Thanks for responding Glenn!

Provided you implement the Tayloe mixer with sufficient baseband
bandwidth (probably about 20 KHz for NBFM), you can mix, say, a 2 meter
FM signal to baseband and demodulate it with an audio discriminator
(some sort of audio frequency to voltage conversion scheme). Mix the
signal to be either exclusively in the upper or lower sideband of the
output. So, for example, a NBFM sig at 146.000 MHz mix with the Tayloe
mixer set to 145.990 MHz and select the upper sideband. The upper or
lower sidebands are, as you probably know, selected by phase shifting
and summing circuits following the Tayloe mixer (implemented in
software in SDRs). Note that SDRs like the Flex-Radio SDR 1000 do not
process incoming signals near 0 Hz anyway, but mix the desired signal
centered around about 11 KHz (I think).
So the phase shifter is NOT for flatten the group delay variance (source is
the Tayloe mixer low-pass)?

Is multiplying I with Q enougth to demodulate FM as in a quadrature
demodulator?
I cannot find a suitable theory page to look for.


The Tayloe mixer is a passive mixer terminated in large capacitors, and
similar performance can be obtained at VHF, UHF and microwaves with two
FET ring mixers driven with quadrature signals and also terminated with
capacitors (that is, no wideband transformer on the mixer outputs, but
capacitors followed by HiZ input differential audio amps). I've done
this with the Peregrine Semiconductor FET mixers.
Thank you for given the link to Peregrine. I read the datasheet. How do you
mix it?
At the moment I prefer the Tayloe mixer because of it's simplicity.
BTW: They have a nice low-power consumption PLL being compatible to
National.

At TI I found nice tinylogic capable of switching like a 4066 down to 500ps
....
Should be possible to run the mixer at 150MHz with it.

Only familiar with the dsPIC, though the others sound okay. The dsPIC
would work, though its 12 bit A/D doesn't give a lot of dynamic range.
Plenty for NBFM though, especially since you can run the signal(s)
through a limiter first. For NBFM sixteen bit DSP is sufficient.
How much dynamic range do I need? I thought about 100dB?
If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.

regards -
Henry

Henry Kiefer wrote:

So the phase shifter is NOT for flatten the group delay variance (source is
the Tayloe mixer low-pass)?
If I understand your question correctly, that is true. The IQ output
from the Tayloe mixer (or any IQ mixer) is not upper and lower
sideband. To get those you must do further signal processing, which
usually involves shifting the I and Q channels ninety
degress with respect to each other then summing or subtracting the
channels depending on which sideband you want.

Is multiplying I with Q enougth to demodulate FM as in a quadrature
demodulator?
I cannot find a suitable theory page to look for.

I can't either. The IQ signals multiplied do not make an FM quadrature
detector as the phase does not really shift much over the NBFM
bandwidth. There may be a clever way to extract FM more direcly from
the IQ channels without first detecting a sideband, but I don't know
it.

Thank you for given the link to Peregrine. I read the datasheet. How do you
mix it?

Maybe the wrong part? I'm talking about the PE4140 FET ring mixer.
You use two, driven with LOs ninety degress out of phase with respect
to each other.

At the moment I prefer the Tayloe mixer because of it's simplicity.
BTW: They have a nice low-power consumption PLL being compatible to
National.

At TI I found nice tinylogic capable of switching like a 4066 down to 500ps
...
Should be possible to run the mixer at 150MHz with it.

Yeah--digital just keeps getting better. Good luck.


How much dynamic range do I need? I thought about 100dB?
If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.techniques,

You need good dynamic range up to the limiter. This can be done with
analog circuits as discussed. After the limiter an A/D converter is
not even needed in theory--a fast-running timer hooked to a digital
port could do the trick. I don't remember any loss by removing AM in
the limiter (since all the the information is contained in the
frequency of the signal), but my theory is in the distant past, back
when FM meant 'funny math'.

Regards,
Glenn

On 12 Sep 2006 13:34:58 -0700, "MadEngineer"
wrote:


Henry Kiefer wrote:

So the phase shifter is NOT for flatten the group delay variance (source is
the Tayloe mixer low-pass)?
If I understand your question correctly, that is true. The IQ output
from the Tayloe mixer (or any IQ mixer) is not upper and lower
sideband. To get those you must do further signal processing, which
usually involves shifting the I and Q channels ninety
degress with respect to each other then summing or subtracting the
channels depending on which sideband you want.

Is multiplying I with Q enougth to demodulate FM as in a quadrature
demodulator?
I cannot find a suitable theory page to look for.

I can't either. The IQ signals multiplied do not make an FM quadrature
detector as the phase does not really shift much over the NBFM
bandwidth. There may be a clever way to extract FM more direcly from
the IQ channels without first detecting a sideband, but I don't know
it.

FM the problem is not responding to amplitude changes but frequency
changes. So any system that can count and measure frequency and
render a pattern based on changing frequency.

Synthetic PLL or simple a software PLL. would do it.
The synthethetic PLL approach means a software oscillator locked
to a varying frequency external signal. The error word generated is
the demodulated signal (apply to D/A or use raw).

How much dynamic range do I need? I thought about 100dB?
If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.techniques,

Dynamic range is one of those the more the better but, many things
like noise eat away at it. These days a radio with 80db is good
and 90DB excellent, 100 Db is attainable.

You need good dynamic range up to the limiter.

You need good dynamic range up to the first selectivity that can
remove offending close in signals.

Then you can limit hard. If you limit before or without adaquate
selectivity you will have intermodulation problems.

