QRZ! Ham Radio 3/QRZ Ham Radio Callsign Database - Volume 3.iso/digests/homebrew/940087.txt

Internet Message Format | 1994-06-04 | 12KB

Date: Tue,  5 Apr 94 04:30:23 PDT
From: Ham-Homebrew Mailing List and Newsgroup <[e-mail]>
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Subject: Ham-Homebrew Digest V94 #87
To: Ham-Homebrew


Ham-Homebrew Digest         Tue,  5 Apr 94       Volume 94 : Issue   87

Today's Topics:
                              advice fm
                         G QRP CLUB AT DAYTON
                             Power meter
                               RADIOKIT

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policies or positions of any party.  Your mileage may vary.  So there.
----------------------------------------------------------------------

Date: Mon, 04 Apr 1994 18:31
From: ihnp4.ucsd.edu!swrinde!gatech!newsxfer.itd.umich.edu!nntp.cs.ubc.ca!unixg.ubc.ca!news.mic.ucla.edu!MVS.OAC.UCLA.EDU![e-mail]
Subject: advice fm
To: [e-mail]

>That's not the problem. You can easily shift the tuning to cover the
>required range, but broadcast FM is 75 kHz deviation FM while aircraft
>transmissions are AM. So you have to build a different detector, and
>get rid of the limiter in the FM receiver to demodulate AM properly.
>It'll still be noisy because the IF bandwidth is way too wide, 200 kHz
>instead of 12 kHz, but it will sort of work after you redesign about
>half the radio. On the other hand, a purpose built aircraft band AM
>receiver isn't expensive.

I've retuned a cheap AM-FM portable to the aircraft band.  It
sortof works, the sounds is low but audible, the wide bandwidth
brings in lots of noise and the tuning is picky (but the wide
bandwidth helps with the tuning).

Apparently the cheap FM detector does respond to AM.

I retuned the front end -- mainly changed the RF OSC coil to have
fewer turns.  I think it tunes from the high end of the FM
broadcast band to somewhere slightly above the 2 meter band.  I
also twisted the adjustments in the IF cans for peak output.
Probably narrowed the IF a bit (but not anywhere near enough).

PS: Very few 2 meter stations are actually audible.

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Date: Mon, 4 Apr 1994 22:17:06 +0000
From: ihnp4.ucsd.edu!usc!howland.reston.ans.net!pipex!uknet!demon!gqrp.demon.co.uk![e-mail]
Subject: G QRP CLUB AT DAYTON
To: [e-mail]

Attention Homebrewers Going to Dayton >>>>
You are invited to attend the Forum:
--------------------------------------------------------------------------
Building Your Own Equipment : George Dobbs G3RJV & Rob Mannion G3XFD
Friday April 29 - 1515 to 1645 - Room 2
---------------------------------------------------------------------------
and to join us at the G QRP Club Booth with its selection of Kits,
Publications and QRP related items
Much of it designed for the homebrewer
                          ................. and a few English voices
--
George Dobbs G3RJV
G-QRP Club

------------------------------

Date: 4 Apr 1994 14:55:55 -0500
From: ihnp4.ucsd.edu!usc!howland.reston.ans.net!vixen.cso.uiuc.edu!moe.ksu.ksu.edu!matt.ksu.ksu.edu![e-mail]
Subject: Power meter
To: [e-mail]

Hi:

I want to make a power meter to measure signals in the -30 to +30 dBm
range. I would like the measurements to be fairly accurate. I'm thinking
about using a diode detector. Does anyone have any suggestions for
doing this. Any comments or references would be welcome.

thanks,
nam

[e-mail]

                  ()     When all you have is a hammer ...
     #######======||
                  ##               ... everything looks like a nail!

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Date: 4 Apr 1994 20:41:29 GMT
From: ihnp4.ucsd.edu!swrinde!elroy.jpl.nasa.gov!ncar!stout.atd.ucar.edu![e-mail]
Subject: RADIOKIT
To: [e-mail]

Has anyone done business with a company called RADIOKIT
recently?

I have not seen any advertising from them in the last few
years.....They used to carry some useful parts for homebrew
projects as well as a line of kits.

Any info on current phone number for them and/or your
experience with them would be appreciated.


Thanks!

Chip Owens,  NW0O
Boulder, CO







--
Chip Owens ([e-mail])
NW0O  (boatanchors forever!)

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Date: 4 Apr 1994 16:49:25 GMT
From: elroy.jpl.nasa.gov!sdd.hp.com!hpscit.sc.hp.com![e-mail]
To: [e-mail]

References <2ng30c$[e-mail]>, <2nlebi$[e-mail]>, <2nmnbq$[e-mail]>
Subject : Re: How to do PSK demodulation?

In article <2nmnbq$[e-mail]>, Donald J. Miller <[e-mail]> wrote:
>
>Once again:  it is NOT necessary for PSK to have a constant envelope.
>If you disagree with this, go talk to one of your HP friends that are
>working on the test equipment for North American digital cellular.
>NADC is Pi/4 DQPSK and it does NOT have a constant envelope.  Folks

As I said, this is a semantic discussion.  I don't agree with their
terminology, that's all.

