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 Post subject: Help with motorboating
PostPosted: Sun May 15, 2011 1:10 pm 
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Hi guys,

I'm building a regen based on this Kitchin design: Image.

The audio volume is too low for me because I am using a low-noise shielded loop antenna (http://replay.web.archive.org/20060715194542/http://www.greertech.com/hfloop/mymagloop.html) which greatly improves the S/N ratio, but which lowers the signal levels significantly.

I figured I could either add another wideband RF preamp or an AF preamp. I decided to go with the AF preamp because I supposed it would be easier to get working correctly. I added a 2N3904 preamp that I found in another direct conversion receiver design. The amp works well and is quiet, but unfortunately when I turn up the volume more than about 25%, I get motorboating in the form of ~900Hz raspy oscillation.

The entire audio section circuit is as follows. Note the LM386 section is slightly different than the design in Kitchin's circuit (based on another LM386 amp I saw elsewhere with an anti-hiss modification). The 4.7 uF audio in to the preamp comes off of the detector's source terminal, and the AF out goes to the headphones or speaker.

Image

The LM386 power amp is built on a separate board from the 2N3904 preamp, which in turn is built on a separate board from the main board housing the regenerative detector and RF preamp. There is a 47uF bypass on the 12V DC bus both on the main detector board as well as on the LM386 power amp board.

Any suggestions as to how to fix this motorboating? I already have two .01uF capacitors across the main volume control, as suggested by the original Kitchin article.

Alternatively, would a broadband RF preamp be a better way to boost the signal levels due to the signal loss caused by the low-noise shielded loop antenna?


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PostPosted: Sun May 15, 2011 1:33 pm 
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Location: The Netherlands
For more responses about your question, ask it also at the Yahoo Group about regenerative receivers http://groups.yahoo.com/group/regenrx/messages.


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PostPosted: Sun May 15, 2011 1:57 pm 
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That anti-hiss mod just about made me nuts when I tried it. This schematic shows what I ended up with. http://qrp.kearman.com/images/fet-regen-schematic.png The article for that radio is here: http://qrp.kearman.com/html/lc-regen01.html It's tame now. I also put the audio stuff on a separate board, and was running it off a 12-V gel cell battery.

Over the years I've developed a real dislike for the 386, which is probably not justified. The LM380 will blow your ears off, but it doesn't like 9-V battery supplies, either. I do like lots of audio, because years of sitting too close to the stage at rock concerts did a job on my hearing. :cry: Where I, and others, probably get into trouble with the 386 is when we try to make it do more than it was designed to do.

73,

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PostPosted: Sun May 15, 2011 6:04 pm 
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Location: Sonoma County, CA
I built the original version of the Kitchin receiver and it works well. No motorboating.

In my experience, motorboating is usually related to power supply impedance, so a large electrolytic (47 uF should be OK) very close to the Vcc pin on the LM386 is very useful. Board layout could also be an issue. I never did the hiss ckt. and always use very short traces for the 10 uF "gain boost" cap.

I don't think you need all of those 0.1 uF caps on the input; I don't think it will affect motor-boating.

Rich

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PostPosted: Sun May 15, 2011 6:14 pm 
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Motorboating also occurs when the regenerative stage is oscillating. In stead of the normal whistles you hear when you advance the regen control, you hear a nasty motorboating sound of about several hunderds of Hz when the regen stage is oscillating. The pitch of this motorboating sounds changes when you turn the dial of the tuning capacitor over the band. It mostly happens on the medium wave band.

This phenonema has something to do with the RC time constants of the coupling capacitor and the bias resistors of the next stage. I think that it has to do with charching and discharging the coupling capacitor between the regen stage and the next stage. And it mostly happens when you extract the audio signal from the source or the emitter of a regen stage. What can I do to avoid this problem?


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PostPosted: Sun May 15, 2011 6:25 pm 
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DrM wrote:
This phenonema has something to do with the RC time constants of the coupling capacitor and the bias resistors of the next stage. I think that it has to do with charching and discharging the coupling capacitor between the regen stage and the next stage. And it mostly happens when you extract the audio signal from the source or the emitter of a regen stage. What can I do to avoid this problem?

Putting too much load on the detector could be the problem. A smaller coupling cap or an additional, high-input-impedance buffer stage may help.

Changing the operating conditions of the detector, so that it goes into oscillation at a different emitter or source current, might also work. An easy way to test that would be to change the supply voltage, either higher or lower. HTH.

73,

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PostPosted: Sun May 15, 2011 7:01 pm 
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qrp -- sometimes I've gotten rid of strange oscillations using the 386 by putting a low value resistor (100 - 1000 ohms) between your C8 point and the battery terminal. I've assumed that the resistor prevents oscillation currents from reaching the battery and it's internal resistance - and forces them to go to ground through C8 & C7. (??)

Let us know if you find something that works.

73, Dan


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PostPosted: Sat May 21, 2011 12:56 pm 
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Solved the motorboating. I inserted 2.2k between R5 and the positive rail. Inserting resistance, up to 3.3k, above the LM386 (above C8) didn't have any effect except making the motorboating frequency lower.

Interestingly the motorboating behavior seems to depend on the FET used for the detector as well. Instead of the MPF102 I used a 2SK241, which I had previously successfully used in another very sensitive Armstrong regen. I experimented with resistances as described above until the motorboating stopped. Then I replaced the 2SK241 with the original MPF102 and the motorboating was back again. Since I've already optimized the set with the 2SK241 I'll be using that transistor for now. The 2SK241 is a MOSFET while the MPF102 is a JFET, but the 2SK241 still seems to work very well with the circuit values from the original design, though I read that JFETs may be lower noise then MOSFETs. Not sure if that makes a difference at HF though with the band noise being the limiting factor for sensitivity.


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PostPosted: Sat May 21, 2011 4:24 pm 
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Changing R5 is an interesting and un-expected solution.

However, now the zener current is cut in half and is well below the Izt recommended on the data sheet.

http://knol.google.com/k/electronic-cir ... chapter-7#

It's possible that the zener voltage is not where it was before and the change in voltage applied to the FET is what is changing the FET operating point?

Rich

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 Post subject:
PostPosted: Sat May 21, 2011 5:43 pm 
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The R5 I'm referring to is the R5 in my revised audio section schematic, not the R5 in the original design. The R5 I'm talking about forms part of the decoupling network of the 2N3904 preamp that I added.

Also I just got done adding a diode limiter to the audio output: a "voltage divider" whose top half is a 100 ohm resistor and whose bottom half is a pair of antiparallel 1N4148 diodes; headphone audio output is taken off the bottom of the 100 ohm resistor. It works remarkably well and has doused my sense of urgency in investigating AGC. With the extra volume boost provided by the preamp, the headphone levels could get dangerously high, but now the limiter is protecting my ears.


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