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 Post subject: Simple modulated crystal controlled 455 kHz oscillator...
PostPosted: Jan Mon 15, 2018 3:03 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5281
Location: Indianapolis, IN
Hello All,

Had been wanting to build a crystal controlled 455 kHz oscillator for alignment for quite some time. Found several circuits on-line and originally was going to pick the best features from a couple and combine them for my own---then ran across this basic circuit which was posted by a few different people with minor variations and decided to further modify/simplify for my needs and to use parts I had on hand (had recently purchased ceramic resonator for another design). Borrowed schematic from one of those posts and modified using Paint to show parts that I used and removing others which were unnecessary. I'm also using a good quality 12VDC regulated power supply module instead of 9V battery called for in original.

Ran out of time today to perform alignment, but did check with a well-aligned Zenith Royal 705 and the nice clean tone came through well when output wire brought next to radio. Am sure looking forward to using it !

Modified schematic and pic below. Plan is to eventually mount inside a metal enclosure...

John


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ARF 455 kHZ IF ALIGN OSC SCHEM.jpg
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ARF 455 kHZ IF ALIGN OSC TOP.jpg
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 Post subject: Re: Simple modulated crystal controlled 455 kHz oscillator..
PostPosted: Jan Mon 15, 2018 5:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36523
Location: Livermore, CA
John

Everyone should use something like this for IF alignment. Sure helps speeding up the procedure.

I made a similar tester with a 4 gate CMOS chip. This made a 455 ceramic resonator oscillator and audio oscillator. I was looking for a way to give complete isolation between tester and radio. Marc suggested using an IF Transformer on the output.

Unit operates off a 9 volt battery. In use you will find there isn't much difference between 455 and 465 kHz. Almost no effect on dial calibration.

To be exact ceramic resonators can be shifted in frequency by changing capacitor values.

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Norm


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 Post subject: Re: Simple modulated crystal controlled 455 kHz oscillator..
PostPosted: Jan Mon 15, 2018 3:11 pm 
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Joined: Aug Wed 26, 2009 1:50 am
Posts: 1350
Location: Mount Pleasant UT 84647 (from Coventry, UK)
That looks like a useful tool. Takes up a lot less space on the bench than a signal generator.


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 Post subject: Re: Simple modulated crystal controlled 455 kHz oscillator..
PostPosted: Jan Tue 16, 2018 3:42 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5281
Location: Indianapolis, IN
Thanks Guys !

Norm---Is that the IC circuit which uses a 455 kHz ceramic filter in output stage and includes a trim-cap in resonator leg ? If so, that is one that I was considering... Would think that the IF transformer would be a good solution for isolation and most of us would have at least a couple around to play with.

Colin---wanted to keep it as small as possible, but left a bit of space on board for modification. Am planning on mounting inside a small metal box, possibly with a BNC connector for output. Still haven't tried it, so don't yet know if the output will need attenuation when coupled to radio. Am gearing up to do that 66T1 as soon as possible...

John


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 Post subject: Re: Simple modulated crystal controlled 455 kHz oscillator..
PostPosted: Jan Wed 17, 2018 2:07 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5281
Location: Indianapolis, IN
Since I don't have a freq. counter, took this to work today to check frequency on the calibrated lab scope with measurement capability before using---modulated output read between 453 & 457 kHz... Think it'll do...

John


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 Post subject: Re: Simple modulated crystal controlled 455 kHz oscillator..
PostPosted: Jan Wed 17, 2018 3:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2160
Location: Saskatoon
John,
Your RF frequency is probably accurate to within less than a kiloHertz, but the audio modulation would likely confuse the frequency counter, thus giving a reading that's slightly off. Your circuit should work fine as is.

For the most part you don't need to worry about trimming the resonator, as long as you use the recommended fixed value loading capacitors. That will give you resonator's design frequency to a better accuracy than you'll ever need for radio alignment work.


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 Post subject: Re: Simple modulated crystal controlled 455 kHz oscillator..
PostPosted: Jan Thu 18, 2018 2:08 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5281
Location: Indianapolis, IN
Thanks Bob---that's just what I thought when observing the waveform on scope. The original circuit I found called for 68 pF caps, but have seen others using 100 pF---had two new "dipped" 82 pF silver-mica in drawer, so gave them a try. Unit works perfectly---added an 18 ohm resistor across output temporarily to align my Motorola 66T1 tonight to attenuate signal to usable level. Will probably use a 100 ohm pot when I find suitable enclosure...

John


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