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#1
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would like advice on idea for class E SSB Exciter
Hi,
I'm considering making a class E amplifier for single-sideband output, but from what I understand, modulating the plate voltage produces double-sideband. I'd like to avoid that if I can. I'm thinking of taking regular audio, digitally processing it into single-sideband using a soundcard, and then digitizing that to 8-bits. I'd then create 8 class E amplifiers that put out 1/8w, 1/4w, 1/2w, 1w, 2w, 4w, 8w, and 16 watt outputs for the same signal; modulating each one as CW in accordance with the audio bit. I'd combine the outputs through a power combiner. Questions: 1) Would this scheme be possible for creating SSB by parallel switching of CW amplifiers in accordance with a digitized audio signal? 2) Would I need to create assymetrical power combiners to combine the amplitudes of the different signals, or could I get away with simply making a symmetrical 8-way combiner. Thanks in advance, The Eternal Squire --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.543 / Virus Database: 337 - Release Date: 11/21/03 |
#2
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In concept, it should be possible to generate SSB digitally at any frequency
and any power level. Clearly, it is possible to generate low-level SSB with a PC sound card, but the output frequency is typically limited by the speed at which the card can sample, digitize and process the audio. This speed limitation caps the output at around 15-20KHz with typical sound cards. The output power delivered to the load is purely a function of how much voltage the D-A converter can deliver to the load. If, as you suggested, one took the 8-bit digital signal (pre-D/A converter) amplified each bit in 8 binary-weighted 'lossless' voltage (not power) drivers, and summed the output in a purely resistive load, then one would have a high-power, high efficiency SSB exciter. Here are the problems I see: 1) Speed. The A/D, DSP, and D/A have to be able to process audio samples at twice the highest RF output frequency (2.5-3.0 times in practicality) 2) Precision: The high power weighted binary output drivers need to produce very accurate voltages 3) Spectral purity: The output spectrum will have aliases and 'birdies'. The number and amplitude of these is a function of the number of bits in the digitizing process, and the accuracy of the high-power D-A at the output. 4) Complexity: Enough said about this. A more practical way to get a similar result might be to generate low-level SSB in a high-speed DSP, and amplify the analog result it in a class-B amplifier whose bias is dynamically adjusted by the DSP processor in anticipation of the instantaneous RF level. This should produce a highly-quality signal with good overall power efficiency. Joe W3JDR Check out my project page: http://mysite.verizon.net/jdrocci/. "The Eternal Squire" wrote in message ... Hi, I'm considering making a class E amplifier for single-sideband output, but from what I understand, modulating the plate voltage produces double-sideband. I'd like to avoid that if I can. I'm thinking of taking regular audio, digitally processing it into single-sideband using a soundcard, and then digitizing that to 8-bits. I'd then create 8 class E amplifiers that put out 1/8w, 1/4w, 1/2w, 1w, 2w, 4w, 8w, and 16 watt outputs for the same signal; modulating each one as CW in accordance with the audio bit. I'd combine the outputs through a power combiner. Questions: 1) Would this scheme be possible for creating SSB by parallel switching of CW amplifiers in accordance with a digitized audio signal? 2) Would I need to create assymetrical power combiners to combine the amplitudes of the different signals, or could I get away with simply making a symmetrical 8-way combiner. Thanks in advance, The Eternal Squire --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.543 / Virus Database: 337 - Release Date: 11/21/03 |
#3
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In concept, it should be possible to generate SSB digitally at any frequency
