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Author Topic: Can an I.F. signal be used like a local oscillator?  (Read 7402 times)
DXTUNER
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Posts: 72




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« Reply #15 on: May 13, 2013, 09:21:31 PM »

In order to get it to work properly, you need to make it degenerative (not enough gain to oscillate) on the fundamental and 3rd overtone around 69 MHz (only odd overtones work) but with enough feedback on the 5th overtone that it oscillates. When you can hear a signal near 115 MHz but NOT near 23 or 69 MHz, then your oscillator is probably working correctly.

Thanks a million for the tip, you explain that very clearly. I'm going to check those areas tomorrow night, before I do anything else.
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DXTUNER
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« Reply #16 on: May 14, 2013, 04:31:44 PM »

Ok, WB6BYU, here's my results:

-- A strong signal at 23 MHz (technically, 22.980 MHz). So strong at this frequency that it contained a rapid, jackhammer-like 'pulse'.
-- A fair-strong signal at 45.960 MHz, we can just call this 46 MHz.
-- A fair-strong signal at 68.935 MHz, considered 69 MHz for the simple purpose.

Here's a kicker: A really strong signal at 160.855 MHz, actually its stronger than the 115 MHz (114.980 MHz) I get. I'd have to assume it's a result of the 115 MHz (oscillation?) plus a 45 MHz I.F. in my scanner. But I'm just guessing here.

The 22.980 MHz signal was copied on my Tecsun PL-600 portable SW radio. In addition there was an identical signal at 22.065 MHz, using the same crystal, just not quite as strong. I'll assume it's the 22.980 MHz minus 2 multiples of that radio's 455 kHz I.F. Again, that's just a guess.

Ok, so are you suggesting that the crystal isn't good enough, or that the circuit isn't, because it doesn't suppress the 69 MHz oscillation? Just so you know, the crystal I'm using (marked "115") I pulled from an old 115 MHz VHF converter. I hope to hear back from you before I proceed expanding the circuit.
« Last Edit: May 14, 2013, 05:03:50 PM by DXTUNER » Logged
WB6BYU
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Posts: 13332




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« Reply #17 on: May 14, 2013, 08:18:27 PM »

Yes, I think your crystal is oscillating on the fundamental frequency of 23 MHz.  You'll
still hear the other harmonics, including the one at 115 MHz. 

The frequency is probably off slightly because the crystal is operating in a fundamental
mode rather than an overtone.

For proper operation you need to have it oscillating on the 5th overtone at 115 MHz: 
you should NOT hear a signal at 23 or 69 MHz in that condition.  That's not to say that
the crystal is bad:  overtone oscillators are not always trivial to adjust.  I think you just
need to work on the oscillator a bit to get it to work on the proper overtone.


And, yes, I think the 22.065 MHz response is an image in the receiver:  I've been fooled
by that before when I was trying to adjust one of my rigs to the 10m band, and ended
up with the VFO off by 910 kHz!
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DXTUNER
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Posts: 72




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« Reply #18 on: May 14, 2013, 08:56:48 PM »

Thanks. Well, I could try that Butler 5th overtone circuit, but I'm reluctant to. Its more complex than I desire right now. Because, if it doesn't work for some reason, then I'll have to back-track through 20 parts, as opposed to, say, five. If you follow me. Like last night, for example, when I was playing with that simple test circuit, it didn't work/oscillate at first, so I went for the obvious suspect: the capacitor across C-E. I changed the value and then it worked.

I think I've seen simple overtone circuits where a tank is put at the collector, resonant to the overtone that's wanted. Perhaps I'll try something like that next.
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WB6BYU
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« Reply #19 on: May 14, 2013, 09:37:35 PM »

Simplicity is not always determined by parts count in a circuit:  often what is more
important is the reliability with which the circuit will operate without needing adjustments.

With tube circuits there was always an effort to squeeze maximum performance out
of as few stages as possible, because each extra stage not only added the cost of the
tube, but also the current draw for the filament and plate.  But with transistors there
is little need for that - adding an extra 2 transistors really doesn't cost much.

