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Author Topic: IF Transformers  (Read 3227 times)
KE7VHG
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Posts: 3




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« on: September 09, 2008, 10:32:19 PM »

I have several boxes of IF transformers with house part numbers on them.  I have tested each type and determined that I have some 455 KHz, and 10 MHz units.  I only have one type of 455 KHz unit, which is the one I happen to need right now.  I want to make a three stage 455 KHz IF section, but I only have the one type which is at that frequency.   I think that normally there are three slug colors used in a broadcast band receiver but what really is the difference in each of the three transformers? Is the color just for identification of the stage number?  They all seem to be tuned to 455.   What gives?

Thank you.

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AC5UP
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Posts: 3835




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« Reply #1 on: September 10, 2008, 03:37:08 AM »

If you're talking about the miniature shielded coils typically seen in Japanese radios over the past 20+ years, the AM oscillator is the red slug, the AM IF can(s) use yellow slugs and the FM IF can(s) are white.

With that said, I pulled out an analog AM/FM tuner & stereo demod pc board assembly I ratholed 25+ years ago and it's like a rainbow on the slugs... The FM oscillator slug is white, there is a dark green slug between the AM oscillator and IF (first IF matching transformer?), near the front end I see an orange slug I have no clue about. The FM IF's appear to be the light blue jobbies near the dark blue and black slug coils. The FM multiplex section has two larger cans in orange and medium green that are likely the 38 & 19 kHz traps while the Siamese twin cans with black slugs are probably the FM detector coils.

Bottom line here is there may be a color standard I'm not aware of that's universal across the industry, but judging by the example I'm looking at I'd suspect it's a case of the coil manufacturer(s) color coding by frequency more than function.

(?)

BTW: If you have a local thrift shop they tend to be a good source of generic radio parts. Locally the going rate for an AM/FM clock radio is a dollar or two depending on condition. Sometimes I see older stuff with double tuned IF cans, but they're becoming scarce.
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WA3SKN
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Posts: 5457




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« Reply #2 on: September 10, 2008, 04:07:30 AM »

I would say "How old are your parts?".
There may be a color code now, but I know I have parts with NO color code on them!  back in "The Good Old Days" there was no color code, then each mfgr had their own color code, now maybe there is an agreed to color code... but I have been out-of-the-loop too long!
I can only recommend hooking up the (grid?) dip meter and checking for the tuned frequency, or inductance of the coil.
73s.

-Mike.
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KE7VHG
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Posts: 3




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« Reply #3 on: September 10, 2008, 08:27:26 AM »

I looked at an AM only receiver I have here and in operation the Freq Counter and scope show three IF stages, and of course a LO (red).  Two of the IF stages are yellow and one is black(not painted) but all three are definitely 455.  I guess maybe the black one has different response skirts or something. I believe I'll build my if strip with identical cans (since I have them on hand anyway) and see how it goes.  My only apprehension is that I may end up with too steep of skirts and smash the audio range to a tiny thread; but I suppose I could stagger tune the coils a bit.

BTW I did not mention this before but I'm helping my son with a school project.  We are making a home brew Superhet radio with hopefully fairly decent selectivity.

Thanks everyone, your input was very enlightening.

73,
-Keith
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WB6BYU
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Posts: 13143




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« Reply #4 on: September 10, 2008, 10:04:53 AM »

Generally the difference is the turns ratio and/or the
impedance that the coil is designed to work into.  The
last IF coil is usually intended to drive a diode detector
and has a lower output impedance than the others.

If you are building your own IF strip you can design it
to match the impedances of the transformers you have.  I'd
start by driving the low impedance side with a known
signal level and measuring the output side with a scope
to get an estimate of the turns/impedance ratio.  Typically
these are designed for bipolar transistor circuits, with
impedances suitable for collector and base circuits.

For example, if you go to this page from the Mouser catalog:

http://www.mouser.com/catalog/635/982.pdf

it shows many of the transformers available from Xicon.
Most have primary impedance in the tens of thousands
of ohms and secondaries in the 500 to 5K ohm range.


One approach you might try is to use the transformers
to step up the voltage and FET stages as impedance
transformers between them:  apply the signal to the
secondary and feed the primary to the gate of a FET.
The secondary of the next transformer goes in the drain
lead of the FET.  Because of the low drain load the FET
itself doesn't provide much voltage gain and the circuit
can be quite stable.
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KE7VHG
Member

Posts: 3




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« Reply #5 on: September 10, 2008, 02:21:14 PM »

Good thought on the FETs, I was planning on some 2N2222s but i have some 2N3823 FETs around I could try it with. I could then amplify the filtered output with a coil and BJT to boost the gain before detection.

Thanks for the ideas.

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W9GB
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Posts: 2616




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« Reply #6 on: March 16, 2013, 11:07:21 PM »

Quote from: KE7VHG
Is there a color code for each IF stage?
Each IF transformers is self resonant with an impedance max at predefined frequency.
The resonant frequency can be adjusted by turning the colored ferrite core.
http://hem.passagen.se/communication/ifcan.html

In an ordinary radio, you will most often find 4 types of IF-cans.
For the FM section, the IF frequency is 10.7MHz.  
The color of the slug in this CAN is most often PINK.

For the AM section, the IF frequency is 455kHz.
RED - Oscillator. With 30pf - 300pf = 1MHz to 2MHz
YELLOW - First 455KHz IF filter transformer
White - Second 455KHz IF filter transformer (not always used)
Black - Last 455KHz IF filter transformer
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KB1GMX
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Posts: 760




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« Reply #7 on: March 17, 2013, 08:31:50 AM »

This is for traditional AM bandwidth (6-12khz) IFs at 455khz using transistors.
That is to say the secondary winding are optimized for the base of a transistor
running in the under 1ma range.

For the AM section, the IF frequency is 455kHz.
RED - Oscillator. With 30pf - 300pf = 1MHz to 2MHz

YELLOW - First 455KHz IF filter transformer; matching secondary for the 1st if.

White - Second 455KHz IF filter transformer (not always used);  usually lighter loading
for selectivity. also has primary tap for neutralization.

Black - Last 455KHz IF filter transformer; closer coupled for the detector diode.

What makes the 455khz IF cans differnt is the secondary winding may have more
or less turns on it to match the next stage and the primary will have a different tap
for feedback (neutralization) and collector impedance matching.  If you do not need
or use those features they are effective identical.

If you are using FET IF stages then you do not use the secondary as the fet would
like to see a higher impedance to get decent gain.   If you need narrower or more
controlled bandwidth just use multiple "cans" as top coupled filter.  Any one of the three
(Y/W/B) will work for that as its only the primary used.


Allison
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