[TenTec] hiss hiding

[Duane - N9DG]

--- Steve N4LQ <n4lq at iglou.com> wrote:
> The Triton through the Omni series were all quiet
> receivers. Narrow xtal 
> filters were added to the Triton to make it an Omni. On the
> Omni C, a small 
> transistor amplifier kicks in when using the 500hz cw
> filter. This adds just 
> a touch extra noise. Otherwise, the Triton and Omni are
> neck and neck for 
> noise. Being single conversion rigs, they had few mixer
> stages thus less opportunity for noise.

The 580 Delta is almost in this same camp though a dual
conversion design. I found its main limitation (other than
poor AGC) is that the audio chain is rather noisy. Ten Tec
radios of this vintage I believe all used the ubiquitous
LM380/383 series ICs for their audio amplifiers. Theses
aren't very good audio amps if you look their specs sometime.
I'd bet that the RX audio of all TT radios of this vintage
could be improved upon with an audio PA update.

> Then came the Corsair. Passband tuning was added. To
> achieve this feat meant 
> another mixer and another IF stage. The extra IF at 6.3mhz
> eliminated the 
> 17m spur but added noise and lots of it in the form of hiss
> or white noise. 
> In the original Corsair you just had to live with the
> noise.
> Then came the Corsair II. Still had the extra noise but
> they added a tone 
> control. Turning this control to minimum treble cut down on
> the hiss.....some.

Like the models that precede it I've also concluded that it
is the audio chain that accounts for 80% of this hiss in a
Corsair series radio. The Corsair has a number of op amps
that make up the active audio filtering, I suspect that the
cumulative noise and distortion of those parts is not
insignificant. They too use a somewhat noisy LM38x series
audio output IC. My experiments last fall using the Corsair
II as a 9MHz IF feeding a 9MHz detector (I/Q baseband audio)
feeding DSP IF processing chain (PC sound card with SDRadio
software) seemed to confirm this. I did not however try
tapping the 9MHz after the PBT tuning but I suspect that
there would be minimal differences than when I tapped the
9MHz signal immediately after the first 8-pole 9Mhz filter.

> They came the Omni VI. Still hissy but now we have DSP to
> help hide the 
> hiss. Works pretty slick as far as hiss hiding goes.

After some playing with a recently acquired Omni VI (price
was too right, couldn't pass it up) I found a new variation
of audio chain limitations. From what I can tell the A to D
is only 14 bits, I've never been convinced that 16 bits is
enough for high fidelity audio, so needless to say I have
reservations about 14. The Omni VI does however use a more
modern audio PA IC; it is noticeably cleaner than the
LM38x's. It is a TDA8611A, if you look up its specs it is
considerably better than the LM38x.  The Pegasus (Jupiter and
Orion too I presume) use the TDA7056B. I always found the
Pegasus to have excellent audio. The Peg's (Jupiter too) main
shortcomings are the high-ish 1st LO phase noise and high
frequency first IF, - therefore suffers from the typical wide
roofing filter dynamic range limitation. Can't say I've
noticed digital noise artifacts for the way I use the Pegasus
though. 

> I played with the Flex-Radio SDR for a few weeks. I got the
> same buzz. It 
> was as if my ears were directly connected to the ether!

I also attribute much of the SDR-1000's *stellar* audio
fidelity to the fact that you use an inherently clean audio
chain by virtue of it being a modern PC sound card. Almost
all such sound cards perform well for high fidelity music
reproduction purposes so it is no accident that a radio based
on them would have very good audio too.    

> Crazy thing was 
> useless as a transmitter and flaky as a june bug though.
> It's SINGLE CONVERSION!

Its software is changing weekly, at least the experimental
code branch is. It is not the same radio
performance/behavior-wise that it was as little as two months
ago. Fun stuff.

If you dig deep enough and get your head wrapped around the
inner workings of the SDR-1000's QSD you will realize that it
isn't really a mixer at all, it is actually a "sampler" that
provides an audio baseband version of a small slice of RF
spectrum. So in reality its analog RF section is a "Direct
Conversion" radio in the truest sense of the word.

> Some day I hope others follow this 
> lead and perfect it.

It's just a matter of time before almost all radios will
include many of the design concepts and approaches that the
SDR-1000 is now pioneering.

> Bottom line here is; too many mixers spoil the radio.

Yepp.

Duane
N9DG



		
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