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Old 17th Nov 2006, 2:16 am   #1
adibrook
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Default superhet

I recently finished another short wave regen, this time using alot of screening (lots of recycled bean cans and coax) which works quiet well. Its alot more stable because of the extensive screening used everywhere.

So...now that i have built some simple RF things, and got them working, i'v been thinking of havign a go at a simple superhet.

After reading some stuff about them i think i finally realise how they work. Hopefully, if i understood it, a superhet looks somethign like this.

http://img.photobucket.com/albums/v4.../superhet1.gif

The triode oscillator is pretty straight forward to me, exept the output is taken from the grid. I guess if it was taken from the anode the voltage would be way too high for the mixer.

The mixer can be several things, from a tetrode upwords (and maybe even a triode?) but a hexode heptode or octode are usually used. I also know that sometimes one valve is used for the mixer and osc, utilising a grid pretending to be the oscillator anode.

As far as i know, the trimmers C1 and C2 are there so that they local oscillator is allways exactly at the main frequancy minus the IF frequancy. If the world was perfect, and the ariel tuning coil and the osc coil and theyr capacitors were exactly spot on than technically the trimmers wouldnt be needed. But because the world ISNT perfect, and the coils caps and tuning caps have slight differences, the trimmers are needed to make sure that the gap between both the areil coil and oscilaltor coils resosnat frequancy is thesame.

C3 is a ''padder'' cap. I dont quiet understand how it works, expt that its got somethgin to do with the charectaristics of lc circuits, and it ''corrects'' them so they have the right charectaristics...or soemthgin like that.

Anythign wrong so far?

I'm thinking of building a simple superhet. I'm enclined towards a seperate triode oscillator for now, to keep things less fiddly and easier to debug...and a pentode or hexode mixer.

thanks...
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Old 17th Nov 2006, 9:30 am   #2
Mike Phelan
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Default Re: superhet

Sounds good, Adi - a few comments:
If you are using a hexode, it may as well be an ECH?? triode-hexode. Millions of circuits available. Easiest way to go.

If you go a different way, remember that the mixing has to be non-linear; to get the sum and difference of the two frequencies they have to be multiplied, not added.

The reason for the padder is for tracking; often they are adjustable.
C3 is in series with the LC cct, Ct in parallel.
At the HF end of the band, with C2 open, C3 has little effect but Ct great effect, so you adjust the latter; at the LF end, things are reversed - Ct has little effect and C3 a lot.
If C3 is fixed, you would align the coil at the LF end and Ct at the HF end.

Make sense?
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Old 17th Nov 2006, 1:23 pm   #3
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Default Re: superhet

Thanks.

A quick dig in the junk box turned up a couple of ECH81's a CCH35 and an X63, either of which could work i suppose. But i'm more enclined towards the ECH81 since they have a seperate triode oscillator, and are 9-pin and therefore easier to work with. Usually i would prefer octal valves for 1st prototypes since they are physically bigger, but for this 9 pin is easier i think because the valve holder can be soldered directly to metal, so I can use a recycled can for screening.

Any idea roughly what value the ''padder'' shoudl be? I think both trimmers C1 and C2 can be about 50-100pf, but the padder i dont know. In schematics i'v seen of real radios the padder is 470pf and is a fixed cap. but since i will probably be using homebrew (and therefore not ajustable) coils, an adjustable padder is needed.

Another thing that i'm not too sure about is IF frequancy. if i recycle IF transformers from a scrap radio, they woudl peak at the donor radio's IF, which is usually 445KHz

Am i right in assuming that radios use thesame IF sections for both SW and LW? So when you change bands the only thing that changes is the ariel and oscillator tuning coils? So in a SW superhet the IF is ALOT lower frequancy than the main frequancy.

So, for SW reception, the ariel and oscillator tuned circuits can have pretty much identicle coils, with the difference being adjusted out by the trimmers?

say if the signal recieved is 10MHz (10,000KHz), the ariel tuning coil would need to be tuned to 10,000KHz and the oscillator coil would need to be tuned to 9,555KHz. The difference is very small compared to the overall frequancy and therefore a small trimmer should be able to do it allright.

Sound right so far?

Thanks.
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Old 17th Nov 2006, 11:19 pm   #4
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Arrow Re: superhet

Hi Adi.

In a superhet - such as the type we have here - it is usual to have the local oscillator on the HIGH side of the required receive frequency. This is because it makes the ratio of aerial tuning freq. span to local osc. freq. span smaller than if the local osc. was on the low side.
This, in turn, makes the freq. difference (which is the IF, of course) error smaller, (errors between the sig. freq. tuned cct. and the osc. tuned cct. are known as "tracking errors") and so the output from the mixer stays closer to the nominal IF than would otherwise be the case - which for broadcast sets is usually in the range 450 Kc to 470 kc. [470 kc. was commonly used by Philips valve radios in the '50s. 450 / 455 kc. is very popular with sets of USA & Japanese origin. 465 kc. is the most common for UK radios]. There is no real reason to chose any particular nominal IF over any other in this range for this type of simple superhet.

Padder capacitor.
If the aerial tuned cct. and the oscillator tuned cct. were each simple L/C parallel tuned ccts., it would be possible to achieve a zero tracking error for 2 signal freqs. only. By introducing a padder capacitor in the oscillator circuit, it cuts down the swing of the local osc. tuning capacitor and produces 3 points that result in zero tracking error. A la, this is known as 3-point tracking. It is usual to aim at a zero tracking error towards the LF end of the band - usually set by adjustment of the "L" in the circuits, and another point towards the HF end of the band - adjusted by the "C" of the ccts. - hence the trimmers. The padder is usually not adjustable. Its value is calculated in advance to give a zero tracking point somewhere towards the centre of the band.
The net result is that between the LF point and the mid-band point, there is a positive tracking error, and between the mid-band point and the HF end there is a negative error. The skill of the design is to shuffle all the available variables (L, C and freqs.) to minimise the errors - but with 3-point tracking, they cannot be eliminated.
There is a lot more to superhet design than this - but I'm sure you'll agree with me that that lot will do for now!

Good luck.

/ Al.
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Old 18th Nov 2006, 2:03 pm   #5
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Default Re: superhet

Thanks.

So, if i was recieving a 10,000KHz signal with a 450KHz IF, than the local oscillator would be running at 10,450KHz. Then the IF would be Local frequancy minus main frequancy...right? So the difference of signals is used...not the sum.

Last edited by adibrook; 18th Nov 2006 at 2:28 pm.
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Old 18th Nov 2006, 11:55 pm   #6
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Wink Re: superhet

Hi Adi,

In a word, "yes"!

The IF is the difference between the local osc. freq. and the signal freq. - in most common broadcast receivers.
This difference can be produced two ways: the local osc. minus the signal freq., - as in our case - or the signal freq. minus the local osc. freq. If the local osc. freq. is above the signal freq., the local osc. is said to be on the "high" side (of the signal freq.) - and vice versa for the "low" side. As you've gathered, it is the so-called "mixer" in a superhet that performs this "arithmetic".

So, in your example, a signal being received on 10,000KHz with an IF of 465KHz will have the local osc. on (10,000 plus 465) KHz = 10,465KHz. But can you spot the snag here? Suppose there is another signal on 10,930KHz. In the mixer, this will mix with the local osc. signal at 10,465KHz and produce the difference of 10,930kHz (this second signal) minus 10,465KHz (the local osc.) = 465KHz. The same IF! As far as the IF amplifier is concerned, this is a perfectly "acceptable" signal - and it will amplify it and pass it on to the rest of the set - and you get two detected signals coming out of the loudspeaker - the one you want, and the one you don't!
In practice, the tuned circuit in the mixer grid will reject most of this unwanted signal, since it will be tuned to the freq. of the signal we want - but if the unwanted signal is very strong, it will "get through" - and cause the interference. As we go higher and higher in frequency, the selectivity of a simple tuned ciruit gets less and less. So there is a much greater danger of this problem above, say, 15 MHz than there is at 3 MHz.

This is the one major drawback to the superhet principle: it will always respond to two signals for one setting of the local osc. freq.

