|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
1st Sep 2014, 6:45 pm | #1 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
|
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.
__________________
When electrons move, things happen. There's nothing you can do that can't be done. |
1st Sep 2014, 7:09 pm | #2 |
Guest
Posts: n/a
|
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.
|
1st Sep 2014, 8:17 pm | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
|
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
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
1st Sep 2014, 8:48 pm | #4 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
|
Re: Doubts about IF coils
Soviet had never been conventional. Here some other examples of that configuration on the VEF206 and SelenaB210 radios
__________________
When electrons move, things happen. There's nothing you can do that can't be done. |
1st Sep 2014, 9:21 pm | #5 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
|
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. |
2nd Sep 2014, 12:17 am | #6 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,787
|
Re: Doubts about IF coils
I always think that Soviet era radios are a bizarre combination of overengineering and poor construction.
|
2nd Sep 2014, 7:42 am | #7 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
|
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. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
2nd Sep 2014, 1:10 pm | #8 | |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
|
Re: Doubts about IF coils
Quote:
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
__________________
When electrons move, things happen. There's nothing you can do that can't be done. |
|
2nd Sep 2014, 2:00 pm | #9 |
Guest
Posts: n/a
|
Re: Doubts about IF coils
I can't help, I have never had one not oscillate so never had the need to fault find.
|
2nd Sep 2014, 2:35 pm | #10 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
|
Re: Doubts about IF coils
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.
|
3rd Sep 2014, 11:22 am | #11 | |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Doubts about IF coils
Quote:
Al. |
|
3rd Sep 2014, 2:13 pm | #12 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
|
Re: Doubts about IF coils
It should oscillate at another frequency, I guess.
I will try a Hartley oscillator with another coil.
__________________
When electrons move, things happen. There's nothing you can do that can't be done. |
3rd Sep 2014, 6:26 pm | #13 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
|
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
__________________
When electrons move, things happen. There's nothing you can do that can't be done. |
4th Sep 2014, 1:25 pm | #14 | ||
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Doubts about IF coils
Quote:
Quote:
Al. Last edited by Skywave; 4th Sep 2014 at 1:31 pm. |
||
4th Sep 2014, 1:38 pm | #15 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
|
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.
__________________
When electrons move, things happen. There's nothing you can do that can't be done. |
4th Sep 2014, 1:51 pm | #16 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
|
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
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |