Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Regenerative Receiver -- Advice Sought  (Read 13547 times)
JAHAM2BE
Member

Posts: 277


WWW

Ignore
« Reply #15 on: June 21, 2013, 08:13:18 AM »

The antenna is currently just 15 meters of insulated wire draped in the trees. The ground is an iron radiator pipe. The plan is to put up a coax-fed fan dipole resonant on 40 and 20.

One thing to keep in mind is that your little oscillating regenerative detector is actually transferring energy to the antenna where said energy then gets radiated. This is less of a concern when using a short antenna, but a full-size resonant dipole could conceivably cause interference as your QRPp signal gets radiated from the dipole.

That's another reason to have an RF stage with good reverse isolation: it can help prevent the oscillating detector's signal from reaching the antenna and the outside world. If you read Kitchin's articles you'll see that he highly recommends such a stage. It's very easy and non-critical to add.
Logged

KB1WSY
Member

Posts: 804




Ignore
« Reply #16 on: June 21, 2013, 09:57:39 AM »

That's another reason to have an RF stage with good reverse isolation: it can help prevent the oscillating detector's signal from reaching the antenna and the outside world. If you read Kitchin's articles you'll see that he highly recommends such a stage. It's very easy and non-critical to add.

Thanks. BTW does Kitchin have tube designs? I did a quick search but no luck. Where is most of his stuff, is it in QEX? (Perhaps I can find it in the ARRL archive.) I do have a copy of his 1998 QEX article.

73 de Martin, KB1WSY
Logged
W4OP
Member

Posts: 436


WWW

Ignore
« Reply #17 on: June 21, 2013, 08:57:43 PM »

Electric Radio magazine has to be the best for  vacuum tube regen designs. The latest issue has a wonderful article by Bruce Vaughn (SK).

Dale W4OP
Logged
JAHAM2BE
Member

Posts: 277


WWW

Ignore
« Reply #18 on: June 21, 2013, 10:35:19 PM »

Thanks. BTW does Kitchin have tube designs? I did a quick search but no luck. Where is most of his stuff, is it in QEX? (Perhaps I can find it in the ARRL archive.) I do have a copy of his 1998 QEX article.

No tube designs that I've seen, but I'm sure there are plenty of other resources for those. I'm not a tube guy, so I can't offer much specific advice there.

As for Kitchin's articles, I'm aware of several different venues he's published in: Electronics Now, Communications Quarterly, QST, QEX, ARRL handbooks, and CQ magazine. One particular article that stands out in my mind (apart from the one you already mentioned, the 1998 QEX article) is his Communications Quarterly article in Fall 1995, titled "Regenerative Receivers: Past and Present". That article covers the basics of regenerative receiver design and includes discussion of several design alternatives.

This following links contain a list of many, if not most, Kitchin articles: http://groups.yahoo.com/group/regenrx/message/502
http://qrp.kd4ab.org/1999/990421/0066.html

The last article I am aware of was in February, 2010, in CQ Magazine.

Although Kitchin has published excellent work on regens, it's important to note that his design decisions (e.g. almost always using a single active device that serves both as Q-multiplier and as detector, almost always using a throttle capacitor to control regeneration) are not the only way to achieve a high-performance regenerative receiver design. But reading his articles can give you a very good start, after which you can then sift through the multitudes of regen designs and discussions on the Internet.
« Last Edit: June 21, 2013, 10:39:12 PM by JAHAM2BE » Logged

G3RZP
Member

Posts: 4715




Ignore
« Reply #19 on: June 22, 2013, 04:20:08 AM »

I wouldn't expect phase noise to be that much of a problem in a regen rx, bearing in mind the overall performance, although it can be a problem in direct conversion receivers.

1/f noise in MOSFETs again can be a problem: it can be calculated though, and I suspect you wouldn't see the effects of it in an HF receiver because of the external noise. GaASFETS are very bad, which is why the phase noise performance of GaAS dividers is so poor. CMOS, on the other hand is very good, outperforming ECL - see Rohde's book on frequency synthesisers. GEC-Plessey Semiconductors did some very low noise high frequency (GHz) ECL dividers using special design techniques: I don't know if they are still available from the 'new' Plessey Semiconductors. On a CMOS chip for hearing aid receivers, we didn't get any 1/f noise problems from the very low IF despite a fairly low gain front end - partly  necessary because of the very low current availability.

As far as radiation is concerned, it was historically a problem with BC rx's in the 1920s and 30s: in Europe, there's a blanket radiation limit for all except broadcast rx's (but including automobile broadcast receivers) of -57dBm below 1 GHz and -47dBm above 1 GHz. That was a bit of a problem for direct conversion pagers at times. (Does anyone have a pager these days except in hospitals?)

So I would agree that an RF stage is desirable: a dual gate MOSFET is pretty good, although I have found a cascode good for reverse isolation.
Logged
KB1WSY
Member

Posts: 804




Ignore
« Reply #20 on: June 22, 2013, 06:21:11 AM »

JAHAM2BE, G3RZP: OK you have convinced me. I will add "an RF stage with good reverse isolation." I have two design requirements and the first one will probably strike both of you as crazy:
(1) Must be done entirely with discrete components (no ICs) that were available in 1968. I am scouring the literature!
(2) Not essential but nice: an RF gain dial to provide yet more control over those strong, overloading signals.

Additionally, today I am starting some major modifications:
(1) (Already done) replaced the main tuning capacitor with a NOS Hammarlund MC-50-S. The leaf spring on the old one was noisy, and although I re-tensioned and cleaned it a couple of times, the fix turned out to be temporary so we must consider that spring to be shot.
(2) Move the antenna connection down to the back apron and install an ANT air trimmer on the front panel (adjusting the antenna coupling on a band by band basis makes a big difference).
(3) Reconfigure regeneration controls by adding a rotary switch to provide a range of values for C9.
(4) Build yet more band coils, with various degrees of looser coupling and a range of tighter bandspreads (I have some "rescued" 5-pin Millen forms for that purpose).
(5) Build an aluminum cabinet, incorporating a "trapdoor" for coil-changing. A/C hum is currently a major problem in my shack, and tests have determined that a cabinet eliminates it completely, at the cost of a small decline in sensitivity.
(6) Eventually I will add a historically accurate additional AF stage to drive a speaker, and add a switch to toggle between 'phones and external speaker. I much prefer using the 'phones but I want to be able to demonstrate "ham radio" to the increasingly incredulous family and friends who wonder what Martin is doing in his "shack." I tried plugging in some amplified computer speakers to the 'phones output but it is much too weak (presumably because of impedance mismatch).

What I am *not* doing is messing with the JFET gate resistor to provide "damping." I did hours of tests last night (by installing a 1 meg pot across the gate resistor and trying lots of different values) and found that damping was (a) unnecessary and (b) had strange and unpredictable results. Without any "damping" at all, I am now getting good results on all three targeted bands: 80, 40, 20. Very cool.

Just for kicks, I also tested 15m and 10m and found the regeneration was uncontrollable regardless of what I did with C9 or with "damping." Either there was no oscillation of the detector, or if you advanced the control further it went into a "motorboarding" sound i.e. not a howl, but more like the "clicks" you would hear from a Geiger counter. Also, strangely, when I fed 21MHz and 28MHz signals from my RF signal generator at maximum level, I was unable to copy them on the receiver. Anyway it's no problem, since I have no plans to work 15m or 10m for the time being. However it will be interesting to see whether adding an RF stage helps (in theory, it should make quite a difference on these higher frequencies, I think).

All of these changes are being done in a reversible way so that the new stuff can be removed and the original ARRL design restored (that's because when I build my transmitter, I want to take a few color photos of a "historically exact" ARRL Novice station). The current front panel will be removed and left as-is; I am installing a new panel for Version 2 of this set.

Uhhhh ... does this ever end???

73 de Martin, KB1WSY
« Last Edit: June 22, 2013, 06:30:45 AM by KB1WSY » Logged
JAHAM2BE
Member

Posts: 277


WWW

Ignore
« Reply #21 on: June 22, 2013, 09:44:26 AM »

Just for kicks, I also tested 15m and 10m and found the regeneration was uncontrollable regardless of what I did with C9 or with "damping." Either there was no oscillation of the detector, or if you advanced the control further it went into a "motorboarding" sound i.e. not a howl, but more like the "clicks" you would hear from a Geiger counter. Also, strangely, when I fed 21MHz and 28MHz signals from my RF signal generator at maximum level, I was unable to copy them on the receiver. Anyway it's no problem, since I have no plans to work 15m or 10m for the time being. However it will be interesting to see whether adding an RF stage helps (in theory, it should make quite a difference on these higher frequencies, I think).

