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 Post subject: Make Your Own IF Transformers.
PostPosted: Dec Thu 10, 2009 6:03 am 
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http://www.angelfire.com/electronic/funwithtubes/IF_Can-1.html

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 Post subject:
PostPosted: Dec Thu 10, 2009 10:53 am 
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OK, let's see here.
This guys 'transformers' aren't really transformers, they are LC circuits that are tuned to (hopefully) the IF you need. He talks about excessive distortion on weak signals, oscillation that can only be corrected with changing the cathode resistor of a tube (which tube?), and the bandwidth being too narrow.
Kinda sounds like the reason for using real IF transfomers.

Back to square one?


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PostPosted: Dec Thu 10, 2009 2:58 pm 
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Johnnysan wrote:
OK, let's see here.
This guys 'transformers' aren't really transformers, they are LC circuits that are tuned to (hopefully) the IF you need. He talks about excessive distortion on weak signals, oscillation that can only be corrected with changing the cathode resistor of a tube (which tube?), and the bandwidth being too narrow.
Kinda sounds like the reason for using real IF transfomers.

Back to square one?


Don't want to get into a big argument here, but in fact this is a transformer. There is magnetic coupling between the two chokes and the coupling is adjusted by changing the distance between the two chokes. Exactly the same physics as a traditional IFT. The critical coupling coefficient is equal to 1/Squareroot(Q1*Q2) and is the point of maximum gain without double peaks in the response. Actually a clever way to get around having to wind your own coils.

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 Post subject:
PostPosted: Dec Thu 10, 2009 6:08 pm 
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If you really want to make your own IF transformers.....

http://www.lindsaybks.com/bks7/rexp5/index.html

They are big and bulky but they are home-made IF transformers.

Rich


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 Post subject:
PostPosted: Dec Thu 10, 2009 8:38 pm 
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If you read that guys article carefully you will see all the drawbacks to his design; probably why engineers have NOT used this method for the last 90 years.


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 Post subject:
PostPosted: Dec Thu 10, 2009 9:10 pm 
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Johnnysan wrote:
If you read that guys article carefully you will see all the drawbacks to his design; probably why engineers have NOT used this method for the last 90 years.


I did read the article carefully and the distortion and narrow bandpass response occured with the Meissner IFT not his homebrew transformer.
This is quoted from the Website
Quote:
A Commercially Made IF Transformer.
I began with a NIB NOS IF transformer that I have several of on hand. It was made by Meissner and the part number is either 16-5740 or 276841-26. Personally I'm betting on the shorter number. The box is clearly marked input transformer. A photograph of the resultant sweep is shown below. 455 kc is at the center of the screen and the calibration is 10 kc per div.


He goes on to say in the same section:
Quote:
As you can see the bandwidth is the narrowest of all the transformers tested. In IF response the -6 dB points are used rather than the -3 dB points as in audio work. Two transformers should add together so for a single transformer the -3 dB points should theoretically be used. Since the detector is seriously loading the transformer in the plate circuit of the IF amplifier the one in the grid circuit is probably providing most of the selectivity.

I decided to drop a couple into my 5 tube superhet. Narrow bandwidth, higher Q, and higher gain all go together. The amplifier oscillated. I had to increase the cathode resistor to 1 k ohm to tame it. A well laid out and properly shielded IF amplifier would probably not need this gain reducing feedback. The resistor is not bypassed.

The tuning was quite narrow as you might expect. These would be excellent for DXing on the AM band or short wave listening. The Meissner company was known for making ham equipment so these transformers may have been designed for use in communications receivers rather than broadcast radios.

The one thing I noticed was excessive distortion on extremely weak signals. This may be due to the fact that I was using an input transformer to drive the diode detector. An impedance buffer might help. An infinite impedance detector might also be used but I don't know how to derive AGC from such a detector. The bandwidth is too narrow to make a high-fi tuner with these transformers.


His next section goes on to explain that the bandpass of his transfomer is much wider and has excellent audio quality.

Quote:
Transformers Made with The Original 1.2 mH Pie Wound Chokes.
The first picture below is the one I was using for the input transformer and the second was the output. I thought I had swept them out but perhaps the flicker on the old oscilloscope made it hard to see what was going on. As you can see I had them seriously over coupled. I wonder what they would do if critical coupling were used.

I didn't make any changes except to slightly readjust the trimmers to account for the difference in capacitance between the radio and the test circuit. When used in the radio, rejection of adjacent channels was quite good. Weak signals were not distorted and the amount of noise did not seem excessive. Although I did not connect it to a high fi system, its frequency response must have been as good as the station was transmitting.


Quote:
Below are the sweeps of the OSE coils. The first one is the input transformer and the second one the output. Channel 2 of the scope is the Y axis and you can see the progressively lower Q as the input has to be set to a more sensitive range to keep the display the same height. The output of the sweep generator was not changed throughout these tests.


Adjacent channel rejection was quite good and there was no audible distortion on weak signals. One of the things I would like to do is build an AM broadcast receiver with delayed, amplified AGC so I could tune the dial without having the other hand on the volume control. I also want to use the PLL VFO from OSE to make a digital local oscillator so I could know exactly what channel I am listening to. So much to do, so little time.

Meantime remember, life is short so eat your dessert first and always listen to tube radios


I was unaware that this particular method for an IFT was used 90 years ago, especially since ferrite had yet to be invented.

The only difference in his design (as compared to modern IFTs) is the fact that you can adjust the coupling and both tuned circuits, whereas with modern traditional IFTs, they are permeabilty tuned with fixed coupling.

