Re: analogue PLL
- From: "colin" <colin.rowe1@xxxxxxxxxxxxxxxxxx>
- Date: Sun, 02 Sep 2007 10:08:27 GMT
"Chris Jones" <lugnut808@xxxxxxxxxxxxxxxx> wrote in message
news:13dk0flpcuph4d4@xxxxxxxxxxxxxxxxxxxxx
colin wrote:
"Chris Jones" <lugnut808@xxxxxxxxxxxxxxxx> wrote in message
news:13djb2och4sr2e@xxxxxxxxxxxxxxxxxxxxx
colin wrote:
Hi,
Im looking at the idea of using an analogue PLL
to add a fixed offset to a variable frequency.
but using a ramped offset to the phase error in the feedback loop
instead of using a mixer to add the frequency offset wich has problems.
to avoid problems at the limits of the phase error range
it would probably be best to do this in increments of 90'
by some simple divide by 4 logic.
the input frequency is ramped from 1-2ghz,
and the offset is ~500khz.
alternatives such as fractional divison pll and dds arnt without there
cons.
is this something that has been done before ?
Colin =^.^=
As the other posters suggested, yes it has been done before, the offset
loop
is one of the standard methods used in GSM mobile phone transmitters.
Basically what you are doing is putting a mixer in the feedback path to
the
divider (or after the divider if you prefer, but that's more noisy).
If you tell us what you are trying to achieve, and your performance
requirements, then we may be able to suggest the best approach.
Chris
laser modulation frequency is ramped from ~1ghz to ~2ghz quite quickly,
reflected signal is mixed with suitable LO to acheive ~500khz IF
not much higher as its an optical mixer.
phase versus frequency gives good corelation to reflected distance.
=> heterodyne lidar.
some error in frequency can be compensated for,
but theres a fair bit of feedthrough of any sidbands in the LO or RF
signal.
Colin =^.^=
Could you give a little more detail? (I am unfamiliar with the system.)
As I understand it, you are generating a modulating signal swept from 1GHz
to 2GHz as well as an LO signal that is always 500kHz above (or below) the
modulating signal. You then get some signal from a photodiode and mix
this
with the LO signal to get something around 500kHz, but slightly higher or
lower because the received modulation is slightly higher or lower than the
transmitted modulation signal. Is that correct?
yes thats pretty much spot on, the reflected laser signal is mixed with
the LO in a APD with the LO modulating the HV bias,
this cuases multiplication inside the gap of the APD wich works very well as
a mixer,
the 500khz signal is very strong compared to say a signal converted at
100mhz,
as the RF signal mixes with the LO before it is attenuated by the junction
capacitance,
yet still has the benefits of large phase change with small distance change.
as the RF frequency is swept the phase of the reflection changes
proportional to distance
the total phase change over the sweep represents the distance.
the output from the mixer changes by the same phase,
relative to the offset between RF and LO.
500khz is the minimum desirable offset as
the output lets through all sorts of crap from flourescent lights etc,
so to have a DC IF as Tam suggested is interesting but would need
something like a double balanced or quadrature mixer to properly reject
this,
im not sure how you would arange this, maybe a custom device?
ot two APD with out of phase LO ?
the APD are quite expensive it would be cool to think of a way to get a
cheap
photodiode to mix optically.
to convert the RF signal directly to electrical
results in far to poor SNR.
although it is posible to tune the detector output
this makes it dificult for wideband
however it would allow me to use a 10mhz IF wich
I have considered wich would make the signal generation quite easy.
You mentioned in another post that there might be problems with the two
signals (LO and modulating signal) locking together. In my experience
this
would not be a problem as long as you have enough money and space to place
the oscillators in separate shielding enclosures, with individual power
supply regulation and filtering, and individual isolation buffers to
prevent RF from one oscillator getting back into the enclosure of the
other
oscillator. You will find that having a wide loop bandwidth for the loop
that locks the two oscillators 500kHz apart will be very beneficial. You
want it to have plenty of gain left at 500kHz.
yes maybe but I was looking for s simpler way than spending lots of money or
time.
to have gain at 500khz means having quite a high comparison frequency,
this makes it dificult to find a N/R ratio wich gives 500khz offset.
the oscilators were moved to seperate boards with triple isolation buffers,
the PLL,VCO,laser and driver were made into a very tiny well sheilded
module.
using PHEMT for vcos driven very hard made it considerably better still,
but the mixed signal was still very distorted,
so they were still trying to pull together.
although im not too sure how this distortion would effect the result.
maybe it could be compensated out.
It might conceivably be easier to build something where the difference
between the modulating and LO signal is not a constant offset, but rather
where the ratio between the modulating and LO signals is constant. e.g.
modulating signal sweeps 1GHz to 2GHz, LO sweeps 1.0005 to 2.001GHz. This
way, you could just use two wide bandwidth frac-N synths which you program
and lock with (say) a 10MHz reference frequency from a DDS, and then sweep
the reference up to 20MHz, keeping the two synths locked the whole time.
You would have to deal with the variable IF frequency somehow, perhaps in
DSP.
this would rule out using an IF filter, wich im using a std ceramic 455khz.
A dsp to do the IF would be an idea,
but if I cld get the two frequencies easily and cheaply it would be nice.
the surest way so far would be to use a DDS and PLL for each,
but theyre a bit expensive and power hungry for fast ones,
and does seem a bit of overkill.
I initialy used just ~100mhz from 2 DDS wich worked well,
but trying to scale this up to 2ghz was dificult
I tried multipliers but this proved quite dificult.
maybe using a DSP to process the signal would allow me to use a lower
frequency.
but I think I might try this ramp idea first maybe simulate it,
should be able to do the comparison at least 100mhz wich should make any
lock in problems easy to deal with
I think I could make 4 PDs 1 for each quadrant in a gate array.
If I could use say 10ghz I would only need to ramp it over 1ghz wich would
only be 10%
this would make things easy, but 10ghz laser modulation
is probably way beyond what I can attain practically,
at the moment im using a cheap $2 laser pointer diode,
I was surprised it worked at 1ghz
2ghz is probaly pushing it a bit realy its droped off quite a bit,
but it does give a good wide change in phase from small distances.
the rest of the system works fairly well,
thanks to various help from this forum,
especialy to whoever sugested using half a torch reflector
to collect the reflected light to the detector wich is put sideways on,
and allow the laser to be in the same path by pasing through the hole in the
center.
but its only something I do in spare time,
so I tend to leave it for ages once I come accros a sticky problem,
until I think of a neat solution like maybe this one.
Colin =^.^=
.
- References:
- analogue PLL
- From: colin
- Re: analogue PLL
- From: Chris Jones
- Re: analogue PLL
- From: colin
- Re: analogue PLL
- From: Chris Jones
- analogue PLL
- Prev by Date: Re: Op amp distortion
- Next by Date: Re: Op amp distortion
- Previous by thread: Re: analogue PLL
- Next by thread: Re: analogue PLL
- Index(es):
Relevant Pages
|
Loading