If the set has trouble going into regeneration at higher, frequencies I wouldn't expect an additional RF stage by itself to fix the problem. It may be that stray capacitances and inductances (due to long lead lengths and non-compact layout) are preventing oscillation at upper HF. Or the impedance of the random-wire antenna could happen to be excessively loading down the tank at higher frequencies (in which case an RF stage would help).

Another thing you can do once you've added the RF stage is to make it into a fixed-frequency self-oscillating converter stage, thus giving you a simple superheterodyne receiver with a tunable IF stage. That may be easier than trying to get your regen to oscillate directly at 21 or 28 MHz. AA1TJ shows how to do that here: http://aa1tj.blogspot.jp/2011/08/1950s-transistorized-ultra-simple.html . He even uses 1950's-era transistors, so that would fit in with the theme of your project. Smiley

I'm not sure about the reverse-isolation of the AA1TJ RF amp circuit, but since it's a superhet, the IF stage (your regen) will be oscillating at a different frequency than the received frequency, so any oscillator energy that does manage to work its way upstream through the RF amp will be greatly attenuated by the front-end tuned circuit (tuned to the reception frequency, not the regen's oscillation frequency). So maybe the spurious emissions would be OK in this case.
« Last Edit: June 22, 2013, 09:48:47 AM by JAHAM2BE » Logged

G3RZP
Member

Posts: 4715




Ignore
« Reply #22 on: June 23, 2013, 12:13:18 AM »

That is what was called a 'supergainer' receiver back in the 1930's. A tube version is in the 1946 ARRL handbook. Has a lot of advantages.
Logged
KB1WSY
Member

Posts: 804




Ignore
« Reply #23 on: June 23, 2013, 04:50:46 AM »

That is what was called a 'supergainer' receiver back in the 1930's. A tube version is in the 1946 ARRL handbook. Has a lot of advantages.

Lew McCoy, W1ICP, had an article in the July 1963 issue of QST for building the "Novice RS-3," a "Three-Tube Regenerative Set for Beginners." I read that last year and actually obtained all the parts for it before being sidetracked by life in general. I also became more dubious about the regen as a realistic rig for QSOs, but I have changed my mind after experiencing the reception possibilities of my little 3-transistor set. The RS-3 was quite popular in its day, apparently. The first stage is a 6U8A with one half as the oscillator and the other half as the mixer. The regenerative detector is one half of another 6UBA. The second half of the tube is the first audio stage, and then there's a 6AQ5 as the final audio stage.

Looking over the article again, it's another one of those projects that is presented for "beginners" but that's a very relative term!! I think I may be ready for it now that I've built the transistor set, so I'll add it to the very long list of pending projects. (Searching on the Internet last year, I found quite a few reminiscences by OTs who remembered building this set but couldn't make it work. One of them had installed the bandset capacitor in the bandspread circuit, and vice versa.) Among other things there is not just one set of plug-ing coils, but two: one for the tuning and one for the oscillator. Plus, a commercial IF transformer that is unobtainium, but I managed to find an equivalent.

I suppose it may be worth revising the design in view of Kitchin's 21st Century advice about regens in general. Or perhaps I should just build it as-is and then tinker.... If you are an ARRL member the article is available in their archive.

73 de Martin, KB1WSY
Logged
KB1WSY
Member

Posts: 804




Ignore
« Reply #24 on: June 23, 2013, 06:19:06 AM »

If the set has trouble going into regeneration at higher, frequencies I wouldn't expect an additional RF stage by itself to fix the problem.

Strangely enough, when I tested it again, perhaps more carefully this time, I had no trouble getting it to oscillate "controllably" on 15m and 10m. The motorboarding sound was gone and all I got when advancing the control too far was the familiar, controllable howl. However the set is still dead on those two bands: no signals audible and it is unresponsive to modulated tones from my RF signal generator. I get the general impression that it is just completely overloaded on those bands and this is shutting down the detector completely (there is plenty of whooshy RF noise, just no signals -- even last night amid all the field day mayhem that I could hear loud and clear all over the other bands). Anyway it's something to put aside for the future; it has no practical effect on my station plans, but it is still an interesting puzzle.

One lagniappe: I also tested it on 160m last night and copied quite a lot of phone and CW. It was very noisy, but I assume that's not uncommon for "top band" reception. Unlike the last time that I tried 160m, there was no intereference from BC stations and no need to use the BC filter.

