I was on a call the other night with someone who asserted that you

didn’t need an I & Q representation

for a direct-conversion receiver, and that I and Q could be

synthesized later from a real-mode-only

baseband signal.

I know that very-early (1940s) direct-conversion receivers didn’t use I

and Q signal chains, but they were

typically used for demodulating AM signals, where the +/- frequency

ambiguity wouldn’t have been

an issue.

So, my feeling is that you *absolutely need* the I and Q “form” in order

to disambiguate +/-

frequencies when dealing with direct-conversion baseband signals.

Who’s right?

–

Principal Investigator

Shirleys Bay Radio Astronomy Consortium

http://www.sbrac.org

On 27.02.2011 17:28, Marcus D. Leech wrote:

I was on a call the other night with someone who asserted that you

didn’t need an I & Q representation

for a direct-conversion receiver, and that I and Q could be

synthesized later from a real-mode-only

baseband signal.

…

So, my feeling is that you *absolutely need* the I and Q “form” in order

to disambiguate +/-

frequencies when dealing with direct-conversion baseband signals.

Who’s right?

As long as you receive the complete signal bandwidth, you can create the

I/Q form later.

Of course you need the double sample rate, if there’s only the real

“baseband”

representation. I call it baseband, but you can also call it IF with the

lowest

possible IF frequency. Strictly speaking it’s not a “direct-conversion”

receiver,

since there is a fixed IF in the middle of the baseband spectrum.

The “data rate” is the same for both, one has double sample rate but

only

half the sample size (real vs. complex numbers).

Complex baseband (I/Q) reconstruction:

- Hilbert transform eleminates the (symmetric) negative frequencies
- Frequency shifting the IF frequency to zero by multiplying a complex

exp(-j*2*pi*f_IF*t)

This is standard in digital down converters (DDC).

The TVRX-Board is working this way. According to the schematic,

only the bipolar A channel is connected to the tuner chip, a real input.

Other Dboards use both A/B inputs for separate I/Q channels.

I think both variants have their advantages and disadvantages.

On 02/27/2011 06:16 PM, Moeller wrote:

- Hilbert transform eleminates the (symmetric) negative frequencies
- Frequency shifting the IF frequency to zero by multiplying a complex

exp(-j*2*pi*f_IF*t)

This is standard in digital down converters (DDC).

The TVRX-Board is working this way. According to the schematic,

only the bipolar A channel is connected to the tuner chip, a real input.

Other Dboards use both A/B inputs for separate I/Q channels.

I think both variants have their advantages and disadvantages.

Right, for a non-zero IF, it’s easy to see how to do the Hilbert

transform and convert to I+Q, provided the

sample rates are correct.

But for a zero-IF, direct-conversion, with only a single baseband output

(single mixer), I don’t see how you

can make it work.

–

Marcus L.

Principal Investigator

Shirleys Bay Radio Astronomy Consortium

http://www.sbrac.org

On 28.02.2011 00:22, Marcus D. Leech wrote:

But for a zero-IF, direct-conversion, with only a single baseband output

(single mixer), I don’t see how you

can make it work.

A real valued zero-IF “universal” (modulation independent) receiver does

not exist.

I think you have the a demodulating receiver in mind that relies on

symmetry in the baseband spectrum, like for AM. In this concept,

“baseband”

is the real valued audio baseband. For digital modulations it doesn’t

make sense.

The real valued representation with IF at half of the one-sided signal

bandwidth

can be called “real baseband”, in contrast to the “complex baseband”.

Same data size, same content, same bandwidth, just shifted in spectrum.

On 02/27/2011 06:41 PM, Moeller wrote:

A real valued zero-IF “universal” (modulation independent) receiver does not

exist.

I think you have the a demodulating receiver in mind that relies on

symmetry in the baseband spectrum, like for AM. In this concept, “baseband”

is the real valued audio baseband. For digital modulations it doesn’t make

sense.

The real valued representation with IF at half of the one-sided signal bandwidth

can be called “real baseband”, in contrast to the “complex baseband”.

Same data size, same content, same bandwidth, just shifted in spectrum.

Yup, that’s pretty much what I said in my initial post on the subject.

The 1940s-era direct-conversion

receivers were designed specifically for things like AM, where the

+/- frequency ambiguity didn’t

matter.

Yup, placing the IF at Fs/4 makes sense in that you can later do a

Hilbert transform and convert to complex.

But if the IF is at zero, you lose.

–

Marcus L.

Principal Investigator

Shirleys Bay Radio Astronomy Consortium

http://www.sbrac.org