We particularly see this with transistor biasing:  initial circuits used just a high value
resistor in the base circuit, but it needed to be adjusted for each specific transistor
due to the range of characteristics.  Many modern circuits use 3 resistors:  one in the
emitter and two as a voltage divider on the base.  That might seem like more
complexity than necessary, but it allows the transistor current to be set fairly accurately
over a wide range of devices without requiring any adjustment.

The same is true of overtone oscillators:  I have a lot of different circuits to choose
from.  One problem with the "simple" ones is that they often depend on parasitic
capacitance or other stray coupling for proper feedback:  if you use a different type
of transistor it might not have the same characteristics.  I've discovered over time
that my circuits work more consistently when I used explicit components rather than
relying on circuit strays, as that allows me to adjust them if needed.  And again, it
makes the circuit more likely to work.

One thing I like about the Butler is the ability to short out the crystal and adjust
the tuned circuit until you get a signal on about the desired frequency:  that makes
it easier to adjust than just fiddling with bits here and there to see what happens.

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DXTUNER
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Posts: 72




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« Reply #20 on: May 15, 2013, 05:34:24 AM »

Understood. Thank you.
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WB6BYU
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Posts: 13332




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« Reply #21 on: May 15, 2013, 07:49:06 AM »

Actually, from looking at the original oscillator circuit again it might be working
as a harmonic oscillator rather than an overtone oscillator.

A harmonic oscillator is designed to oscillate on the fundamental, with one
or more tuned circuits that extract just the desired harmonic from the original
signal.  That would mean that it really is oscillating on 23 MHz, but the final
tuned circuit is tuned to 115 MHz to emphasize that frequency to be sent
on the the mixer.  Harmonic oscillators are usually easier to get going (though
the tuning of the output network has to be set properly to get the right
output) but the output is generally lower and all the other harmonics from
the fundamental frequency will also show up in the output to some degree
(which can lead to extraneous responses.)

The output stage is tuned by the 35pf trimmer capacitor and L4.  If you've reduced
the capacitor to get the circuit to oscillate, then the tuned circuit probably is not
resonant at 115 MHz, and you wont' get the proper mixer injection.  It may be that
if the circuit is tuned properly you would actually get overtone operation.


The test oscillator circuit is clearly a fundamental oscillator - it contains no tuned
circuits.  You need some sort of tuning in an overtone oscillator because the gain
is generally higher at the fundamental frequency than at the desired overtone,
and some sort of frequency selectivity is required to restrict oscillation only to
the overtone.  With a harmonic oscillator you need a way of extracting just the
desired harmonic.  So either approach will require a tuned circuit of some sort.

A 2N2222 probably isn't the best transistor in this application:  it has a typical fT of
300MHz, and a good rule of thumb is to use a device up to 1/10 of the fT, though
they can be pressed into service at higher frequencies.  I've found some samples
of 2N2222A that will oscillate as high as 150 MHz, but a proper VHF transistor
such as 2N5179, 2N3563, or MPS-H10 would be more likely to give good performance.
Look for something with an fT of at least 600 MHz.

Common FETs such as the MPF-102 or 2N3819 will also work, and any of the
oscillator circuits (including the Butler) can be modified to use FETs instead of
bipolar transistors with a savings of a bias resistor or two.
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DXTUNER
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Posts: 72




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« Reply #22 on: May 15, 2013, 03:46:39 PM »

A 2N2222 probably isn't the best transistor in this application:  it has a typical fT of 300MHz, and a good rule of thumb is to use a device up to 1/10 of the fT, though they can be pressed into service at higher frequencies.  I've found some samples of 2N2222A that will oscillate as high as 150 MHz, but a proper VHF transistor such as 2N5179, 2N3563, or MPS-H10 would be more likely to give good performance. Look for something with an fT of at least 600 MHz.

