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PostPosted: Sun Dec 22, 2013 11:50 am 
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Joined: Thu Apr 14, 2011 1:10 am
Posts: 232

Tuning down from 12MHz to 9MHz using the TriStar regenerative receiver circuit.
I hope it is not too much like watching paint dry, but probably it is.


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PostPosted: Sun Dec 22, 2013 2:01 pm 
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Joined: Mon Jul 11, 2011 8:36 pm
Posts: 62
Location: Newcastle upon Tyne - UK
Interesting. It works well. I read threads about the Tristar circuit while your video was playing. Lots of ideas there that I will need to learn more about. Thanks for the info.

Tony
G0BZB


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PostPosted: Mon Dec 23, 2013 12:46 am 
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Joined: Thu Apr 14, 2011 1:10 am
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I compared it with this online SDR radio http://websdr.ewi.utwente.nl:8901/ and I seem to be able to receive more radio stations per band.
I presume then that the sensitivity is in the microvolt range if not at the sub-micro volt range. People experienced with regenerative receivers will notice that you can tune within a few 10's of Hz of even a strong AM carrier before you get frequency lock. You can also hear the effects of a very narrow bandwidth on the audio quality. Further evidence of the narrow bandwidth is the seashell sound effect you can get tuning across an AM station.
The lack of frequency lock makes it a good choice of circuit for SSB reception. The circuit also has very low frequency drift.
I used permeability tuning in the radio with ferrite material. If you used a variable capacitor there would be less need to adjust the regeneration controls.
With the SDR you don't hear the effects of phase noise. With the regenerative receiver when you are listening to weak to intermediate strength signals you can hear the background noise increase and decrease as the signal increases and decreases. This is due to phase noise to amplitude modulated noise conversion acting on the carrier. I suppose it is fortunate that the conversion is proportional to signal strength. Of course when you set the regenerative receiver to the just oscillating state you will hear a constant amount of background noise as the oscillation is converted to AM noise.


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PostPosted: Mon Dec 23, 2013 4:30 am 
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Joined: Tue Jan 04, 2011 1:52 am
Posts: 261
Location: Toronto
I'm playing with a tube design right now, but I'm also planning to build a solid-state regen in January, as a portable. Initially i was thinking of one of the Kitchin designs (http://n9sja.com/2012/11/15/k8tnds-take ... hin-regen/) but your Tristar seems sophisticated and interesting too.

A few questions:
- have you tested what frequency ranges the design works at? Do you have a list of suggested L1, L2 and C3 values for different ranges?
- do you have suggestions and warnings for the physical layout of the circuit? I will probably use dead-bug or "Manhattan" style of building on a piece of copper-clad board.

Thanks for sharing your design.


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PostPosted: Mon Dec 23, 2013 3:05 pm 
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Joined: Thu Apr 14, 2011 1:10 am
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There are a few components that affect the frequency range of the circuit. The RFC inductor in the AM detector is the most likely to cause trouble. It can in fact be replaced with a suitable resistor. If the RFC is operated near its self-resonant frequency you will get a great increase in gain and unfortunately associated instability. That is the only component that can really cause trouble. At lower frequencies (100KHz to 1MHz) there could be an advantage to increasing the emitter coupling capacitors from 100nF to say 1uF ceramic.
In more recent versions of the circuit I have used independent biasing of the individual transistors using voltage divider resistors. That seems to be preferable. I always use a voltage regulator chip to power the circuit. There is a 1nF:100pF 10 to 1 capacitor tap at the resonant circuit. You could change that to 330pF:33pF for greater tuning range. The limit there is the base emitter capacitance of the transistor. You couldn't do 33pF:3.3pF for example. The effects of that capacitor tap are more or less frequency independent, I guess it would be ok down to 100KHz, If not you could do 10nF:1nF.


Last edited by seanvn on Mon Dec 23, 2013 10:52 pm, edited 1 time in total.

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PostPosted: Mon Dec 23, 2013 10:29 pm 
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Joined: Tue Jan 04, 2011 1:52 am
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Location: Toronto
Useful info! Thanks.


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PostPosted: Mon Dec 23, 2013 11:14 pm 
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Joined: Thu Apr 14, 2011 1:10 am
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I looked a little at how to bias the transistor connect to the resonant circuit, to minimize the noise contribution from that source. You can just use a 2 resistor voltage divider to bias the transistor. Those biasing resistors can be quite low in value (10k,20k) without loading the resonant circuit because the capacitor tap does a big impedance transformation.
You can bias it from a fixed voltage source (heavily filtered) through a RFC. That can suppress any residual mains hum (but you should be careful not to let the RFC act as a magnetic pick-up, or it will add hum).
Or you can bias it through a very high value resistor (eg 1 or 2 Mega Ohms) connected directly the positive supply rail. The advantage of that is that the noise contribution from the biasing resistor is heavily filtered by the 1nF capacitor and restricted to below about 100Hz. The disadvantage is that biasing is then Hfe dependent.

I didn't find anything critical in terms of layout, dead bug, ugly, Manhattan, PCB, eyelet board, should all be fine. I presume it should be small enough in extent that the decoupling capacitors are effective. Shielding is important or you will get mains hum, however it doesn't need to be fully enclosed. An open metal box is ok, you just need to suppress the electrostatic field around the circuit board. In terms of construction I'd like to buy a vinyl cutter/plotter. I think you can even buy copper foil material to use with it, to make some sort of PCB, but probably not a very good one.


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