RF Feedback

[W1RKW]

What's a tell tale sign of RF feedback?

[W1AEX]

Squealing at low or high audio frequencies, runaway modulator current, irregular 60 cycle hummy background noise, audible on-the-edge-of-oscillation that sounds like ringing on audio peaks. Those are a few of my favorites Bob!

[KD6VXI]

Blisters on your microphone hand and / or lips.

--Shane

[W7TFO]

If done on purpose (correctly) in a high-level rig, less noise and distortion.

[w1vtp]

Blisters on your microphone hand and / or lips.

--Shane

Been there!!   'specially while using loop modulation

[K5UJ]

If by "RF feedback" you mean tx RF getting into your baseband audio gear and distorting the audio, you can spot it when you monitor yourself off-air and the audio sounds fuzzy and unclear on an antenna but is much less so or not at all on a dummy load.  I could be mistaken about this but I think solid state gear, both audio and RF, is more vulnerable to this.  If this is what you are asking about we can continue with methods for eliminating it.

Rob

[W1RKW]

What I was thinking was more of RF output getting into RF input and how to recognize it.  The reason I ask, I built a solid state linear amplifier for the Retro75, a Communications Concepts EB63.  It's a broadband design.  My input and output jacks are next to each other but are connected with shielded cable.  I didn't think about separating them until now.   I'm having trouble with linearity.  Input looks nasty as does output.  I'm beginning to think it's the load presented to the Retro75 that is the issue but just wanted to know how to recognize RF feedback.  Maybe I don't have an RF feedback issue but an issue with the Retro75.

[KD6VXI]

That amp has the input and output out of phase, thank goodness....

If linearity is suffering, check the bias.  About .7 to .75 will turn the 454s on enough to not have a problem.  Also, you could have a problem with RF NOT being decoupled enough from the collectors, and getting back into the retro75 (possible) or into the bias of the amplifier, causing problems.

And the thing Motorola forgot was the negative feedback.  Try a .01 ceramic disc and a 68 ohm 2 watt resistor from collector to base on BOTH transistors.  I'd say that's probably the problem...  That amp will be MUCHO unstable (especially on the low end of the operating spectrum, where you happen to be having problems) depending on the ferrite they used because they didn't use any type of feedback.


I run a 16 X 2SC2879 amplifier.  Playing with the values of negative feedback got it a lot more stable and tame on the low end of the spectrum, whereas before it only wanted to play on 20 - 10.

--Shane

[W1RKW]

Don't know if this is any indication but here's what's happening, Retro75 has a nice carrier when the amp isn't keyed (straight through to 50ohms) but as soon as the amp is keyed input linearity goes out the roof.

[W1GFH]

What's a tell tale sign of RF feedback?

That's like asking "How can I tell if I've been shot with a .45" or "How can I tell if she's pregnant". Don't worry, YOU WILL KNOW IT.  

[ka3zlr]

Ah Yeah.. as to the The Retro

Umm,.. what ever any owner of the little bugger does.. DO NOT Pair up the Final in that little thing Umm,.. the Tranny,..it gets a Little Bit Real Hot Real Fast LOL.....Yessir and the Audio chip Lets a little bit of Smoke out..LOL..Careful on that one..
Ahh... Recieve is Good in a strong environment butt could use a stage of Gain.. just a L'ill..just a Tad...

But Over all after the l'ill bugger settles down a bit it's a neat little toy...

Oh an Be careful about the Fet's...

undersolderlingly yours

Zed.L.R.

[W1RKW]

I used a different RF source into the amp and got good results, a nice clean wave form. So I'm ruling out the amplifier as the problem, at least for now.  The Retro  75 seems to be the issue at this point.

[K1JJ]

Bob,

Try some type 45 ferrite cores on the coax between the driver and linear.

Also, do you have more than enuff drive available so that you can add a 50 ohm non-inductive resistor across the linear amp input to provide a more stable load for the driver?   Another version of this might be a 25 ohm in series with the linear input.  This will provide some isolation but increase the input impedance to 75 ohms or so.

If you're not sure about RF in the audio yet, try a .001 cap across the audio input to the rig - and a 500 ohm in series with the same audio input lead. They can both be put inside the audio connector that plugs into the rig.

T



BTW, here's a Mouser order I recently placed for 0.5" I.D. and 0.25 ID cores. The .5" are good for coax and the .25" for cables.  I used all 40 in the shack as a preventive measure. This is now an RF-in-the-audio-proof shack, caw mawn.

