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

One thing I want to add. I have built tayloe detector polyphase
network receiver before and I know,
that we need I and Q to cancel out the image. But how the heck AM and
FM is done?

I just have a glue, that the I or Q signal is mixed to a higher
frequency and detected in traditional way,
but in software. Though, is there reason to mix it to a higher
frequencyif we are doing it in software?
Ok, that was the glue about FM, but AM, I can't even have a glue...

Thanks,

Janne, OH1GTF

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

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

On Mar 4, 1:03 am, "OH1GTF" wrote:
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

Hi Janne,

Yes, IQ receivers are fun. We typically use the complex frequency
domain to describe their operation, and in that domain they are very
elegant.

I just want to correct a misunderstanding: I or Q alone are not
sufficient to demodulate AM or FM except in the special case of AM
mixed so the carrier amplitude and phase are exactly at DC 0 volts.
You need both I and Q together just as you do to reject unwanted
sidebands in other modes.

My previous post was meant to say that it doesn't matter if you mix an
AM or FM signal to baseband or to some audio IF such as 20 KHz, as
long as you have both I and Q a software algorithm to demodulate it is
possible.

73,
Glenn

Sorry about not deleting unnecessary stuff in the last post--

AM and FM behave the same as other signals in an IQ receiver and if
you use I or Q alone they have the same 6dB loss I mentioned. You can
receive and demodulate AM and FM using only I or Q alone, but just as
with other signals you need both I and Q together to be able to reject
the unwanted mixing sideband and to recover all the power from your
desired signal.

73,
Glenn

"OH1GTF" wrote in message
oups.com...
One thing I want to add. I have built tayloe detector polyphase
network receiver before and I know,
that we need I and Q to cancel out the image. But how the heck AM and
FM is done?

I and Q give you phase information; sqrt(I^2+Q^2) gets you the amplitude of
the signal. Now... AM is not phase-modulated, right? So **assuming you can
synchronize your carrier to the incoming carrier**, "I" will be the original
signal and Q will be zero, so sqrt(I^2+Q^2)=I -- poof, only I needed! If you
chose Q, you'd just get zero (ideally) out of the receiver... but since you're
presumably in control of the local oscillator, you can just advance or retard
it 90 degrees and now Q is in-phase with the signal and I is zero. Hence AM
can be made to work with either I or Q... although it's not really
recommended, since -- if you have an IQ demodulator anyway -- you can build
SSB receivers as well, which is useful.

The tricky part is that "synchronizing to the incoming carrier" bit: If the
receiver and transmitter have the exact same frequency but phase offsets of X
degrees, the result is that I receives the original signal mulitplied by
cos(X) and Q receives the original signal multiplied by sin(X). (This is just
the general case of what I described above where things were 90 degrees out of
phase.)

Notice that phase and amplituide, while connected by phase=arctan(Q/I) and
amplitude=sqrt(I^2+Q^2), are two separate, uniquely "identificable" "things"
that you can transmit. This is taken advantage of in, e.g., "compatble" AM
stereo broadcast standards: In Motorola C-QUAM, for instance, I is set to
1+L+R whereas Q is set to L-R. If you run through the math, the amplitude of
this is not 1+L+R, but it is "close enough" if L-R is relatively small, and
hence compatibility with traditional (envelope) receivers is maintained, while
allowing a synchronous receiver to dig out the full stereo information.

---Joel

On 5 Mar 2007 03:53:20 -0800, "MadEngineer"
wrote:


I just want to correct a misunderstanding: I or Q alone are not
sufficient to demodulate AM or FM except in the special case of AM
mixed so the carrier amplitude and phase are exactly at DC 0 volts.
You need both I and Q together just as you do to reject unwanted
sidebands in other modes.

If you have frequency lock but not phase lock, the phase detector in a
phase locked loop will output a DC voltage proportional to the phase
difference. When using the Q multiplier as a phase detector, just
adjust the oscillator phase to bring the averaged Q output to zero and
you will get the demodulated signal from the I multiplier.

Costas loops have been used for AM detection, so you might find some
ideas from that direction too.

Paul OH3LWR

From: "Joel Kolstad" on Mon, Mar 5
2007 9:24 am

"OH1GTF" wrote in message

One thing I want to add. I have built tayloe detector polyphase
network receiver before and I know,
that we need I and Q to cancel out the image. But how the heck AM and
FM is done?

