I have done this at lower frequencies. It ends up being quite a bit like the method with mixers. That gain you want to apply to each phase detector's output is a sine function. This takes the gain smoothly through zero as the phase detector goes through its maximum value.
Offset PLLs are a piece of cake. The reason is that you can filter off all the mixer spurs in the control voltage, leaving the output very clean. The biggest problem is that when using sinusoidal PDs, offset PLLs can lock up on the wrong sideband. PFDs solve this problem (we had a thread on this some time back), but aren't that easy to do at 2 GHz.
The usual approach is to use an auxiliary mixer driven in quadrature, so you can distinguish +pi/2 from -pi/2 (and hence USB from LSB) by looking at the sign of the output from the quadrature mixer.
offset PLL you refer to is that as a specific/distinct topology/layout ?
I dont know if you refering to some other threads where ive explored posible ways to do this, the thing that makes it difficult is the closeness of the frequencies they tend to lock together.
so the loop needs to respond quite quickly.
I was thinking of using a ecl prescaler to bring it down to the 100mhz region, and a digital phase detector so the phase signal is idealy linear to phase.
Further thinking I realise I can use an xor type PD, and a triangle waveform, ofc I would need to reverse the polarity of the error amplifier, this would avoid discontinuties of a ramp but im not clear in my mind how it would behave for locking etc.
quadrature is difficult to generate well over a wide range at that frequency, with a good quadrature mixer system I could generate the 2nd signal directly however ive tried this and the result isnt a very clean signal, it probably would be ok at lower frequencies or narrow range or if I paid a lot of attention to layout, although this could be cleaned up with a PLL.
you can get ecl PFD, the problem is here that they will sense one frequency is always higher. it would be posible to use combination of FF and xor type to have 1 PD for each quadrant.
a gate array might be usefull here, it could generate the appropriate pwm for offset frequency waveform too.
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.
The earliest fractional-N synthesizers (those which used analogue compensation) worked by applying a ramped offset to the phase detector output e.g. DANA Digiphase.
If he is doing the sweep by changing the value of some /N counter, he could use a second PLL whose input is the original binary frequency value plus some offset obtained from a digital full adder. You add a constant proportional to the IF frequency.
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?
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.
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.
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.
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.
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.
You reload synchronously with the counter roll-over. Thus avoiding a big step in the edge.
...Jim Thompson
--
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| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
If you change the divide ratio whilst the PLL is running, there is quite a risk that it will go out of lock for a while. If the reference comes from a DDS then it can be swept in a way that makes it phase coherent and the PLL stays locked the whole time.
Nah, you use a VCO, mix it with the 1-2 GHz reference, low-pass filter the resulting IF output, and then run a 1:1 PLL at the offset frequency. You can use a frequency-phase detector at the offset frequency. By changing the sign of the loop gain, you can make a USB/LSB switch. (Search back for a discussion between Jim Thompson and me on this very subject.) Offset PLLs are actually really nice, and no dividers are required.
Right, that's what every sigma-delta frac-N synth does. I am not sure how practical it would be on the common pre-fabricated int-N PLL ICs where the frequency is shifted in serially. Hopefully they latch it in a way that the timing can be made to work. Certainly in an ASIC or FPGA it could be made to work.
Ive tried this, the problem is the loop bandwidth is necessarily less than the offset frequency, it needs to have reasonable gain at the offset frequency so that it can overcome the 2 VCOs from pulling eachother into lock.
If you have enough stray coupling to overcome the action of a frequency-phase detector, man do you ever have layout problems. With a double-balanced mixer, I might almost believe it, but no way can you argue with a 4046.
The topology I'm talking about is this one (copied from our previous go-round here):
I made such a prototype with 2 vco modulules, mixer, and 4046 on a small board, it would pull so bad it wouldnt lock at anything less than 10mhz offset, I moved the vco to 2 seperate boards, with sheilding, triple buffers etc it worked ok at a few mhz but although it will lock at 500khz the signal from the mixer is very distorted.
im looking for a solution where I can put everything on the same board again.
both vcos are amplified and fed to optical devices wich makes it difficult to shield totaly.
I think if I have an xor type PD and a flip flop wich detects wich vco is ahead and steers the xor output into pos/neg charge pumps that will give me a phase error ramp wich covers 360' obviously il need to prescale the input a bit, cplds seem to be able to cope with ~500mhz. wich still allows me plenty of loop gain at many Mhz.
it would be best to avoid the discontinuities in the ramp so inverting the signal will allow me to keep to +/- 90'
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