>have, and continue to ship what they call BPSK systems that fall
>under your definition of ASK.  To be consistant with your stand that
>ONLY the phase changes in PSK, one would conclude that only the
>amplitude changes in ASK, right?  A phase change at the zero crossings
>would not be allowed.  It has always been my impression that ASK

I am allowing "negative" amplitudes, so 180 degree phase shifts
(and only 180 degree phase shifts) are allowed in my definition of
ASK.  Again, my definition.

>was like sending data into an AM modulator -- complete with the
>offset so that over 100% modulation was avoided.  It should be
>possible to get NRZ data directly from an envelope detector without
>playing transition games.

The envelope detector cannot distinguish negative from positive
polarity so it doesn't work here.

>Vendors such as Alpha Industries produce "Bi-Phase modulators" as
>standard products for implementation in BPSK modulators.  These products
>have NO Q CHANNEL INPUT, so it would be impossible to generate a
>constant envelope signal with them.

A true phase modulator should have only one input, "phase".
What I don't know about that specific one is if the phase port is
linear or digital.  Modulators with I and Q inputs are for QAM.

To produce true BPSK, as I define it, take your NRZ data, run it
through a finite bandwidth pulse shaping filter, and then run it
into a linear, *analog* phase modulator (with no ideal switches).

To produce true ASK, as I define it, take your NRZ data, run it
through a finite bandwidth pulse shaping filter, and then run it
into a linear, *analog* bipolar amplitude modulator (a double balanced
mixer for example).  If you don't like the bipolar aspect, bias
the data so it is unipolar.

As the bandwidth goes to infinity, the systems become more and
more alike.

If you don't watch what you're doing, you can easily get a combination of
the two with neither constant envelope nor constant (absolute value of)
phase.  For example, following a BPSK modulator with a bandpass filter
produces AM via PM/AM conversion.  Moral, do all filtering at baseband.
(That comment ought to produce so controversy!).

>This "symantic nit" has turned into a rather lively discussion, it
>would appear.  I have checked the books at my disposal and can't
>really find any that have drawn the lines between modulation formats
>in the manner you describe.  What sources have led you to these
>conclusions?  I would like to check into them.

Essentially, these are my own ideas; you won't find any real discussion
of this in books, as far as I know.  You'll just have to keep reading
Usenet to keep up :-)  I have read every book I could
get my hands on about this subject, and they all gloss over the
details of implementation.  As I said in another posting, try Feher's
books and John Bingham's book on modem design for starters.
(Ever notice how you never see a *schematic* in those communications
theory books, only block diagrams.)

>~~ [e-mail] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Rick Karlquist N6RK
[e-mail]

------------------------------

Date: Mon, 4 Apr 1994 14:24:34 GMT
From: ihnp4.ucsd.edu!library.ucla.edu!europa.eng.gtefsd.com!emory!wa4mei!ke4zv![e-mail]
To: [e-mail]

References <2mriqq$[e-mail]>, <2nn1ge$[e-mail]>, <[e-mail]>ry
Reply-To : [e-mail] (Gary Coffman)
Subject : Re: advice fm

In article <[e-mail]> [e-mail] (Trevor Strohman) writes:
>In article <2nn1ge$[e-mail]>, Bob Witte <[e-mail]> wrote:
>>
>> I think other posters have pointed out that this will NOT be a simple mod.
>> There are many cost-effective solutions to listening to the aircraft band
>> if that is the goal here. Check your local Radio Shack catalog.
>>
>
>Why?  If it's an analog FM radio, couldn't you put an inductor or
>capacitor in series with the tuning system to change the frequency, or
>am I way off?

That's not the problem. You can easily shift the tuning to cover the
required range, but broadcast FM is 75 kHz deviation FM while aircraft
transmissions are AM. So you have to build a different detector, and
get rid of the limiter in the FM receiver to demodulate AM properly.
It'll still be noisy because the IF bandwidth is way too wide, 200 kHz
instead of 12 kHz, but it will sort of work after you redesign about
half the radio. On the other hand, a purpose built aircraft band AM
receiver isn't expensive.

Gary
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

------------------------------

Date: Mon, 4 Apr 1994 14:16:17 GMT
From: ihnp4.ucsd.edu!swrinde!emory!wa4mei!ke4zv![e-mail]
To: [e-mail]

References <2n6vv4$[e-mail]>, <[e-mail]>, <2nd871$[e-mail]>
Reply-To : [e-mail] (Gary Coffman)
Subject : Re: How to do PSK demodulation?

In article <2nd871$[e-mail]> [e-mail] (Donald J. Miller) writes:
>Gary Coffman ([e-mail]) wrote:
>
>: You can use a simple discriminator detector, run it's output through
>: a slicer that switches at zero crossing, use a Costas loop or squaring
>: loop to recover clock, and sample the slicer waveform at bit centers to
>: data detect.
>
>Wrong.
>
>Costas and squaring loops are used for CARRIER recovery, not clock
>recovery.  Discriminators have been used to recover PSK where optimum
>performance is not required, but synchronus detection is far better.

Wrong. :-)

Costas loops, or somewhat simpler varients, can be used for clock recovery,
the WA4DSY modem does for example. And discriminator detectors are superior
to synchronous detectors when lock up speed is critical, as it is in fast
packet modems. There is a 6 db S/N penalty, but that's usually acceptable.
The trick is to use a tracking data detector after the discriminator so that
frequency errors up to +/- 5 kHz are compensated.

Gary
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
534 Shannon Way             |    Guaranteed!      | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     |                     |

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End of Ham-Homebrew Digest V94 #87
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