This unwanted signal is called the "image" frequency, and the usual way of reducing this problem in broadcast radios is to make the set more selective before the signals get to the mixer - and this is usually done by inserting additional tuned circuts - tuned to the freq. of the signal we want - in an amplifier before the mixer. This is usually called the RF amplifier stage - or RF amp. for short. You can always tell if a set has a RF amplifier: it will have a 3-section tuning capacitor: one for the RF amp., one for the mixer grid and one for the local oscillator. Trouble now is, of course, you've got the additional problem of making THREE tuned circuits "track" over the required frequency range of interest - but like they say, there ain't no such thing as a free lunch!

Advanced, communications-type receivers get a lot more complicated than all that - although they are still "superhets", and the arithmetic stays the same - but that can wait until another time!

Glad to be of assistance.

Al / G8DLH
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Old 19th Nov 2006, 5:58 pm   #7
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Default Re: superhet

Quote:
Originally Posted by G8DLH View Post
Advanced, communications-type receivers get a lot more complicated than all that - although they are still "superhets", and the arithmetic stays the same - but that can wait until another time!
After the study of an Boat Anchor Receiver we will make something different in the design of a homebrew all tube FM Radio. But this will make a lot of Fun and give a lot of experience. I will build one
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Old 19th Nov 2006, 10:26 pm   #8
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Question Re: superhet

Quote:
Originally Posted by 6AL5W View Post
After the study of an Boat Anchor Receiver we will make something different in the design of a homebrew all tube FM Radio. But this will make a lot of Fun and give a lot of experience. I will build one
Hi!
Just out of interest, what is the Boat Anchor Receiver that inspired you?
And again - just out of interest - what will be the significant differences in your homebrew design?

Al.
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Old 20th Nov 2006, 7:30 am   #9
6AL5W-Martin
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Default Re: superhet

hello Al,

i will make the homebrew Radio with a multiple stage mechanical adjust of the windings, there will not be an variable capacitor to do that. This is smaller in size and the part what is doing the adjust is self made, so it becomes exactly.
excuse my small english..

my Boatanchor is an Collins 51j4, this make the complete Tuning like that. I will post pictures to see what i mean.

Martin
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Old 20th Nov 2006, 8:29 am   #10
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Default Re: superhet

so, this is the pictures from restauration of my old 51j4.

51j4.
http://budoclub-biedenkopf.de/51j4/1.jpg

inside, not cleaned...
http://budoclub-biedenkopf.de/51j4/2.jpg

while cleaning the roll (with window cleaner, carefully, its only old paper with a rest of transparent paint)
http://budoclub-biedenkopf.de/51j4/3.jpg

front
http://budoclub-biedenkopf.de/51j4/4.jpg

this is the inspriation for the tuning of the FM Radio ;-)
http://budoclub-biedenkopf.de/51j4/5.jpg

downside of 51j4
http://budoclub-biedenkopf.de/51j4/6.jpg

it`s living.
http://budoclub-biedenkopf.de/51j4/7.jpg

the capacitor must be changed, problem in RF Gain Adjust
http://budoclub-biedenkopf.de/51j4/8.jpg

so you have seen my 51J4

Martin
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Old 20th Nov 2006, 9:04 am   #11
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Default Re: superhet

Quote:
Originally Posted by G8DLH View Post
Hi Adi.

In a superhet - such as the type we have here ... There is a lot more to superhet design than this - but I'm sure you'll agree with me that that lot will do for now!

Good luck.

/ Al.
That is a really lucid explanation, Al, and much better than I could have supplied!
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Old 20th Nov 2006, 7:16 pm   #12
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Thumbs up Re: superhet

Quote:
Originally Posted by Mike Phelan View Post
That is a really lucid explanation, Al, and much better than I could have supplied!
Thank you.
In previous employment, I used to be a technical lecturer / instructor. If I had my time again, I'd go into the teaching profession - maths and physics. Too old to change now.

Aside:
When I was a kiddie just starting out in the radio / electronics "game", there was no-one I could turn to for help. With all due respect to Adi - and not wishing to sound condescending - I think that I can feel "where he's at - and going to" - so I simply want to help him on his way. The most satisfying return that a teacher can receive is finding an enthusiastic & inquisitive pupil.

