Simplifications to the SM0VPO BJT regenerative receiver

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Re: Simplifications to the SM0VPO BJT regenerative receiver

Post by qrp-gaijin on Thu Mar 27, 2014 1:35 am

Admin wrote:Forgive me being sceptic but do you have such a circuit actually working you could point you iPhone video camera at?

The closest circuit that I actually built is the last circuit shown in my last posting, which is also the circuit shown in the Youtube video I linked. As you can see in the video, the receiver works. That circuit has the collector and base at the same DC potential, as both are tied to Vcc through only an inductor.

It will be some time before I will have an opportunity to build the ultra-simplified version of your circuit with collector and base at the same DC potential. When I get around to it I will post the results. Based on my success, and reports and Youtube videos of others' success, with bipolar transistor regens with collector and base at the same DC potential, I see no fundamental reason that the ultra-simplified version of your circuit should not work.

Consider my last circuit:


Notice the collector is coupled to the tank via a link winding. The base is coupled to the tank by raising it above RF ground with the RFC and connecting it to the low-impedance side of the tank inductor (Cv at 1640 pF has lower impedance than the varactor diode D1 with max capacitance 500 pF). This circuit works in hardware, as you can see in the Youtube video I posted.

But this is in fact very similar to my proposed modification to your circuit:

Here, the collector again is coupled to a the tank via a link winding - the same as in my circuit. The only major difference with my circuit is that the base is connected not directly to the tank inductor's low-impedance side, but is instead inductively coupled to the tank via a low-impedance link winding. It should work, provided that the coupling coefficients of the link windings to the main tank coil are high enough, and that the phasing of the link windings is correct.

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Re: Simplifications to the SM0VPO BJT regenerative receiver

Post by Admin on Wed Mar 26, 2014 7:00 pm

> sorry for the long post.

Do not be sorry, it was a real pleasure to read it. Forgive me being sceptic but do you have such a circuit actually working you could point you iPhone video camera at?

This is the sort of thing I would have expected at the beginning of next month, but I can see a certain logic in it that sort of appeals. I must read your text and follow your links once more and study them well.

But this is the sort of think I just LOVE. Please keep it up :-)

Very best regards from Harry

(with no workshop to speak of)

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Re: Simplifications to the SM0VPO BJT regenerative receiver

Post by qrp-gaijin on Wed Mar 26, 2014 6:22 am

Admin wrote:
Interesting modification. If you follow the DC circuit you will see a direct DC short from the collector to base of the transistor. No-mater how you look at it there MUST be a bias resistor and a coupling capacitor.

As surprising as it may seem at first, it is indeed possible to make RF oscillators (and regenerative receivers) where there is a DC short (through an inductor) from the collector to the base of the transistor.

The following patent describes the operation of a similar circuit where the collector and base are at the same DC potential: 
https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US4287489.pdf 

To quote the patent:


Since the collector and base electrodes are at the same DC potential, when the circuit is oscillating the voltage across the inductor 15 causes the transistor to alternately operate in its saturated region and in its active region. When the collector to base junction of the transistor is driven into a forward biased state, the gain of the transistor decreases, limiting the collector to base forward excursion to about the same voltage as the transistors forward bias base to emitter voltage.

As I mentioned, I did run a circuit simulation on the above simplified circuit and it did oscillate in the simulator. I have some degree of experience building oscillators based on circuit simulations and I feel optimistic that the above design will indeed oscillate. Note however that the coupling coefficient between the link coils and the main coil must be very high for this to work, suggesting the use of a toroidal core inductor with very high mutual inductance.

Another example of an oscillator/regenerative receiver with collector and base at the same DC potential can be seen here: 
http://www.wehner.org/electro/short/



The famous Russian ham V. Polyakov (known widely for his "Polyakov mixer") also described a similar ultra-simple regenerative receiver operating with collector and base at the same DC potential:
http://amfan.ru/avtodiny/prostoj-regenerator/
http://amfan.ru/avtodiny/prakticheskaya-sxema/
(use Google Translate to translate the Russian into your language)


I've built a couple of regenerative receivers with collector and base at the same DC potential and they really do work. Here's an example of a receiver I designed and built, with collector and base at the same DC potential:

http://theradioboard.com/rb/viewtopic.php?f=4&t=5140


Here's a Youtube video of my receiver in action:


Sorry for the long post!


Last edited by qrp-gaijin on Wed Mar 26, 2014 7:02 am; edited 2 times in total (Reason for editing : added Vackar-style circuit)

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Re: Simplifications to the SM0VPO BJT regenerative receiver

Post by Admin on Mon Mar 24, 2014 8:10 pm


Interesting modification. If you follow the DC circuit you will see a direct DC short from the collector to base of the transistor. No-mater how you look at it there MUST be a bias resistor and a coupling capacitor.



Now this simplification I do like.

I used to have a huge dislike for DC through coils, especially when coils could be changed by adding a plug and socket arrangement. DC causes noise when sockets are disturbed, especially if the metal is a little bit tarnished. There should be absolutely no problem with a single-band receiver, and if it is so simple then you could make more than one receiver to cover more than one band - one RX = 1 band :-)

Keep up the good work, this is good stuff.

Very best regards from Harry - SM0VPO

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Simplifications to the SM0VPO BJT regenerative receiver

Post by qrp-gaijin on Fri Feb 28, 2014 10:40 am

I've been doing some work with bipolar transistor regenerative receivers lately and was reviewing existing designs. I found your design at http://www.sm0vpo.com/rx/regenrx.htm and believe your circuit can be simplified without loss of performance.

The original schematic is:


My proposed changes are to remove the choke and blocking cap on the collector link winding, and to run DC over the collector link winding:


I think this should work fine and save two components.

Another thing that might be possible - but that I am not so sure about - would be trying a similar trick with the base link winding: remove the 1M5 collector feedback bias resistor, remove the base 1 nF capacitor, and run the base directly through its link winding to the bottom of the 1k8 resistor.


This will place the base and collector at the same DC potential, but oscillation is possible under these circumstances (for instance connecting the tank inductor directly between the base and collector and using Vackar pi-network feedback with 180 degree phase inversion). As I said I'm not so sure if this further simplification will work in this specific circuit, but it seems feasible to me, and would save 2 more components.

---

edit: I took some time to run some circuit simulations (using QUCS software) and the results look good: oscillation does appear possible with the last, simplest circuit variant I posted:


And in fact, the circuit can oscillate even with a supply voltage as low as 1 volt (note however reduction of R1 and increasing of R3):


Note my simulation assumes coil losses at RF are only 1 ohm, which is somewhat optimistic, but I still think it is realistic to believe this circuit variant can oscillate. I suppose this circuit could be called a Reinartz oscillator using a BJT in common-emitter configuration.

Audio output volume may not be so good, though - with the base and collector tied together with only a low RF impedance between them, it's possible that any detected AF at the collector is negatively fed back to the base, which could reduce audio output. However, I have seen a YouTube video of a regenerative detector that uses a somewhat similar biasing arrangement that seems to work okay.

I hope this information might be interesting to you or your readers. Smile


Last edited by qrp-gaijin on Sat Mar 01, 2014 5:02 am; edited 2 times in total (Reason for editing : added simulation results of ultra-simplified circuit)

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