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IQ demodulator PFD

C

colin

Hi,
With an IQ demodulator (lt5515) and a continuos RF(2->2.2ghz) and LO input,
I can tell if the RF is higher or lower then the LO by testing if Q leads I,
and a fairly crude instantaneous phase diference too by mixing them with a
dc offset added to one.

However what I realy want to do is to tell if the RF is higher or lower than
LO+IF(455khz)
and have an instantaneous phase difference.
I assume if I mix each of the above IQ signals with IF in a IQ modulator
this will let me acomplish this,
is there an easy way to implement this ?

I wish to use this to lock the LO.
I know there are other ways to lock a LO but im looking into this way,
as i need to generate the IQ signals anyway.

Colin =^.^=
 
J

John Woodgate

dated Thu said:
However what I realy want to do is to tell if the RF is higher or lower
than
LO+IF(455khz)
and have an instantaneous phase difference.
I assume if I mix each of the above IQ signals with IF in a IQ
modulator
this will let me acomplish this,
is there an easy way to implement this ?

If I've understood you correctly, I don't think there is even a
difficult way of doing it. You want to generate a 2.200455 GHz signal by
one stage of mixing? Even with quadrature mixing, I think it's pretty
ambitious.
 
J

Joerg

Hello Colin,

With an IQ demodulator (lt5515) and a continuos RF(2->2.2ghz) and LO input,
I can tell if the RF is higher or lower then the LO by testing if Q leads I,
and a fairly crude instantaneous phase diference too by mixing them with a
dc offset added to one.

However what I realy want to do is to tell if the RF is higher or lower than
LO+IF(455khz)
and have an instantaneous phase difference.
I assume if I mix each of the above IQ signals with IF in a IQ modulator
this will let me acomplish this,
is there an easy way to implement this ?

I wish to use this to lock the LO.
I know there are other ways to lock a LO but im looking into this way,
as i need to generate the IQ signals anyway.

At the risk of having misunderstood you there could be two methods:

a. Do the full Hilbert thing after the mixer. 90 phase shifters and all.
Doesn't have to be very precise since you'd only have to see in which
sideband the RF shows up. IOW check where is it stronger. So you
probably could get away with a few degrees of phase error, reducing
complexity in the Hilbert. If you aren't familiar with this topic look
up "phase method" under SSB in a ham radio book (the ARRL Handbook is
usually a really good deal).

b. Do the locking uC-assisted. Have it scoot up the LO a wee bit. If the
beat becomes lower then the RF was above, else it was below. Then track.
Have it do a sanity check once in a while if you expect wild changes.
 
C

colin

Joerg said:
Hello Colin,


At the risk of having misunderstood you there could be two methods:

a. Do the full Hilbert thing after the mixer. 90 phase shifters and all.
Doesn't have to be very precise since you'd only have to see in which
sideband the RF shows up. IOW check where is it stronger. So you
probably could get away with a few degrees of phase error, reducing
complexity in the Hilbert. If you aren't familiar with this topic look
up "phase method" under SSB in a ham radio book (the ARRL Handbook is
usually a really good deal).

b. Do the locking uC-assisted. Have it scoot up the LO a wee bit. If the
beat becomes lower then the RF was above, else it was below. Then track.
Have it do a sanity check once in a while if you expect wild changes.

Hmm maybe I need to explain it a bit better,
perhaps I'd better do a block diagram,

______ ______________
[RF vco]---| |Q------>[IQ mod]-->| |--- ph err
|lt5515| | | | bit of logic |
[LO vco]---|______|I------>[IQ mod]-->|______________|--- f error
| |
| |
[455khz*2]--[divide/2]--IFQ--' |
'-not-[divide/2]--IFI-----'


The RF is sweep scanned so the LO needs to keep up.
The idea is to try and do the LO tuning (almost) instantaneously,
hopefully this will solve the problem with the vcos pulling each other into
lock once they get within 1Mhz
Its for my heterodyne Lidar.

Colin =^.^=
 
J

Jim Thompson

On Thu, 31 Aug 2006 23:19:59 GMT, "colin"

[snip]
Hmm maybe I need to explain it a bit better,
perhaps I'd better do a block diagram,

______ ______________
[RF vco]---| |Q------>[IQ mod]-->| |--- ph err
|lt5515| | | | bit of logic |
[LO vco]---|______|I------>[IQ mod]-->|______________|--- f error
| |
| |
[455khz*2]--[divide/2]--IFQ--' |
'-not-[divide/2]--IFI-----'


The RF is sweep scanned so the LO needs to keep up.
The idea is to try and do the LO tuning (almost) instantaneously,
hopefully this will solve the problem with the vcos pulling each other into
lock once they get within 1Mhz
Its for my heterodyne Lidar.