73 de Martin, KB1WSY
Logged
AF6QA
Member

Posts: 5




Ignore
« Reply #25 on: June 23, 2013, 09:46:17 AM »

Hi - A 2 tube set I've been accumulating parts for is at:  http://www.qsl.net/k5bcq/REGEN2/regen3.html  Tom
Logged
KB1WSY
Member

Posts: 804




Ignore
« Reply #26 on: June 28, 2013, 06:41:31 AM »

One thing to keep in mind is that your little oscillating regenerative detector is actually transferring energy to the antenna where said energy then gets radiated. This is less of a concern when using a short antenna, but a full-size resonant dipole could conceivably cause interference as your QRPp signal gets radiated from the dipole.
That's another reason to have an RF stage with good reverse isolation: it can help prevent the oscillating detector's signal from reaching the antenna and the outside world. If you read Kitchin's articles you'll see that he highly recommends such a stage. It's very easy and non-critical to add.

I've decided to use the RF stage circuit from Kitchin's 1998 QEX article schematic for "a high performance [regenerative] shortwave receiver." His RF stage only uses a handful of components, all of which I have already, apart from the transistor. He's used a 2N2907 (PNP) and I'm planning to substitute its NPN complement, the 2N2222A. This is historically OK because that transistor was available by the early 1960s, and there is nothing "modern" about Kitchin's implementation of the RF stage. I will mount this little circuit on the new "back panel" of my set and make it switchable in-out, to enable experimental "before and after" comparisons.

Even if it makes no difference in performance, it should, as you said, reduce radiation from the antenna. What I am mainly looking for however is a more robust way to attenuate strong signals and thus prevent overload. The 1,100k series pots that I have put across the antenna input currently have only a moderate effect, even when set to zero ohms (antenna shorted to ground!). To demodulate really strong signals I have to use a whole arsenal: open the antenna trimmer so that it has only residual capacitance, set both attenuation controls to zero ohms, and play with C9 to lower the oscillation threshhold. Fortunately this is almost entirely an issue with SSB stations and I'm not planning to use that mode, but occasionally there is an issue with strong CW signals too, especially on 20m.

I am intrigued by your suggestion (seconded by G3RZP) that I should install a converting RF stage, turning the rest of the set into a tunable IF and getting a simple superhet in the bargain! That is something I may try in the future, although there are other (tube-based) receiver projects on the drawing board that I should really turn my attention to now.

73 de Martin, KB1WSY
« Last Edit: June 28, 2013, 07:20:03 AM by KB1WSY » Logged
WB6BYU
Member

Posts: 13335




Ignore
« Reply #27 on: June 28, 2013, 08:22:06 AM »

Quote from: G3RZP

... I have found a cascode good for reverse isolation.



I can recommend this approach.  I use a cascode RF amplifier in front of my hand-held
80m DF receivers, which use a 6" to 8" loop antenna.  I chose that initially for the
direct conversion designs to minimize oscillator radiation from the antenna, and have
demonstrated that there is more radiation from the oscillator circuitry than from the
antenna.  Even with my newer superhet designs I've kept the cascode preamp because
it provides good sensitivity and convenient gain control.

It only takes a couple parts more than a conventional stage - an extra transistor
and bias divider resistor.  I've used the 2N2222 or equivalents, though you can use
just about anything.  There may be an advantage to using an FET in the input circuit
instead if you want a high input impedance - I calculate around 1500 ohms for my
typical bipolar circuit.

You can find the circuit in the Sept. 2005 issue of QST.
Logged
KB1WSY
Member

Posts: 804




Ignore
« Reply #28 on: July 19, 2013, 02:18:54 PM »

So I added a very simple 2N2907 grounded-base RF stage based on Kitchin's design. It has made no audible difference to the set's performance, when doing direct in/out comparisons. Nor has it made any difference to the set's unusual behavior i.e. the setting of the FET's source capacitor remains critical, but the important thing is that as long as this value is adjusted when changing bands, the radio can pull in ham traffic on all bands 160m through 20m. Over the past 24 hours I've even started hearing a few signals on 15m and 10m but nothing that is identifiably a ham transmission.

The main thing I guess is that the RF stage is providing isolation and should thereby help prevent unwanted radiation when I upgrade my antenna to full "ham size."

You can find the circuit in the Sept. 2005 issue of QST.

Thanks! I found the article and will consider trying that approach, too. Perhaps it will provide better control over sensitivity/gain.

73 de Martin, KB1WSY
« Last Edit: July 19, 2013, 02:25:42 PM by KB1WSY » Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!