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Better sound from an SDR (such as a Flex) or other TX that uses a balanced modu




 
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Author Topic: Better sound from an SDR (such as a Flex) or other TX that uses a balanced modu  (Read 2440 times)
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WB2CAU
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« Reply #50 on: March 05, 2017, 01:16:26 PM »


 Maybe a board if there is enough interest.


Add me to the list of ops with interest in purchasing a blank PCB.  

I had boards made for an old PE project from this guy last year.  I was very impressed with the quality.  His prices are very reasonable.  All he needs is a good clear photo print of the board layout.  

http://www.farcircuits.net/

Although I've made numerous projects using perfboard, it's a PIA.  An etched board makes a much nicer, neater project, and greatly reduces the possibility of a wiring error.   The possibility of a wiring error increases with complexity.  This circuit is complex enough where a wiring error is possible with perfboard.  That's why I'd gladly pay for a printed circuit board.
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W2PFY
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« Reply #51 on: March 05, 2017, 05:13:28 PM »

.
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WA2SQQ
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« Reply #52 on: March 06, 2017, 09:14:34 AM »

... would it be the last thing in the line after the compressor limiter or before it?

Great question ... My audio chain is balanced throughout, and I see that this is only "line" out. Not sure that it makes that much difference feeding the radio with unbalanced line out. Also have one more question - why two phase reversal switches, @ input and output?

Already started building mine.
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KD6VXI
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« Reply #53 on: March 06, 2017, 09:23:24 AM »

1.  Add a phase splitter with a tl082 or 72 op amp on the output.

2.  Phase switch on in and out because when your audio signal goes through the Low Pass Filter you can have phase reversal.

--Shane
KD6VXI
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WA2SQQ
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« Reply #54 on: March 06, 2017, 10:01:52 AM »

1.  Add a phase splitter with a tl082 or 72 op amp on the output.
--Shane
KD6VXI

I'll have to research that one - thanks.
Since I had several 074's, I decided to use those throughout. The unused sections could probably be used for the above. I don't think there would be any audible difference using "line" vs a balanced output. Thanks
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Steve - K4HX
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« Reply #55 on: March 06, 2017, 11:47:57 AM »

You want the polarity (phase is an incorrect term) on the input so that the limiter is working on the side of the signal with the smallest amplitude. The switch on the output is required to ensure the side of the signal with the largest amplitude is modulating the transmitter in the positive direction. Without these two switches, much of the effectiveness of the processor could be lost.
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WA2SQQ
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« Reply #56 on: March 06, 2017, 11:56:37 AM »

Thanks for the education ...
Very much appreciated
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IN3IEX
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« Reply #57 on: March 07, 2017, 04:48:55 AM »

DC coupling to the modulator, I suppose...
There is a need to check this point.

G.

I picked up a Flex 5000 over the weekend at the Marlboro flea.  The price was right, so it followed me home....

Anyway, I put the thing on the air, hooked to my normal audio system that I use with the class E rigs.  The Flex has incredible frequency response, and is quite clean.  However, there was one problem - no negative peak limiter.  The result of this is the "folding back" AKA wavelets problem when one exceeds 100% modulation.  Such rigs don't really overmodulate - they fold back.   The resultant distortion is particularly annoying in standard AM detectors (fine in a sync detector)....

The flex has a particularly good low frequency response (down to single digits) so a negative peak limiter is very practical to use here, and there won't be any waveform "tilt".  So, I built a negative peak limiter followed by a 6 pole filter with the 3dB corner at 5.7kHz, and it works just great !!!



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steve_qix
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« Reply #58 on: March 12, 2017, 08:18:30 PM »

You want the polarity (phase is an incorrect term) on the input so that the limiter is working on the side of the signal with the smallest amplitude. The switch on the output is required to ensure the side of the signal with the largest amplitude is modulating the transmitter in the positive direction. Without these two switches, much of the effectiveness of the processor could be lost.

Steve is correct.  The NPL shown works properly for the Flex and with class E rigs because they are set up such that positive going energy generates a positive peak.  This may not be the case with all transmitters, in which case a phase selector on the output would be required.  An easy solution would be to use a TL074 instead of a TL072 in the input, and use one of the other op amps in the TL074 as a unity gain inverting amplifier.   Then add a switch to select between the inverted and non-inverted output.
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« Reply #59 on: March 13, 2017, 09:20:26 PM »

Sorry guys, I don't know what I was thinking.  The negative peak limiter ALREADY has an output phase switch included (I guess I anticipated this need, and put it into the design).  So ignore everything I said circuit-wise in the previous post  Roll Eyes

The circuit is all-ok as is.

Regards,  Steve
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