and any power level. Clearly, it is possible to generate low-level SSB with a PC sound card, but the output frequency is typically limited by the speed at which the card can sample, digitize and process the audio. This speed limitation caps the output at around 15-20KHz with typical sound cards. The output power delivered to the load is purely a function of how much voltage the D-A converter can deliver to the load. If, as you suggested, one took the 8-bit digital signal (pre-D/A converter) amplified each bit in 8 binary-weighted 'lossless' voltage (not power) drivers, and summed the output in a purely resistive load, then one would have a high-power, high efficiency SSB exciter. Here are the problems I see: 1) Speed. The A/D, DSP, and D/A have to be able to process audio samples at twice the highest RF output frequency (2.5-3.0 times in practicality) 2) Precision: The high power weighted binary output drivers need to produce very accurate voltages 3) Spectral purity: The output spectrum will have aliases and 'birdies'. The number and amplitude of these is a function of the number of bits in the digitizing process, and the accuracy of the high-power D-A at the output. 4) Complexity: Enough said about this. A more practical way to get a similar result might be to generate low-level SSB in a high-speed DSP, and amplify the analog result it in a class-B amplifier whose bias is dynamically adjusted by the DSP processor in anticipation of the instantaneous RF level. This should produce a highly-quality signal with good overall power efficiency. Joe W3JDR Check out my project page: http://mysite.verizon.net/jdrocci/. "The Eternal Squire" wrote in message ... Hi, I'm considering making a class E amplifier for single-sideband output, but from what I understand, modulating the plate voltage produces double-sideband. I'd like to avoid that if I can. I'm thinking of taking regular audio, digitally processing it into single-sideband using a soundcard, and then digitizing that to 8-bits. I'd then create 8 class E amplifiers that put out 1/8w, 1/4w, 1/2w, 1w, 2w, 4w, 8w, and 16 watt outputs for the same signal; modulating each one as CW in accordance with the audio bit. I'd combine the outputs through a power combiner. Questions: 1) Would this scheme be possible for creating SSB by parallel switching of CW amplifiers in accordance with a digitized audio signal? 2) Would I need to create assymetrical power combiners to combine the amplitudes of the different signals, or could I get away with simply making a symmetrical 8-way combiner. Thanks in advance, The Eternal Squire --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.543 / Virus Database: 337 - Release Date: 11/21/03 |
#4
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"The Eternal Squire" wrote in message ...
I'd then create 8 class E amplifiers that put out 1/8w, 1/4w, 1/2w, 1w, 2w, 4w, 8w, and 16 watt outputs for the same signal; modulating each one as CW in accordance with the audio bit. I'd combine the outputs through a power combiner. Questions: 1) Would this scheme be possible for creating SSB by parallel switching of CW amplifiers in accordance with a digitized audio signal? nope, you will instead need to apply simultaneous frequency modulation to the carrier as well as amplitude modulation. 2) Would I need to create assymetrical power combiners to combine the amplitudes of the different signals, or could I get away with simply making a symmetrical 8-way combiner. you will need an infinite way combiner, the easier way is to frequency modulate the carrier so that it shifts to wherever you want the carrier to be and control the envelope (this is a white lie, the acutal explanation is a little deeper). the dsp software required is not going to be easy. i have written a dsp shell for the sound card, you can download from http://farhan.net.co.nr and play with it. once i am finished with my VHF transceiver i plan implement a hilbert transform using this shell and make a simple SSB exciter with it. - farhan |
#5
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"The Eternal Squire" wrote in message ...
I'd then create 8 class E amplifiers that put out 1/8w, 1/4w, 1/2w, 1w, 2w, 4w, 8w, and 16 watt outputs for the same signal; modulating each one as CW in accordance with the audio bit. I'd combine the outputs through a power combiner. Questions: 1) Would this scheme be possible for creating SSB by parallel switching of CW amplifiers in accordance with a digitized audio signal? nope, you will instead need to apply simultaneous frequency modulation to the carrier as well as amplitude modulation. 2) Would I need to create assymetrical power combiners to combine the amplitudes of the different signals, or could I get away with simply making a symmetrical 8-way combiner. you will need an infinite way combiner, the easier way is to frequency modulate the carrier so that it shifts to wherever you want the carrier to be and control the envelope (this is a white lie, the acutal explanation is a little deeper). the dsp software required is not going to be easy. i have written a dsp shell for the sound card, you can download from http://farhan.net.co.nr and play with it. once i am finished with my VHF transceiver i plan implement a hilbert transform using this shell and make a simple SSB exciter with it. - farhan |
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