Real-only direct-conversion

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(-j2pif_IFt)

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(-j2pif_IFt)

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

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