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 Post subject:
PostPosted: Dec Sun 13, 2009 9:29 pm 
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It would be simple to have selectable bandwidth with that transformer. Couple with a small cap and space the inductors to give you the most selectivity wanted. Then switch in various resistive loads on the secondary to broaden the response. There are several methods to vary the selectivity.

Quote:
If you really want to make your own IF transformers.....



Yeah, that is pretty clunky. Ive made several using old can capacitors and spade lugs; just a different way of restuffing :lol:

Carl


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 Post subject:
PostPosted: Dec Mon 14, 2009 12:58 am 
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OK, try this guys method and see how well it works. He admitted to having to change components to make the radio work properly; in my book, re-engineering a radio to make it work means you are not using a proper substitute part.


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 Post subject:
PostPosted: Dec Mon 14, 2009 1:20 am 
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Johnnysan wrote:
OK, try this guys method and see how well it works. He admitted to having to change components to make the radio work properly; in my book, re-engineering a radio to make it work means you are not using a proper substitute part.


Can you show me where he states he has to change components in the radio to make his IFTs work properly?
Thanks

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 Post subject:
PostPosted: Dec Mon 14, 2009 1:31 am 
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He said he had to change a cathode resistor because of distortion. Then he said something about an 'infinite impedance buffer' (whatever that is) to make a circuit work.


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 Post subject:
PostPosted: Dec Mon 14, 2009 1:45 am 
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Johnnysan wrote:
He said he had to change a cathode resistor because of distortion. Then he said something about an 'infinite impedance buffer' (whatever that is) to make a circuit work.


Yes but that was when he was using the Meissner transformer that had much higher Q and a bandwidth of only 4kHz. He said the Meissner was an input transformer and he was using it as an output transformer at the detector. He was referring to using a detector with infinite impedance, but we know that's impossible. I think he was saying that in jest.

His homebrew transformers had much lower Q (than the Meissner) and bandwidths of between 10 and 20kHz by looking at his scope displays.

I'd like to try this method, but I don't have the chokes.

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 Post subject:
PostPosted: Dec Mon 14, 2009 3:31 am 
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Chuck,

I had seen this article before. It looks like it would work very well -in fact it is exactly like an IF transformer works,

Not sure why the panties are in a wad?

This guys method is the same as any IF transformer - a loosely coupled set of inductors with resonating capacitors?

The old IF cans are still easy enough to get - but in the future it would be a good way improvise.

Pat
N4LTA


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 Post subject:
PostPosted: Dec Mon 14, 2009 4:12 am 
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pbunn wrote:
Chuck,

I had seen this article before. It looks like it would work very well -in fact it is exactly like an IF transformer works,

Not sure why the panties are in a wad?

This guys method is the same as any IF transformer - a loosely coupled set of inductors with resonating capacitors?

The old IF cans are still easy enough to get - but in the future it would be a good way improvise.

Pat
N4LTA


Yep that's exactly what I think as well.

Another point that the author didn't think of is; that you could adjust the Q (make it lower) by bringing the chokes closer to the AL enclosure. The eddy current generated in the AL will reflect back to the primary as a loss and lower the Q. So you could adjust the coupling and Q to get just the response you wanted, assuming the natural Q is high enough in the first place.

I think it's a pretty cool alternative to play with. I may just have to wind some chokes to play around with this.

I've been trying to design IFTs for triodes such as 01As. The low plate resistance really kills the Q, so you have to have very high inductances and relatively high coupling for the pri and sec windings. The guys in the 20s did amazing things with those 01A circuits.

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 Post subject:
PostPosted: Dec Mon 14, 2009 4:20 am 
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Quote:
He was referring to using a detector with infinite impedance, but we know that's impossible. I think he was saying that in jest.


The Infinite Impedance Detector is well known and documented as an exceptionally good one for AM.
A benefit is that it does not load the last IF as does a diode even with the proper IFxfmr. In fact an input transformer is ideal for it.

Carl


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 Post subject:
PostPosted: Dec Mon 14, 2009 4:45 am 
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Burnt Fingers wrote:
Quote:
He was referring to using a detector with infinite impedance, but we know that's impossible. I think he was saying that in jest.


The Infinite Impedance Detector is well known and documented as an exceptionally good one for AM.
A benefit is that it does not load the last IF as does a diode even with the proper IFxfmr. In fact an input transformer is ideal for it.

Carl


Carl,
Is this the same as a plate detector or reflex detector? Apparently must have some type of feedback to get zero input admittance to the detector grid?? Not familiar with this type of detector so I'm all ears.

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 Post subject:
PostPosted: Dec Mon 14, 2009 5:51 pm 
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Lots of references to it in various handbooks, QST, etc. Racal used it in several very high end tube receivers of the 60's.

The Radiotron Designers Handbook 4th Edition refers to it as the Reflex Detector and offers a small circuit analysis. I suspect the issues and cost of proper implementation in a superhet caused it to fall out of favor in the late 30's. However there is quite a bit of discussion lately on implementing it in various vintage communications/ham receivers to improve modulation recovery.

It has also been implemented in SS radios.

Carl


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 Post subject:
PostPosted: Dec Mon 14, 2009 6:04 pm 
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Here is an 'infinite impedance detector' implemented in solid state.

Image

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 Post subject:
PostPosted: Dec Tue 15, 2009 10:14 pm 
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Thanks Carl and Terry,

I did some reading in the RDH 4th ed. and Termans Radio Engineering 1st ed. and they do mention the infinite Z det, but the RDH says it's actually a misnomer.

I see now it's just a cathode follower biased to near cutoff with no signal applied. The the input impedance is just due to the parallel combination of the grid to plate and grid to cathode reactance. Since the gain is less than 1 and no plate load impedance, there is no miller effect so the input impedance is almost purely capacitive.

So not really infinite Z, but about as high as you can get I guess??

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