I and Q give you phase information; sqrt(I^2+Q^2) gets you the amplitude of
the signal. Now... AM is not phase-modulated, right? So **assuming you can
synchronize your carrier to the incoming carrier**, "I" will be the original
signal and Q will be zero, so sqrt(I^2+Q^2)=I -- poof, only I needed! If you
chose Q, you'd just get zero (ideally) out of the receiver... but since you're
presumably in control of the local oscillator, you can just advance or retard
it 90 degrees and now Q is in-phase with the signal and I is zero. Hence AM
can be made to work with either I or Q... although it's not really
recommended, since -- if you have an IQ demodulator anyway -- you can build
SSB receivers as well, which is useful.

Good post, Joel. Let me emphasize some more simplistic points
for other readers:

BOTH I (in-phase) and Q (quadrature) carry the SAME amplitude
variation as modulation, ergo the AM information can be taken
from either one if desired.

Selection of sideband for AM uses the phase DIFFERENCE between
I and Q that, combined with a wideband audio relative phase-
shift network in a linear (algebraic addition/subtraction) mixing
circuit (just op-amps), selects the desired AM sideband.

The tricky part is that "synchronizing to the incoming carrier" bit: If the
receiver and transmitter have the exact same frequency but phase offsets of X
degrees, the result is that I receives the original signal mulitplied by
cos(X) and Q receives the original signal multiplied by sin(X). (This is just
the general case of what I described above where things were 90 degrees out of
phase.)

Heh, heh, while a concise statement of what it is, the above
is going to be a snow job to those not familiar enough with
more-advanced math. I know it got me a few decades ago and
I just finished explaining that to another last month (off-
line with the advantage of some scratch paper to show the
relationships).

Locking the AM carrier to the local detector sub-circuit
oscillator can be done by simply running the whole IF
signal into a LIMITER to blot out the sidebands. The resulting
limited carrier is then used as a reference to lock the local
(detector) oscillator. "Lock" may be simplistic since the
old Motorola MC1330 video detector simply limits the carrier
internally, uses a single L-C external circuit to keep the
internal I and Q references at quadrature, and the two
internal "product detectors" output is linearly added
internally. Blessedly simple circuit, just one 8-pin DIP
and a very ordinary L-C that isn't critical in tuning. I used
that in a bank of 8 pulse detectors having carriers up to
62 MHz with no problems (even crammed into a thin space).

FM detection by similar, but NOT identical means, to AM.
The modulation equations for FM and PM don't allow that
similarity since sideband content is not really close to
AM. That is done in the more familiar "ratio detector"
or "quadrature detector" (sometimes an alternate name)
found in consumer electronics SOCs for FM receivers.
Try as I might this morning I couldn't put together a
text-only simple description of how that critter works. :-(

"Synchronous detection" of AM can be done on the same
model of the internals of the Motorola MC1330 IC with,
perhaps some more finesse applied to the detector's
local oscillator to remove some of the noise on low-level
input signals; not a terribly-important thing nor precise
since the oscillator's lock DC signal can be lowpassed
to around a half-second time constant in the loop filter.

Notice that phase and amplituide, while connected by phase=arctan(Q/I) and
amplitude=sqrt(I^2+Q^2), are two separate, uniquely "identificable" "things"
that you can transmit. This is taken advantage of in, e.g., "compatble" AM
stereo broadcast standards: In Motorola C-QUAM, for instance, I is set to
1+L+R whereas Q is set to L-R. If you run through the math, the amplitude of
this is not 1+L+R, but it is "close enough" if L-R is relatively small, and
hence compatibility with traditional (envelope) receivers is maintained, while
allowing a synchronous receiver to dig out the full stereo information.

C-QUAM follows the FM Stereo system where the demodulated
audio is the sum of Left plus Right. One hears (directly)
both sources for monophonic audio. The supersonic audio
carries Left minus Right audio. Linearly added and
subtracted, the Left plus Left and Right plus Right gets
the individual "ears" separated. The C-QUAM audio does
have a slight stereo distortion on odd selective conditions
that favor one sideband over another locally, typically
the effect of electric power distribution lines close to
the receiver site. Typical on my particular street. :-(

73,

Thanks for the additions, Len!

wrote in message
ps.com...
Heh, heh, while a concise statement of what it is, the above
is going to be a snow job to those not familiar enough with
more-advanced math.

Many textbooks and professors tell you the "bad news" about a phase difference
and that unwanted cos(theta) term... but they seldom mention that what "I
taketh," "Q giveth," and that you haven't actually lost anything yet -- if you
have both I and Q available -- just because you aren't phase-locked. I
suppose this is because, at that point, most textbooks are still hundreds of
pages away from discussing I-Q methods.

A lot of the texts make quite a mire out of the whole "arbitrary band-limited
signal impressed upon a carrier" bit. I think most people are almost better
off ignoring it initially (it seems to come up within the first few chapters
in many texts!) and then coming back to it once they have a good "feel" for
how phase modulation techniques work.

---Joel