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Old 9th Apr 2018, 8:20 pm   #1
G6Tanuki
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Default Quick-and-dirty 1MHz test oscillator.

Like most of you, I've got a few decent crystal-controlled 1MHz oscillators whose harmonics I can trim to zero-beat with WWV and the like - but recently came across a 'packaged' solution which rather more easily fits the bill of a quick-and-dirty oscillator I can take around with me which provides 1MHz-harmonics for general purpose testing of amateur-radio stuff, car-radios, FM tuners etc.

The RS 478-5749 being the heart of it.

https://uk.rs-online.com/web/p/cryst...ators/4785749/

Way cheaper than a traditional 1MHz rock, and it's got the necessary drive-circuitry included. Feed it 5V and get your 1MHz 5V pk-pk squarewave a couple of pins further down.

OK, it's got no way of 'trimming' the frequency so you can't zero it with WWV but for 3.23 it's a lot of value in a small package. Mine I've fitted on a square-inch of Veroboard along with four 1.2V AA-size Nicads of questionable age/chargeability: even running from these two-decades-old cells I can hear harmonics of this little beastie up to 175MHz.
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Old 9th Apr 2018, 10:42 pm   #2
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Default Re: Quick-and-dirty 1MHz test oscillator.

Quote:
Originally Posted by G6Tanuki View Post
fits the bill of a quick-and-dirty oscillator I can take around with me which provides 1MHz-harmonics for general purpose testing of amateur-radio stuff, car-radios, FM tuners etc.
On a weight-for-weight basis, or for its power requirements, I'd say it compares fairly well against a Class D wavemeter in its transit case .

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Old 9th Apr 2018, 11:15 pm   #3
Dave757
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Default Re: Quick-and-dirty 1MHz test oscillator.

Hi,

If you inserted a transformer into the 5v feed and coupled audio into it,
you would have a little pantry Tx in the MW band on 300m. Of course you would
need to reduce the operating voltage to get the output down to sig gen level.

Kind regards
Dave
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Old 10th Apr 2018, 3:42 am   #4
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Default Re: Quick-and-dirty 1MHz test oscillator.

As long as it is high in frequency, you can trim it to some extent by reducing the supply voltage.
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Old 10th Apr 2018, 5:27 pm   #5
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Default Re: Quick-and-dirty 1MHz test oscillator.

Quote:
If you inserted a transformer into the 5v feed and coupled audio into it,
you would have a little pantry Tx in the MW band on 300m.
I perceive this as leading towards the construction of a source of amplitude-modulated 1MHz CMOS-level pulses. If so, I hope most sincerely that anybody following up on the concept will give further consideration to dynamic stability under varying supply conditions, screening, output bandpass filtering etc. The implied use of the "T" word in this context is a bit of an eyebrow raiser, to say the least ...
Best wishes
Guy
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Last edited by Nymrod121; 10th Apr 2018 at 5:29 pm. Reason: consulting F.(2)
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Old 11th Apr 2018, 9:46 pm   #6
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Default Re: Quick-and-dirty 1MHz test oscillator.

Quote:
Originally Posted by Dave757 View Post
If you inserted a transformer into the 5v feed and coupled audio into it,
you would have a little pantry Tx in the MW band on 300m.
I'd bet it would end up fm'ing all over the place.
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Old 12th Apr 2018, 8:36 am   #7
G6Tanuki
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Default Re: Quick-and-dirty 1MHz test oscillator.

I'm going to be investigating the extent to which it can be 'pulled' by varying the supply-voltage, also what effect different load-impedances have.

As something that - fitted in a diddy little box along with a PP3 and a 7805 - provides me with a handy and portable way to quickly 'ping' a radio to see if it receives anything and get a guess at the calibration accuracy, this little chip looks like it has great price/performance/pocketability!
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Old 12th Apr 2018, 10:08 am   #8
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Default Re: Quick-and-dirty 1MHz test oscillator.

Lots of nice harmonics off a square wave as well meaning it will mark every 1MHz.
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Old 12th Apr 2018, 10:37 am   #9
G6Tanuki
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Default Re: Quick-and-dirty 1MHz test oscillator.

Quote:
Originally Posted by MrBungle View Post
Lots of nice harmonics off a square wave as well meaning it will mark every 1MHz.
Indeed: I may look at possibly adding a VHF amplifier to increase the harmonic-strength above 100MHz or so...

{Awaiting attention I have a Hallicrafters S27 receiver that will need a lot of testing, so nice easy-to-access 1MHz 'pips' above 100MHz will be rather handy}.
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Old 21st Apr 2018, 11:17 am   #10
G6Tanuki
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Default Re: Quick-and-dirty 1MHz test oscillator.

I kludged together a circuit and have the little thing oscillating away to itself quite happily. Seems to be about 40Hz low of 1MHz, which I find quite astounding considering there's no means of trimming it.

Harmonics detectable to 30MHz on my RACAL RA217D, but they're much weaker at 145MHz.

I'm thinking of building a MMIC-based broadband amplifier to see if that brings the VHF harmonics up at all.

I will also do some experimentation [heating with a hairdryer!] to test the thermal stability: I have in the past known soldering crystal-leadouts to cause an irreversible frequency-shift (which is a pain when you bought a job-lot of rocks then carefully selected the frequencies of the xtals for an 8-pole ladder filter only to find that after soldering some of them have wandered...)
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