TYPE 45 material clamp-on cores: (snap-on)

1/2" ID:
875-28A2024-0A0
Laird Technologies EMI/RFI Suppressors & Ferrites
280ohms 100MHz
US HTS: 8504.90.0080 ECCN:EAR99  20 20 2.420 48.40
2 1  


1/4 ID:
 
875-28A0640-0A0
Laird Technologies EMI/RFI Suppressors & Ferrites
240ohms 100MHz
US HTS: 8504.90.0080 ECCN:EAR99  20 20 1.070 21.40
 1RoHS: Compliant

Shipping Information Merchandise Total
 69.80
 Handling: 0.00
Freight: 18.67
Tax: 0.00
Order TOTAL: $88.47

  
THIS ORDER IS SUBJECT TO ALL TERMS AND CONDITIONS DISPLAYED AT:
www.mouser.com/saleterms
 

[WA1GFZ]

I've also put a 560 ohm load across the audio input connector.
radio shack snap on beads could be your friend

[W1RKW]

Hi Tom,
Yes, I did the 50ohm non-inductive resistor approach across the input to the amp lastnight. That improved the carrier waveform considerably but still the carrier is not a 'clean' sinewave.   Without the 50ohm resistor the Retro75 is behaving like a noise generator, no discernable waveform at all.  The amp is producing what it sees at its input.  The HW-8 and FT-102 have a good match as it seems with the amp.  Their input waveforms are clean and the output of the amp is clean with these two RF sources. I'm wondering if there is some sort of reflection or impedance issue that is messing with the biasing of the final transistor.

I haven't addressed any potential audio problems yet.  Just trying to get the carrier cleaned up. I suspect that if I get the carrier cleaned up the audio amp will not be a problem.

So I 'm wondering on my approach to clean this up.  Do I mod the Retro 75 final or do I need some sort of matching network between the Retro and amp.

http://smallwonderlabs.com/Retro-75_Instr.pdf    The final PA is on page 10.

Lastly, as far as I can tell the phase relationship of the amp from input to  output seems to be 180 degrees apart so any feed back related to the close proximity of the RF input and RF output jacks will only effect gain potentially depending on the shielding between them. So I don't think I have a detrimental issue here, at least not yet.

[K1JJ]

Bob,

You could try a pi-network between the driver and amp. That will probably solve it. All you need are some small variable caps, like a 365pf multi-section to ground on the input, (1500pf or so)  a roller inductor in series and a 1500pf cap to ground for the ouput of the tuner.


Even  easier, a 'T' network should work just as well. Experiment with clips leads until you find an effective config to build.

You can build one up easily and get a 1:1 match on any freq when using the roller and var caps.

T

[WA1GFZ]

bob,
Try a couple beads on your scope probe lead to make sure it isn't common mode crap getting into the scope.

[AB2EZ]

Bob

I was looking at the documentation on the Communications Concepts web site for the EB63.

Some things are peculiar:

A. The documentation says that the input and output transformers are 16:1 (turns ratio)... but this is clearly a typo. The correct value is 1.6:1. I.e., the 20 ohms of swamping resistance from gate to gate x 1.6 x 1.6 = a 51 ohm input impedance.

What is the turns ratio of the transformers you are using?

B. The capacitance across the primary and the secondary of each transformer appears to be too large. For example, the capacitance across the primary of the input transformer is .001 uF. This capacitor has a reactance of only 40 Ohms on 75 meters (and, of course, less at higher frequencies). Something is "fishy" about those capacitances.

I suggest that you reduce the values of the capacitors across the input and the output of the input transformer by a factor of 10.

Best regards
Stu

[WA1GFZ]

Bob,
I agree with Stu .001 is way too high. CCI amplifiers have problems below about 3 MHz. The transformer inductance is a bit low for 160 meter operation. I'm thinking you could get away with around 200 pf. This will degrade low frequency operation a bit. If you are running a 12 volt final the output transformer is usually 1:4 or 1:5 turns ratio. 1:5 for higher power devices. Both inputs and outputs of bipolars are quite low impedance. FETs are easier to drive. A Bipolar without RF feedback will have an input impedance all over the map. Best to try a 3 dB pad on the input to control the load on the source.

[AB2EZ]

Frank

Yes... since these are bipolar transistors (it would have been better to use 11N90s for 75 meter operation) the input impedance is very low... and that would explain the large transformer turns ratio. The pair of 10 Ohm resistors are not swamping resistors... but just biasing resistors.

Nevertheless, as we agree, the capacitance across the primary of the input transformer is too large... and the Retro75 is probably very unhappy about looking directly into this amplifier.

I agree that a power divider/attenuator at the input might help.

Stu   

[K1JJ]

(it would have been better to use 11N90s for 75 meter operation)  

Stu,

You've probably heard a few of the guys on 75M experimenting with 260's and 11N90's for 75M linear service. How clean do you think these devices are for linears?    It seems commercial  amps go to the great expense of paying $35++ per device for the MRF-150's. Why aren't the 11N90's for $3 each being used there?

No one's run spec analyzer tests on these project amps yet, so I'm curious.

I even had problems getting my eight pill  MRF-150 SS linear amp to be acceptably clean, so this is puzzling.


