G'day
Thanks to all those who wrote emails to me, expressing interest in the idea
of 160m-only direct conversion receivers with DSP 'hearts' and the idea of
integrating a number of these with a single local oscillator, with separate
active antennas for each one, to form a directional receive set-up in a
very small space.
The below is a continuation of the debtate between Tom W8JI and Phil VK6APH
on the 'pros' and 'cons' of such a system.
Such a system is possible with the DSP and PC sound card technology
currently available, but much work is involved in developing one. The
major problem isn't really technical, but that as this is a hobby there is
limited time available (you know, the rest of life that lies outside
topband - work, family, all that stuff).
Unfortunately, VK6APH has a commercial R & D operation to run, which takes
presidence over designing 160m receive systems for me (although I wish it
didn't). Right now, my wife has just come into the shack and reminded me I
have our kids to take to school and if I don't want to do it in my dressing
gown, I had better get off the PC.
These are the realities - and ultimate develoment limitations - we all
work within as 'radio amateurs'.
Vy 73
Steve, VK6VZ
W8JI said:
>I'd be the first to encourage people to spend money and effort to hear
>better, but they should also be encouraged to spend money wisely.
>
>Rather than argue or voice subjective opinions about "what's reasonable" or
>"darned good", it would be nice to see Phil describe why a 40dBm TOI 6-10dB
>NF mixer/receiver system is necessary and how and why it would directly
>change the ability of a fellow with a typical good receiver like a FT1000MP
>in an urban or suburban environment to actually hear better (barring his
>present receiver is defective).
>
>Where can we buy a processing system and software that will let unfortunate
>city dwellers hear without noise, and who will stand behind that result?
>Where can we get one that allows us to use a 40dBm TOI 10dB or better NF
>mixer?
>
>Where can I buy software and a DSP system that will allow me to combine TWO
>antennas that way, let alone four or more??
>
>I'd be willing to buy the software, if it has a money-back guarantee.
>
>As for matching the bandwidth, I disagree it requires a DSP system. A good
>250Hz filter works just fine, and is within a couple dB of anything anyone
>could do. Certainly 25Hz BW and normal CW at normal speeds without ringing
>is a pipe dream. Bandwidth is set by the rise and fall times, not speed.
>With 25Hz BW, I have to have significant rise and fall time (40mS or more)
>unless the skirts are useless.
>
>Knowing the cost of very fast reliable A-D convertors with enough bits to
>process 100dB dynamic range, I'd like to know how to do that with a $100
>sound card. Short of that, what is the actual limit for what we could buy?
>
VK6APH replied:
>First, let's take the sound card that is a vital part of such a system:
Our fellow engineers in the Audio business have been hard at
work pushing the state of the art with A/D and D/A converters. The bad new
is whilst some
PC audio cards claim 24 bits and 135dB dynamic range, this end of the
market costs in the
$1,000-plus range and finding software that can actually use these number of
bits is a challenge. The good news is although these high-end cards are
expensive, they are rapidly dropping in price.
The even better news is since I can live with 96dB of dynamic range (!),
multiple Santa Cruz Turtle
Beach cards do the trick in my phased array. At $55 each, 18-bit sampling
and 16 useable bits represents great price/performance, with the certain
knowledge that the 24bit cards will be at this price level shortly.
A useful review of the current crop of cards is at:
http://audio.rightmark.org/results.html
With regard to: "where can we buy a processing system and software that
will let unfortunate
city dwellers hear without noise, and who will stand behind that result?",
the simple answer is
that you can't yet buy a system that does that - you have to build it.
What I am designing for Steve VK6VZ
is a 'building block' that can form the basis of multiple DC receivers
all running off the same local oscillator, with each one connected to a
different active antenna and the outputs to sound cards connected to a PC.
As I have said before, this kind of system could form a a phased array,
with performance characteristics
that would cost thousands of dollars to build in terms of real-estate and
towers.
I'm not going to sell my software radio system commercially
since:
1. I like to keep my hobby and business ventures separate.
2. To prove the system, the first step is to build up one that can help
Steve VK6VZ do better in Top Band contesting and DXing - see the earlier
emails.
3. If the time is available to complete the entire system, then ultimately
no one will need to buy the associated software since I am happy to give it
away.
We will however keep reporting issues, progress and circuits to the topband
reflector, as the system
progresses. For example, one wonderful problem I have currently is that the
(KISS) local
oscillator for our DC receiver/transceiver appears to have such low phase
noise that we
can't find a way of measuring it!
-------------------------------------------------------------
W8JI said:
>As for matching the bandwidth, I disagree it requires a DSP system. A good
>250Hz filter works just fine, and is within a couple dB of anything anyone
>could do. Certainly 25Hz BW and normal CW at normal speeds without ringing
>is a pipe dream. Bandwidth is set by the rise and fall times, not speed.
>With 25Hz BW, I have to have significant rise and fall time (40mS or more)
>unless the skirts are useless.
VK6APH replied:
>If you are happy to give away 10dB of S/N ratio, then a 250Hz filter will
work fine. But that does not solve the noise problem.
The DSP-based system can determine the appropriate filtering to apply,
given the characteristics
of the received signal and the type of noise for a given block of samples.
For example, it may be appropriate to momentarily apply a wider filter to
minimise the smearing effects of a group of noise pulses.
This dynamic bandwidth switching is either extremely difficult and or
expensive with a
conventional analogue system, but can be implemented in a few 100 lines of
code in a DSP-based system. Again, I am talking about a matched filter, one
that takes into account the rise and fall times of the signal you are trying
to decode and automatically adjusts its characteristics to suit.
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