Yeah, I know. It was either the 2N2222 or a 2N4401, but the latter has a lower fT so I chose the former. I have some J310's, which I know can operate in the VHF range, but so far I've only had success with BJT's, so that's what I used in the test circuit/oscillator. Tonight, however, I'm going to try this FET overtone oscillator, and see what it does for me. Plus its simple. Anyone who knows me, and works with me, knows that I like simple:
(see Fig. 7-5)

http://www.ve7zsa.net/technical/advsgtxt/c7oscillator_b1_r00.htm

I'm going to settle on a oscillator soon, don't want to stay mired in one stage for an uneccessary length of time, and then move on to building the next stage.
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DXTUNER
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Posts: 72




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« Reply #23 on: May 15, 2013, 05:53:24 PM »

Nothing. Zilch. Words can't describe how much I hate FET's. The BJT's might be considered lowly, just for beginners and rubes, but at least I get *something* from them. I'm going back to that simple 2N2222 circuit, put the 5th overtone tank on that collector, and just go from there.
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WB6BYU
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« Reply #24 on: May 15, 2013, 08:49:28 PM »

I'll be happy to send you some VHF bipolar transistors if you need some.  Most
are still packed up from our move, but I'm sure I can dig up something useful.
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DXTUNER
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Posts: 72




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« Reply #25 on: May 16, 2013, 05:47:20 PM »

Wow, that would be fantastic. I really appreciate it. If you like you can e-mail me at lanbergld@vcu.edu.

I'm too busy tonight to mess with the circuit much. But I have a question, if you don't mind: I just got a batch of small parts, including crystals. Can a 25 MHz crystal (I assume its fundamental) be used as an overtone crystal? Say at 5th overtone, 125 MHz? Or does the crystal have to be an overtone crystal? I read something recently that seemed to say a fundamental crystal can't be used at an overtone, but I'm not clear about that. Thank you.
« Last Edit: May 16, 2013, 05:51:23 PM by DXTUNER » Logged
WB6BYU
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Posts: 13332




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« Reply #26 on: May 16, 2013, 06:46:16 PM »

Quote from: DXTUNER

Can a 25 MHz crystal (I assume its fundamental) be used as an overtone crystal? Say at 5th overtone, 125 MHz?



Maybe...

In theory, any crystal can be operated on one of its overtones.  The overtones aren't
exactly on the harmonics - they are mechanical vibrations.  So if you order a 125 MHz overtone
crystal the fundamental won't be exactly on 25 MHz:  they will grind it so the overtone
is on the right frequency.

Sometimes, however, a crystal can be damage or have limited activity (overtone oscillation
requires more feedback).  It might be mounted in such a way that the vibrations are
dampened, etc.  So not every crystal will operate on an overtone (or operate easily),
but many crystals will, even when designed for fundamental operation.  As you get to
the higher overtones (5th, 7th, etc.) it becomes more difficult to get a crystal to oscillate
properly unless it is specifically designed for such use

While you can find some specialty crystals with fundamentals as high as 50 MHz, generally
crystals over about 24 to 25 MHz will be overtone types.  So your 25 MHz crystal might
be a 3rd overtone - you'll have to try it and see.
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DXTUNER
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Posts: 72




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« Reply #27 on: May 16, 2013, 07:53:01 PM »

Ok, I follow you on that. Thanks for explaining it so clearly.
« Last Edit: May 16, 2013, 07:55:19 PM by DXTUNER » Logged
KB1WSY
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Posts: 802




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« Reply #28 on: May 18, 2013, 08:12:11 AM »

Sounds like you have abandoned the idea of using the cable box for IF oscillator. FWIW, I found that the broadband cable modem that happened to be in my shack (it's where the cable comes in from the outside) generated wide-spectrum noise (plus one-frequency images) that virtually blanked out ham HF reception on several bands. In the end I had to lengthen the incoming cableco coax so that I could put the modem in another place a couple of rooms away.

73 de Martin, KB1WSY
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DXTUNER
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Posts: 72




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« Reply #29 on: May 18, 2013, 12:06:30 PM »

Hello Martin,

Yes, I abandoned the cable box idea, and have commenced trying to build a VHF converter using a crystal oscillator, on a small plug in board. I'm getting somewhere. I can now get NOAA Weather radio (162.475 MHz) near 10-meters on my portable shortwave radio, but so far that's it. I think I need to improve the mixer portion.

I'm just making a basic rf input/l.o./mixer type of configuration. I don't have the experience or parts to get too fancy right now.
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