On 3 maalis, 11:48, "MadEngineer" wrote:
On Mar 2, 11:19 am, "OH1GTF" wrote:

Hello mates!

I'm really confused. I just built an I/Q based transceiver (mostly
following the Youngblood articles) and just noticed that when I'm
using either PowerSDR or sradio, I can still demodulate everything if
I plug I or Q connector off from the rx.

I thought that PowerSDR and Sradio are both IQ based softwares. Is it
done just by mixing the audio to baseband. Where those programs need
the phase relation between I and Q. Yet, I noticed that putting the
other component back, the reception improved many db.

Looking for some answers. Thank you!

73 de Janne, OH1GTF

Hi Janne,

I and Q each contain all received signals, and unless your ears are
better than mine you can't really tell whether you're listening to I
alone, Q alone or the full demodulated signal. I and Q are needed
together to determine whether the signals demodulate into the upper or
lower sideband, so if you have signals spread equally across the band,
listening to only one of the channels will fold the upper and lower
sideband signals together and you will receive twice as many signals
in a given bandwidth. This is the same as a simple DC receiver. Note
that your spectrum display will be symmetrical about the baseband
frequency if you disconnect one channel--all signals appear twice--in
the upper and lower sidebands.

In theory, I and Q each contain half the received power, and if only
one channel is present you spread the 3 dB reduced power to two places
(upper and lower sideband) so you will see a 6 dB total loss with
respect to the fully demodulated signal when you disconnect one
channel.

AM and FM demodulation can be done in software regardless of whether
the signal is mixed to a higher audio frequency (say, 11 KHz as some
SDRs do), or mixed right to baseband. For AM at least, the
demodulation algorithm is probably a little simpler when mixed to
baseband. For FM likely the opposite is true. Once you understand
the nature of IQ demodulation a little better the AM and FM
demodulation algorithms may become self-apparent to you.

73,
Glenn AC7ZN



Hi Glenn!

First of all, thanks for the really nice and informative reply.

So basically, what you are saying, is that we only need I or Q to
demodulate
AM or FM. Is I and Q then only needed to cancel out the other image ?
What I would still like to know is, does the same 6 dB rule come in at
the case of FM
and AM ?

This area of ham radio is one of the most exciting things I've been
playing with.
I started with earlier mentioned polyphase network receiver few years
ago and that
encouraged me to go further. Yesterday I tested the TX of my SDR and
it worked
immediately, giving out 85 mW with nothing else that the exciter
itself to a 50 ohm
resistive load. Very impressive!

Thanks,

Janne - OH1GTF