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Ham-Digital Digest Tue, 5 Sep 2000 Volume 2000 : Issue 242
Today's Topics:
Help wanted on understanding modulation theory. (4 msgs)
MFSK Info? (2 msgs)
MURS potential (4 msgs)
Service Manual Icom IC-211
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Loop-Detect: Ham-Digital:2000/242
----------------------------------------------------------------------
Date: Mon, 04 Sep 2000 15:20:18 GMT
From: "Michel Morin" <morinm@csdgs.qc.ca>
Subject: Help wanted on understanding modulation theory.
Hi there all PSK 31 fans.
OK, here is the problem, it concerns some theoretical aspects of
modulation in which I am lost. Supposing that someone sends a 1 khz tone on
29 mhz using USB, then his VFO will indicate 29000.00 khz. If I want to
receive this tone, I will then have to tune my radio to the sending
frequency of 29000.00, am I right so far? Now here is where I get lost.
Let's suppose that this tone is a PSK 31 signal whose frequency will vary 15
hz on either side of the 1000 hz tone. My VFO will indicate 29000.00 but I
will be receiving a signal which is varying 15 hz on either side of 29001.00
khz because the carrier has been suppressed and there is really nothing at
29000.00. What I don't understand is this: what signal, 29000.00 or 29001.00
will be converted and sent to the IF section at the IF frequency? If I have
a filter in the IF section, what I really want is that the 1 khz signal be
right in the center of the filter passband so I can work on that signal. If
29000.00 is converted to the center of the filter, then the 1 khz tone will
be on the edge of the filter and nearly unusable. If I want to use a narrow
cw filter, then the 1 khz tone has to be right in the middle of the filter
passband.
I am sure that there is a flaw somewhere in my reasoning and that I am
missing something that is probably obvious to specialists. The same problem
could be applied to a ssb voice signal and filtering. Please help because I
really am lost here.
73 de VE2CJW
Michel Morin.
------------------------------
Date: Mon, 4 Sep 2000 16:59:14 GMT
From: nomail@rob.knoware.nl (Rob Janssen)
Subject: Help wanted on understanding modulation theory.
Michel Morin <morinm@csdgs.qc.ca> wrote:
>Hi there all PSK 31 fans.
> OK, here is the problem, it concerns some theoretical aspects of
>modulation in which I am lost. Supposing that someone sends a 1 khz tone on
>29 mhz using USB, then his VFO will indicate 29000.00 khz. If I want to
>receive this tone, I will then have to tune my radio to the sending
>frequency of 29000.00, am I right so far? Now here is where I get lost.
>Let's suppose that this tone is a PSK 31 signal whose frequency will vary 15
>hz on either side of the 1000 hz tone. My VFO will indicate 29000.00 but I
>will be receiving a signal which is varying 15 hz on either side of 29001.00
>khz because the carrier has been suppressed and there is really nothing at
>29000.00. What I don't understand is this: what signal, 29000.00 or 29001.00
>will be converted and sent to the IF section at the IF frequency? If I have
>a filter in the IF section, what I really want is that the 1 khz signal be
>right in the center of the filter passband so I can work on that signal. If
>29000.00 is converted to the center of the filter, then the 1 khz tone will
>be on the edge of the filter and nearly unusable. If I want to use a narrow
>cw filter, then the 1 khz tone has to be right in the middle of the filter
>passband.
The signal you receive has to be in the filter passband. The frequency
of the suppressed carrier can be outside the filter passband.
So when you calculate your local oscillator frequency, you should simply
use Fsig-Ffilter or Fsig+Ffilter, and use the 29001.00 for Fsig. The
suppressed carrier (only used as a method to generate the PSK signal using
a USB transmitter) does not matter during detection.
Rob
--
+----------------------------------+--------------------------------------+
| Rob Janssen pe1chl@amsat.org | WWW: http://www.knoware.nl/users/rob |
| AMPRnet: rob@pe1chl.ampr.org | AX.25 BBS: PE1CHL@PI8WNO.#UTR.NLD.EU |
+----------------------------------+--------------------------------------+
------------------------------
Date: Mon, 4 Sep 2000 15:50:24 -0400
From: "Bob Lewis" <rlewis@staffnet.com>
Subject: Help wanted on understanding modulation theory.
Lets say you are running USB with the receiver dial set to 29000.00
Khz. If your IF filter is 2.8 Khz wide, the receiver passband will
extend from just above the suppressed carrier, perhaps 29000.10 to its
upper limit, 29002.90 Khz (The oposite condition exists for LSB
because the desired signal is lower in frequency than the suppressed
carrier). Now, if you want your 1000 Hz (+/- 15 Hz) PSK signal
centered in the recever filter, that filter has to be located (its
center) 1000 Hz above the suppressed carrier.
When specifying the frequency of a PSK signal, we really should
specify the actual RF frequency of the center of the signal. If you're
using a 1000 Hz tone and USB then that will be the dial reading plus
1000 Hz. If you want to set your receiver on that frequency then you
To be continued in digest: hd_2000_242B
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