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PostPosted: Thu Jan 19, 2012 4:53 am 
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KR1S wrote:
You've come a long way since you first showed up here.

That's due in no small measure to the guidance and advice of you and the other fine folks here on TheRadioBoard. Thank you!

Regarding DDS units - have you seen any that can directly output 0-30 MHz quadrature outputs? I read a while ago that this should be possible "real soon now".


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PostPosted: Thu Jan 19, 2012 5:07 am 
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Location: South Florida
qrp-gaijin wrote:
One thing I read a while ago here on TheRadioBoard has been gnawing at the back of my mind though: the fact that a good DC or superhet receiver can hear signals inaudible on a regen due to the regen's higher noise. That bugs me, that a better receiver might hear more than my regens.

Ditch that Wyatt Earp 'tude! :) Even if you build a superhet, somewhere on the planet will be at least one other superhet that hears stuff yours won't. Japan is loaded with hams and propagation to the rest of the world is good now. Believe me, with any JA callsign you will be a lot more in demand than a W1 operating in Florida. What really matters is to get on the air and gain operating experience.

As for quadrature DDS, I don't know of any. I'd think the way to go would be to use two chips controlled by one encoder and work out the phase shift on-board. Someone will probably build such a chip, but applications are limited in the HF range.

73,

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PostPosted: Thu Jan 19, 2012 11:31 am 
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KR1S wrote:
Ditch that Wyatt Earp 'tude! :) Even if you build a superhet, somewhere on the planet will be at least one other superhet that hears stuff yours won't. Japan is loaded with hams and propagation to the rest of the world is good now. Believe me, with any JA callsign you will be a lot more in demand than a W1 operating in Florida. What really matters is to get on the air and gain operating experience.

You raise good points. The reason I want to use a homebrew station is to relive Radio the way the pioneers did it, with self-built equipment. If regens, with all their faults, were good enough for the old-timers, then they should be good enough for me. Somewhere along the way I started to get sidetracked and fascinated with the idea that I actually could build a superhet comparable to some commercial ham equipment. All a question of how much time one is willing to spend.

OK, so maybe it's time to switch gears and work on the tx side to get closer to actually getting on the air. First would be to dust off the unbuilt 2W QRP TX and companion 10W amp kits. Then I need to revive my poor-man's-spectrum-analyzer project (using a simple DC rx, strong mixer, step attenuator, manual sweep oscillator, and PC sound card spectrum analyzer) to measure spurs. Then there's the application/approval process for homebrew gear in Japan, followed by antenna construction and tune-up.

Yes, looking at that list, it does seem I need to get working on the tx side in order to enjoy this solar cycle.


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PostPosted: Thu Jan 19, 2012 2:33 pm 
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Location: Toronto
qrp-gaijin-san, your curiosity, determination and thoroughness are commendable. Must be the influence of Japan on you. :)

From your posts, and from people's responses to your posts, I've learned alot lately. Don't stop posting, please.


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PostPosted: Sun Nov 04, 2012 3:29 pm 
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I found this interesting circuit, described as "a universal sine wave VFO with extreme tuning range (DC-144MHz), oscillating on the fundamental frequency (no mixers), amplitude steady throughout all bands, adequate stability and only one coil changing per band": http://neazoi.com/20KHz-65MHzSineOsc/20 ... ineOsc.htm

This could surely be easily adapted for varactor use. (EDIT: closer reading of the original article indicates that the author did not have success trying to use varactor control for this circuit, getting a distorted waveform. Perhaps the circuit is a bit touchy to get working correctly.) I am, however, starting to have second thoughts about varactors (I think they introduce phase noise, more so than a good quality air variable capacitor would do). But the basic VFO circuit looks very versatile.

Of note is that the amplitude is apparently steady over the tuning range. Can anyone comment on how this is achieved? Might this mean that the oscillator could also be used as a regenerative receiver that requires little regeneration adjustment, similar to vladn's frequency-compensated regenerative receiver?

Finally, does anyone know any similar wide-range analog VFO circuits?


Last edited by qrp-gaijin on Mon Nov 05, 2012 2:00 am, edited 1 time in total.

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PostPosted: Sun Nov 04, 2012 6:19 pm 
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Location: N 64º 41' E 21º 14'
qrp-gaijin, the thing I can imagine with that circuit is quite a bit of harmonics sneaking in, some LC bandpass filtering should be in order at very least.
if anyone has built it I would be interested to see what a spectrum analyser would make of the output.

EDIT
also I think the biasing may need tinkering to get really clean unclipped sine wave, i might be wrong asthings can vary quite a bit between makers and even batches.
when I simulate this circuit it had output not unlike square wave in some circumstances
/EDIT


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PostPosted: Wed Feb 20, 2013 12:57 pm 
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Location: Bristol, SW UK
qrp-gaijin wrote:

I'm having a problem getting a clean tone from the varactor-tuned oscillator. It sounds warbling as if it's being frequency-modulated, perhaps by 60 Hz hum. As soon as I replace the varactor with a variable cap the tone quality improves immensely, so obviously something is modulating the varactor control voltage.



Apologies if this has already been mentioned, I did a brief scan of the thread and didn't see it.

You realise that glass bodied diodes are light sensitive, and that artificial light at 50/60Hz can modulate the signal passing through them?

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PostPosted: Sun Feb 24, 2013 7:25 am 
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DragonForce wrote:
Apologies if this has already been mentioned, I did a brief scan of the thread and didn't see it.

You realise that glass bodied diodes are light sensitive, and that artificial light at 50/60Hz can modulate the signal passing through them?

Right. The varactors I'm using now have opaque bodies, but light modulation is something to keep in mind.

The problem in this case was more-or-less "solved" with using a battery power supply instead of an AC adapter. There was still some slow drift though. I never did get around to building a superhet receiver around this VFO. My interest has shifted back to regens again, for now.

Designing stable wideband varactor-tuned VFOs (or regenerative receivers) seems tricky because high tank voltages can cause varactor self-modulation and warbling even if the power supply is stable.


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PostPosted: Thu Mar 07, 2013 10:53 am 
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This fellow has created a wide-range varactor-tuned VFO (VCO) and explains how he solved the problem of instability caused by oscillator voltage detuning the varactor.

https://sites.google.com/site/linuxdigi ... 1mhz-30mzh

That guy's site is also interesting to browse as he has a lot of homebrew RF circuits.


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