T

[AB2EZ]

Tom

I agree that it is important to consider how "clean" an amplifier is... and that this may be a problem associated with many of the simple linear amplifier designs that are appearing on the air (both in commericially produced products and in homebrew versions)

As you know... there are two complementary issues in that regard. For clarity... I'll state what these are.

The first issue (which is not what I think you are referring to) is whether the amplifier generates harmonics of its intended rf output frequency.

If one is using a broadband output circuit... then getting the amplifier to meet FCC requirements can be challenging. However, if the amplifier has a resonant tank circuit of sufficient Q (such as the series resonant circuit that the Class E folks use) or a selectable low pass filter at the output (which a lot of these types of simple linear amplifiers employ)... or both (which I use with my 1-FET Class E transmitter)...then keeping harmonics under control is a lot easier.

I assume that anyone who builds an amplifier would take reasonable steps to sufficiently attenuate harmonics.

The second issue is whether or not the amplifier produces "splatter". Splatter can be produced in a collector-modulated or drain modulated amplfier by non-linearity of the modulation characteristic... but it can also be produced in a "linear" amplfier by 3rd order rf intermodulation products associated with the rf output vs rf input characteristics. For example, if I have a linear amplifier that is accepting a 3.5 kHz-wide modulated signal as input, centered at 3.885 MHz... then the third order products will include a 10.5 ( 3 x 3.5) kHz wide signal that is also centered at 3.885 MHz. The output circuit, of course, cannot filter this out.

So... what I think you are referring to is this 3rd order product.

I suspect, as you suggest, that many of the linear amplifiers that are appearing on the air have a "wide" signal that is, in fact, associated with 3rd order intermodulation products.

This is certainly something to keep in mind when building one of these amplifiers.... particularly if the input signal is AM-modulated by a fairly wide audio signal, than doesn't have any sharp bandlimiting filters (e.g. +/-7 kHz x 3 = +/- 21 kHz).

Thanks for pointing this out.

Stu    

[WD5JKO]

I have been following the Retro 75 projects for a while. These rigs will find a nice niche on 75m AM.

I have a couple of comments regarding the Retro-75+EB63 combination.

1.) It seems that the PEP output of the Retro75 (~ 11watts pep) will be at least 2X the input requirement of the EB63 (~ 5W pep). So as others have posted, a 3db (at least) attenuator between the Retro and the amp would be helpful. Alternatively adding negative feedback to the amp as Shane suggested will lower the EB63 stage gain, increase the drive requirement, and clean up some of the intermodulation distortion Stu and Tom discussed. Maybe do some of both an attenuator, and RF feedback. Analysis of the EB63 input impedance is warranted.

2.) Since both the Retro75, and the EB63 are powered from 12V, there might be some sort of regeneration between the two units if the power comes from the same source. If so, additional decoupling, and ground considerations should be looked into. The layout of the parts, wire routing, etc. is important. Tom, K1JJ has a bunch of insight with this kind of stuff from his Class E experience. I think Tom eats ferrite beads for breakfast!   

Jim
WD5JKO

[W1VD]

Can't recall seeing good IMD performance (-32 dB 3rd and quickly disappearing higher orders) from devices unless they are specifically rated for linear service or run well below (-3 to -6 dB) their saturated power output levels. This goes for tubes, transistors and FETs.   

[K1JJ]

Can't recall seeing good IMD performance (-32 dB 3rd and quickly disappearing higher orders) from devices unless they are specifically rated for linear service or run well below (-3 to -6 dB) their saturated power output levels. This goes for tubes, transistors and FETs.  

Yes, this is what I'm referring to, the 3rd order IMD of a device. Thanks for the info Stu and Jay.

Lord knows I've been thru many many "linear" amp tubes trying to find one that will give me -35db 3rd in the real whirl. It was difficult and drove me crazy for years. I finally did it, but it took elaborate measures.

I see that most riceboxes of today are good for only about 30-32db 3rd. So the IMD willl never be better than the ricebox's IMD figures when driving an amp - and always worse. Many SS linear amps are good for -25db 3rd. Gawd, when someone is S9 + 60 over, they will be splattering up the band heavily using specs at -25db 3rd or lower.

Anyway, that said, using these 11N90 amps mobile might fly under the radar, as the signal is already down 15db with the mobile whip - and the narrowband whip may even offer some near-channel attenuation due to the whip's VERY narrow swr curve.

I would certainly like to build one up myself using 11N90's, though, from my past experience with MRF-150's, it will not be an easy task if the amp is doing 1500w pep and operating into a fullsize dipole near others...  

Though I heard one guy with a band scope say that he thought the project amp (running 260's) was reasonably clean the other day. I would like to see a real two-tone scientific  spectrum analyzer test and some careful efficiency measuements made to put the mystery to bed...

I know Bob/KBW has one fired up and is getting the bugs out now. I'll post whatever results we can glean from that. Hope it works FB. What a savings it would be to run $3 11N90 super 900V Fets in a linear amp!!!

T