Al.
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Old 20th Nov 2006, 7:19 pm   #13
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Thumbs up Re: superhet

Quote:
Originally Posted by 6AL5W View Post
so, this is the pictures from restauration of my old 51j4.
so you have seen my 51J4

Martin
And a very nice looking radio too!
I've never seen one before.
Thank you.

/ Al.
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Old 20th Nov 2006, 7:33 pm   #14
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Thumbs up Re: superhet

Quote:
Originally Posted by adibrook View Post
I recently finished another short wave regen, this time using a lot of screening (lots of recycled bean cans and coax) which works quite well. It's a lot more stable because of the extensive screening used everywhere.
Screening: indeed. There are probably more problems generated in home-brew r.f. designs by lack of screening (which, per se, includes layout) than any other cause, with (bad / inadequate) decoupling (including earthing) arrangements coming in as a close second.

Keep up the good work!

Al.
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Old 21st Nov 2006, 6:31 am   #15
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Default Re: superhet

Al so you know what I want to do... mechanical adjust in the coils.
I want to have a Antenna Amp Stage with 2 Coils Input, 1 Coil in its output, a oscillator ( may be not only one coil filter, the signal must be very clean from foreign things, so i can use also multiple stage coil filter there), the IF Stages will be fixed at 10MHz, at last Ratio Detector, following a small Audio Amp 6AQ5 or like that May be a little magic eye for tuning beside.
So i think there will be 5 mechanical moved coil filters connected to the tuning knob. The hole radio must be small, like "Philetta", and homebrewed case of coarse also. A lot of work, but this work is the enjoyment.
As tubes, 7pin Miniatures.

Martin
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Old 21st Nov 2006, 7:46 pm   #16
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Default Re: superhet

Thanks for all the info. I appreciate the help. As you said i am only startting out when it comes to RF stuff.

I found two IF cans from thesame radio. I cant remember what theyre from....but i think maybe my Philips radio which i scrapped.

I housed each valve in a mini shassis made from a recycled food can, and connected the ''shassis'' together with coax (only earthed on one side to prevent loops?)

However, so far all i have is a very stable psu and audio amp (EF91 and 6BW6). The amp is the stabelest i'v built for a while..

Ok...now back to some theory. I'v been looking at some block diagrams, and from what i can see a comms reciever and a broadcast reciever only differ in the detecor.

A broadcast reciever has a diode, and a comms reciever has a ''product detector'' with a BFO.

Of course i understand that in reality a comms reciever would be more comlicated than a broadcast reciever...with tighter tuning better circuitry less cost cutting etc. But basically they seem simular.

So, what exactly is the product detector? I knwo what thw BFO is (its for resolving SSB...right?) but what does the product detector do with the IF and BFO signals? I guess its some kind of mixer?

Thanks.
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Old 21st Nov 2006, 11:17 pm   #17
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Default Re: superhet

Hi Adi.

Seems to me that we are starting to drift away from the OP and into the area of comms. receivers: design and features of.
To avoid being accused of going OT, I'm going to open a new Thread in "Everything Else" entitled "What is a Comms. Receiver?" - and try to answer your questions therein.

See you there.

Al.
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Old 29th Nov 2006, 10:39 am   #18
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Default Re: superhet

Quote:
Originally Posted by adibrook View Post
i'v been thinking of havign a go at a simple superhet..
I start now to build a fine little FM Superhet, with homebrew Filters and handwired - all tubes design. This will be a lot of fun, experience and also a great field for making changes and difficults. It comes as complete "how to do", with a lot of pictures, plan and instructions. fine for beginning
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Old 29th Nov 2006, 9:13 pm   #19
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Default Re: superhet

Hi , not sure of their availability in germany of if it should be Philips rather than Mullard, but in the UK Mullard produced a lot of 1 page application notes in the 50's and 60's on valve circuits in radio and TV. These were usally just a single stage circuit such as an IF amp or a discriminator.
Their "Mullard Valve Data Manual" (TV &FM), publishedin 1958 has most of these sheets as well as much other information. It covers a lot of the design of receivers

Ed
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