This can be done with
analog circuits as discussed. After the limiter an A/D converter is
not even needed in theory--a fast-running timer hooked to a digital
port could do the trick. I don't remember any loss by removing AM in
the limiter (since all the the information is contained in the
frequency of the signal), but my theory is in the distant past, back
when FM meant 'funny math'.

If the signal is limited then zero crossings are enough. That could
be expressed as 1bit. Your now working in the time/frequency
domain.


Allison

radio_rookie wrote:

Hello,
I want to know the importance of intermediate frequency in any
receivers. IF was used in Superhet transceivers. My question is why
doesn't anyone use zero IF now a days. What is the problem of brining
the RF signal directly to baseband? Does the IF stage conditions the
incoming signal? What are the advantages of the IF stage?

Just confused. Can anyone throw some light on this?

Thanks.

Direct conversion is used in nearly all modern mobile phones, because it is
cheaper (no IF filters), and because the baseband amplifiers use less
current than the old IF amplifiers used to. The RF performance is not
necessarily as good as a well designed superhet.

It is difficult to make the receiver immune to "AM detection" which is a
problem caused by receiving a strong interfering signal with amplitude
modulation on it, at a frequency other than the one that you are trying to
receive, but which somehow gets turned into a baseband frequency signal
coming out of the mixer, even though it shouldn't. There are plenty of
reasons why this can happen, such as second-order nonlinearity in the
mixer, meaning that a strong interferer coming into the RF port of the
receiver can mix with itself in your mixer and end up on top of the wanted
signal. Another cause could be if there is coupling between the LO
generation circuit and the RF input (in either direction, both are bad.)
Also it is common to get large DC offsets coming out of direct conversion
receivers, and for some modulation formats where you're interested in
frequencies down to DC, that can be a pain. People have pretty much solved
these problems in phones, after a lot of work.

Chris

Hi Glen -

Is multiplying I with Q enougth to demodulate FM as in a quadrature
demodulator?
The IQ signals multiplied do not make an FM quadrature
detector as the phase does not really shift much over the NBFM
bandwidth. There may be a clever way to extract FM more direcly from
the IQ channels without first detecting a sideband, but I don't know
it.
direct way:
I read about something like:
phase = (I * diff Q + Q * diff I)/(I*I + Q*Q)
but I cannot remember it exactly.


Thank you for given the link to Peregrine. I read the datasheet. How do
you
mix it?

Maybe the wrong part? I'm talking about the PE4140 FET ring mixer.
You use two, driven with LOs ninety degress out of phase with respect
to each other.
Yes, the correct part!
But I cannot see a great benefit using your concept. What is better than
others?

How much dynamic range do I need? I thought about 100dB?
If I recall theory I loss 2dB if limiting the signal to remove AM
sensitivy.techniques,

You need good dynamic range up to the limiter. This can be done with
analog circuits as discussed. After the limiter an A/D converter is
not even needed in theory--a fast-running timer hooked to a digital
port could do the trick. I don't remember any loss by removing AM in
the limiter (since all the the information is contained in the
frequency of the signal), but my theory is in the distant past, back
when FM meant 'funny math'.
Everywhere I read about the necessity to remove AM. If the band is used for
FM only, why then remove the not found AM in it?
I ran a simulation with Spice doing FM demodulator concepts comparision. The
difference between limited and not limited FM product detector was not of
significance. Doing the correct low-pass filtering after the detector was of
much higher importance.

- Henry

FM the problem is not responding to amplitude changes but frequency
changes. So any system that can count and measure frequency and
render a pattern based on changing frequency.

Synthetic PLL or simple a software PLL. would do it.
The synthethetic PLL approach means a software oscillator locked
to a varying frequency external signal. The error word generated is
the demodulated signal (apply to D/A or use raw).
A digital detector will have problems if the SNR is to low! If the SNR is
high enought, the all-digital system is just simpler to realize and consumes
lower power.

You need good dynamic range up to the first selectivity that can
remove offending close in signals.

Then you can limit hard. If you limit before or without adaquate
selectivity you will have intermodulation problems.
Superposition prinzip.

"Henry Kiefer" ) writes:

Everywhere I read about the necessity to remove AM. If the band is used for
FM only, why then remove the not found AM in it?

The limiting has nothing to do with receiving FM, it has everything to do
with getting rid of QRN.

Armstrong pursued FM because he wanted some system that was more noise
free than AM. Since FM does not have an amplitude componenet, that means
you can limit to clear out the QRN and some of the fading. The limiting
is what makes FM so appealling. Take out the limiter and the FM demodulator
will still work fine, but there'll be little point in switching to FM.

Michael VE2BVW

"Michael Black" schrieb im Newsbeitrag
...
"Henry Kiefer" ) writes:

Everywhere I read about the necessity to remove AM. If the band is used
for
FM only, why then remove the not found AM in it?

The limiting has nothing to do with receiving FM, it has everything to do
with getting rid of QRN.

Armstrong pursued FM because he wanted some system that was more noise
free than AM. Since FM does not have an amplitude componenet, that means
you can limit to clear out the QRN and some of the fading. The limiting
is what makes FM so appealling. Take out the limiter and the FM
demodulator
will still work fine, but there'll be little point in switching to FM.

OK Michael. So it is for fight against QRN and fading, not primary for being
immune to AM.
As my Spice simulations suggest; the FM demodulator works almost completely
comparable in analog (=not limiting) and digital (=limiting) mode.

What about the performance comparision of PLL demod and quadrature demod
concepts? Seems no much difference?

regards -
Henry