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RE: Designing the Ultimate HF Front End
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by K0BG on June 22, 2009
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Larry, Dave, et. al.,
First, I apologize to the author, as this response is off the direct subject, albeit related.
I make it a policy to never (!) use compression, not even for that rare, nirvana producing, DX entity. I keep my mic gain on the verge of none, I'm careful about getting too close to others, and I always try to make sure the frequency is clear before I call CQ. I'd like to think I do this as a courtesy to others, even though too few others do likewise.
As self-centered as it may appear, my on-air conduct is part of my personal motto; set the example, don't be one. I don't believe I've heard Dave on the air, but I have heard you, Larry, and it appears you too set a good example. I think that is all any of us can do, and I further believe that we all get our just rewards in the end.
As for the rest, they can hide behind all manner of disguise, berate and belittle me for my responses, and even hate me, if that's their intent. The difference is, their reward is here and now in the pitiful world they live in. Sobeit!
Alan, KØBG
www.k0bg.com
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RE: Designing the Ultimate HF Front End
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by WB2WIK on June 22, 2009
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I've never even seen a Hilberling, let alone use one, so can't comment on that. I have used both the IC-7800 and the FTDX9000 and they're both pretty spectacular.
But "ultimate HF front ends" don't have to be very modern to be good. My old Drake TR-7, c. 1978, uses a half octave preselector followed by a balanced mixer, with no preamplifier stage at all, and is as "bulletproof" as anything I've ever used. It does have a crystal roofing filter also, maybe the first one ever implemented in a piece of ham gear (not sure).
I cannot recall anyone ever overloading the Drake. It's also extremely immune to out-of-band (BC etc) interference, and in the Sherwood Engineering reports has the highest "100 kHz blocking" dynamic range of anything ever tested (at least, that was the case last time I looked), at 146 dB. Not bad for 30 year-old technology.
WB2WIK/6
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RE: Designing the Ultimate HF Front End
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by K6AER on June 22, 2009
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Receiver front ends are not a problem in today’s transceivers. Their dynamic range is well over 100 dB with third order intercept in the plus 30 dBm range. Even the cheapest transceivers are more than sensitive for the limitation is the noise floor coming out of the coax.
Mixer, IF distortion and filter phase distortion is much more of a problem. Even the IC-7800, K3 can suffer from Mixer, IF and filter phase distortion. The last 10-20 dB of performance is generally lost due to a poor antenna systems.
Software defined radios are also changing the traditional mindset on receiver design. Every cell phone in use is a software designed radio.
Sill, I would much rather have a beam up high and a cheep transceiver than a 7800 and a dipole.
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Designing the Ultimate HF Front End
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by KC8ZEV on June 22, 2009
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While many of the high end transceivers perform exceptionally well, my elmer taught me many years ago to put more of your resources (doesn't always mean $ but often times translates into that) into your antenna than your rig.
High end rigs cannot make up for a marginal antenna, period.
Even with modern advances in transceiver design, no circuit has yet to replace a poor antenna.
My two cents, for what it is worth, save your resources for the best antenna you can make work for your location. You might be surprised how that "entry level" HF rig performs.
Don't forget about your feedline too!!
73
KC8ZEV
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Designing the Ultimate HF Front End
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by MACKAY3031 on June 22, 2009
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Things I would want...No phase noise, no passband ripple, a pulse RF noise blanker early in the system that works,very high dynamic range, and a very low noise floor.
The amateur who mentioned cleaning up our transmitted signals is spot on!!
And of course, those that mention the critical importance of the antenna system are also right, however...Improvements in receiver design are always welcome.Spectral noise subtraction and 64 bit A/D converters with excellent dynamic range would be cool too.
DE KI4WCA
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Designing the Ultimate HF Front End
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by K4ELO on June 22, 2009
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A quick glance at http://www.sherweng.com/table.html
will show you a lot.
The K3, Flex 5000, and the Orion II are way ahead of what you have mentioned.
73
Wayne
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RE: Designing the Ultimate HF Front End
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by W7ETA on June 22, 2009
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"What order would you assemble the perfect HF front end and why?"
Can one ponder better answers than antenna and transmitter?
Bob
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Designing the Ultimate HF Front End
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by SV9OFO on June 23, 2009
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I also believe that since ADC's climbed linearly up to the 250 MHz range, there are more to come in SDR developments, like the complete removal of LO's as all conversions can now be in the math size. Greater sampling rates provide better accuracy filtering and an enormous bit depth along with huge linearity allows for even better dynamic ranges. Since evolution on the field keeps pulling performance ratings up the scale, the "perfect" receiver is always the next model to show up.
That, of course, leaves space in the receiver for better spectrum management on the RF side. Lower noise design concepts, including less parts for better performance, new types of semiconductors, hybrid components like electronically tunable RF filters and notches with extreme accuracy, and more.
Maybe new technologies could be evolving on managing spectrum on RF side. There are more to come in this field and I feel that we are witnessing a very interesting phase of transition in receiver performance equal to the introduction of tube supergenerative receivers.
My fears for tomorrow include facts like BPL and other sources of industrial in-band QRM that weren't here before, and that manufacturers will invest more on researching ways to get rid of those interference sources than evolving other characteristics.
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RE: Designing the Ultimate HF Front End
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by G3RZP on June 23, 2009
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The problem with cleaning up transmitters is that it's going to cost. Such an approach as a polar or cartesian loop is quite complex, especially the polar loop for multiple bands. Another similar approach is the CALLUM modulator, which uses two limiting amplifiers, fed in phase quadrature from VCOs with feedback and combined in phase quadrature. That needs very careful adjustment of both phase and amplitude balance and linearity in downconversion in the feedback loops, which is a fairly frightening requirement for multiple bands, although maybe an auto calibration might be possible. Possibly pre distortion in the SDR approach may work, but that requires some form of auto calibration when doing band changes (or even major frequency changes within a band). Pre distortion is probably going to be the most effective from the viewpoint of power consumption.
Reverting to receivers (although this applies to transmitetrs too), the phase noise is likely to be the limiting factor. If we have a +30dBm Intercept Point (IIP3) and a 10dB noise figure, then with a 250Hz CW filter noise bandwidth, we have a 113dB dynamic range. To have an equal phase noise limited dymanic range, the phase noise needs to be -137dBc/Hz with one interferer. For two interferers, we need -140dBc/Hz to get the same performance. Now if the transmitter has its IMD products improved to -60dB, the phase noise requirement for a 3kHz bandwidth is -94dBc/Hz for a 3dB degradation, so it should be -104dBc/Hz. But then we need to also look at the transmitter noise figure. Assume that we have a 1mW level at the base band modulator and a 60dB SNR. We then have at 1kW out, a 0dBm noise signal in a 3kHz bandwidth, or -34dBc/Hz. Now assume that outside the base band, we have 80dB of attenuation, we have -114dBc/Hz, so that allows a total transmitter noise figure of around 10dB, which is pretty difficult - especially with complex high linearity power stages.
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RE: Designing the Ultimate HF Front End
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by ZS5WC on June 23, 2009
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My dream rig would be an HF rig with all signal processing handled by valves-something like my FT-501.
Then add up conversion and roofing filter.
All HT dc supplies must be regulated by mosfets and the frequency control must be controlled by very well filtered VCO's locked to DDS'es..
Now that would be my best performing rig..!
William
ZS4L / ZS5WC
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