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 Local Oscillator
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 Return to top of page · Post #: 1 · Written at 2:05:43 PM on 29 October 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Hi Guys,

Since a superheterodyne likes to add and subtract frequencies, I was wondering how one might go about accessing an LO signal?

I would like to count the frequency of the LO to derive the tuned radio station frequency from it, by counting the time between kHz osc cycles with a faster processor.

I imagine this is a signal you might look at with a scope if you suspected the amp was broken, but to check the radio still works.

Could this be made useable at a 5 Volt logic level to read the frequency? Transistor buffered output to the logic PCB maybe?

Cheers, Brek.



 
 Return to top of page · Post #: 2 · Written at 9:32:29 AM on 30 October 2013.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

A guy in our radio club, bought a frequency counter module that he put on a device he made.

That is really all you need.

Coupling would be the issue as you do not want to overly load the Osc. Valves like 6J8 with separate exciter, I would hope, be less prone to problems.

Marc


 
 Return to top of page · Post #: 3 · Written at 10:35:15 AM on 30 October 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Hi Smile
The frequency counter is the part I can make,
with the output already subtracted the intermediate freq.
to display the tuned freq (which is the goal).

The question is more about connecting to the radio without interfering with it.

Was your friend's radio happy to run with the device
connected,
and not mess up the station it's tuned to?


 
 Return to top of page · Post #: 4 · Written at 1:14:54 PM on 30 October 2013.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 830

A cathode follower as a buffer should work. IIRC, the waveform developed on the cathode of a 6BE6 or similar tube is about enough amplitude to feed a TTL gate, once buffered. The cathode follower should be high enough impedance to have little effect on the oscillator frequency.


 
 Return to top of page · Post #: 5 · Written at 5:59:04 PM on 30 October 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Ok, thanks.
I am maybe a bit ahead of myself with the radio not ready
yet, but this is something I can look into now.


 
 Return to top of page · Post #: 6 · Written at 6:03:44 PM on 30 October 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

I thought it might be worth buying one of these and junking it: http://www.jaycar.com.au/productView.asp?ID=AR1741.

They have an LCD display, but still have a tuning gang (variable cap)
doing the tuning, and there are no buttons for tuning either.
However, the radio says it's digital, suggesting the oscs
are synthesised, and the radio isn't going to teach anything.

There is nothing squaring off the wave if I'm correct...
all clocks would still be a sine wave wherever you looked at them (for vintage radio).


 
 Return to top of page · Post #: 7 · Written at 10:38:34 PM on 30 October 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

The radio tuner at it's earliest stage has to be an oscillator,
if it wasn't oscillating, it wouldn't be tuned to a frequency.

I never understood the cyclic nature of the circuit working
like software does, very fast and very procedural.
It's like a revelation, because I made radios, but didn't
understand what was going on, even though a crystal radio
is the simplest oscillator and rectifier.

So it is the charge discharge cycle of the tuner capacitor in
the simplest radio that is running the show really,
the diode is an overrated sidekick just to produce sound.



 
 Return to top of page · Post #: 8 · Written at 2:35:19 PM on 6 November 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Yes,
It's still just an analogue radio with a digital frequency counter.

It's impressive for a $25 radio (the $15 one is discontinued), not what I expected at all:


Image Link

Image Link


 
 Return to top of page · Post #: 9 · Written at 7:58:01 PM on 6 November 2013.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

Crystal set is tuned radio frequency (TRF) there is no oscillator.

You can derive a square wave by using the sine wave to switch a transistor. I have to remember where I saw the circuit within a myriad of books, but it was basic.

I still think just buying the module only is the best bet. There are no clues in some of these radio's as to how the frequency is synthesized, or the counter fed. The other way you may be buying a Pig in Poke.

Why go complex.

Marc


 
 Return to top of page · Post #: 10 · Written at 10:49:27 PM on 6 November 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

I bought this one just out of curiosity.
The frequency counting I want to do with a micro controller.

I tried to flatten this one out on a breadboard,
but my iron actually clapped out part way through!

Image Link


 
 Return to top of page · Post #: 11 · Written at 5:50:03 PM on 6 December 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Not having much luck.
I used a BC549C to buffer the LO signal from my cheap transistor radio,
and the emitter output is still good witht he freq counter,
but not enough to light an LED.
Then I fed that output into a 7404, maybe they are too slow,
it seems I can use resistors to make the output stay on or off
when connected to LED, but the freq counter doesn't run from that output.

I was trying to run an LED to use to opto-couple the LO to an isolated circuit...
otherwise there is noise from the counter.


 
 Return to top of page · Post #: 12 · Written at 10:40:26 AM on 7 December 2013.
Maven's Gravatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

Are you sure the LED itself is capable of responding at LO frequencies? Is the opto-couple designed for audio frequencies too low for the LO?

Maven


 
 Return to top of page · Post #: 13 · Written at 4:30:57 PM on 7 December 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Thanks for the reply Maven,
So far I've got the LED lit with two transistors.
I guess I just needed more amplification.
I can see the signal at the LED with scope & freq counter.

I have read that LEDs are particular about detecting light.
They only respond to a very narrow bandwidth which is
outside of the range of the colour they emit,
so to pair a couple of them requires two deliberately selected colours.

Of course this didn't matter for initial testing done in white light.

I have got some real photo transistors and opto-couplers
now, so ready for a bit of progression.
While I was at Jaycar, I cleaned out their valve stock at 25% Smile
There are some 12AX7's in there.

Wa2ise, I should have acknowledged your advice,
but it sounds like you are suggesting another tube,
which I'd rather avoid. A great part of the project is already solid state.


 
 Return to top of page · Post #: 14 · Written at 5:58:38 PM on 7 December 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

So the transmitting side is something like this now,
just that the LED gets ditched.
Note that the input to the freq counter is taken from the
LED now:

Image Link

This is fun. I don't pretend to know a super heterodyne
inside and out, but I couldn't have done any of this a couple of months ago Smile


 
 Return to top of page · Post #: 15 · Written at 5:12:39 PM on 17 December 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

A ferrite loop antenna works when directly connected to a scope
or freq counter, but my two transistor amplifier does nothing with it,
and a diode inbetween messes up the signal.

Image Link


 
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