation, otherwise the device runs flat-out and makes high levels of distortion.

There is concern that this action degrades the phase-noise performance of an oscillator. Ulrich Rohde (of Rohde & Schwarz) has written several times about this issue.

Anyway, fitting the diode should reduce the amplitude and make the sinewave a better shape. It also reduces the sensitivity to different characteristics of individual FETs

David
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