Colin =^.^=

I'd bet it won't work ;-)

...Jim Thompson
 
C

colin

Jim Thompson said:
On Thu, 31 Aug 2006 23:19:59 GMT, "colin"

[snip]
Hmm maybe I need to explain it a bit better,
perhaps I'd better do a block diagram,

______ ______________
[RF vco]---| |Q------>[IQ mod]-->| |--- ph err
|lt5515| | | | bit of logic |
[LO vco]---|______|I------>[IQ mod]-->|______________|--- f error
| |
| |
[455khz*2]--[divide/2]--IFQ--' |
'-not-[divide/2]--IFI-----'


The RF is sweep scanned so the LO needs to keep up.
The idea is to try and do the LO tuning (almost) instantaneously,
hopefully this will solve the problem with the vcos pulling each other into
lock once they get within 1Mhz
Its for my heterodyne Lidar.

Colin =^.^=

I'd bet it won't work ;-)

maybe, but how much ?

Colin =^.^=
 
J

Jim Thompson

Jim Thompson said:
On Thu, 31 Aug 2006 23:19:59 GMT, "colin"

[snip]
Hmm maybe I need to explain it a bit better,
perhaps I'd better do a block diagram,

______ ______________
[RF vco]---| |Q------>[IQ mod]-->| |--- ph err
|lt5515| | | | bit of logic |
[LO vco]---|______|I------>[IQ mod]-->|______________|--- f error
| |
| |
[455khz*2]--[divide/2]--IFQ--' |
'-not-[divide/2]--IFI-----'


The RF is sweep scanned so the LO needs to keep up.
The idea is to try and do the LO tuning (almost) instantaneously,
hopefully this will solve the problem with the vcos pulling each other into
lock once they get within 1Mhz
Its for my heterodyne Lidar.

Colin =^.^=

I'd bet it won't work ;-)

maybe, but how much ?

Colin =^.^=

Draw up schematics for the "IQ Mod" and "bit of logic" blocks, then
I'll decide how huge the amount ;-)

...Jim Thompson
 
C

colin

Jim Thompson said:
Jim Thompson said:
On Thu, 31 Aug 2006 23:19:59 GMT, "colin"

[snip]

Hmm maybe I need to explain it a bit better,
perhaps I'd better do a block diagram,

______ ______________
[RF vco]---| |Q------>[IQ mod]-->| |--- ph err
|lt5515| | | | bit of logic |
[LO vco]---|______|I------>[IQ mod]-->|______________|--- f error
| |
| |
[455khz*2]--[divide/2]--IFQ--' |
'-not-[divide/2]--IFI-----'


The RF is sweep scanned so the LO needs to keep up.
The idea is to try and do the LO tuning (almost) instantaneously,
hopefully this will solve the problem with the vcos pulling each other into
lock once they get within 1Mhz
Its for my heterodyne Lidar.

Colin =^.^=


I'd bet it won't work ;-)

maybe, but how much ?

Colin =^.^=

Draw up schematics for the "IQ Mod" and "bit of logic" blocks, then
I'll decide how huge the amount ;-)

So you want to see if its gona work or not b4 you place your bet ahaha,
thats making it too easy but oh well, ..

Well the [IQ mod] are just typical IQ modulators with 2 ordinary modulators
summed together,
ie IFQ.Q + IFI.Q and IFQ.I + IFI.I

When the LO=RF+IF, the I and Q signals are 455khz and so when mixed with
455khz the resultant inputs to the bit of logic should be steady value.
when the phase/frequency relationship of the RF-LO changes they should
reflect the error from the desired phase/frequency.
Exactly how its reflected im not 100% sure, ive only imagined this in my
head so far, wich is a little bit tricky for so many stages of mixing.

The [Ph err] just converts the two inputs as if they were quadrature related
(wich I'm hoping they still are) into a phase.
However its still very much all on the drawing board still.

To measure small phase error this is quite easy as its just the eqv of one
input, for larger errors both inputs cld be combined, a.(b+dc) wich would
give a wider range albeit crude at the extremes.

The [F err] just detects wich input is leading with a flip flop, obviously
only makes sense when the phase error is changing quickly enough over > 360'

If it falls into a steady state of LO=RF-IF its liable to get confused,
however this shouldnt be a stable steady state, as the slope of the [ph err]
will be backwards.

Colin =^.^=
 
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