Honored ContributorHi all,
I want to implement a PLL that will lock onto & track an incoming signal of about 30KHz and output a multiple of that locked frequency. I tried using Quartus's PLL megacore function but it wont accept a 30KHz input signal, saying that its too low. Is there something I might be doing wrong or do I have to design my own PLL using the an XOR,NCO & the FIR filter megacores(if thats even possible)???
Honored ContributorLooking at the PLL hardware specification clarifies, that it can't use a 30 kHz reference clock. Without going too deep into details, it's simply impossible for a PLL with a 0.5 to 1 GHZ VCO range.
Designing "your own" PLL is actually an option. But an low frequency equivalent to the FPGA PLL would be rather a basic all digtal PLL than a NCO design dealing with digitized waveform. Although the latter is also feasible, of course.Honored ContributorI understand. My knowledge of PLL's though goes as far as analogue(hence the mention of the analogue equivalent subsystems like NCO & filter etc.).
How does the "basic" DPLL you mentioned differ from the, now seemingly labourous usage of NCO's & such? Another reason I mentioned an NCO is that I was advised (by a microelectronics lover,hence analogue again)that a VCO can output a multiplied version of the locked frequency,which is what I need to do.Honored ContributorA DPLL can also use a VCO equivalent operating at multiple of the reference input, but as the VCO is time-discrete, it's clock edges are coinciding with the system clock events, a higher VCO frequency means more jitter in the output signal. But a NCO is time-discrete as well.
Honored ContributorHi,
You may wish to consider the 4046 http://www.standardics.nxp.com/products/plls/4046/ RegardsHonored ContributorHi again people.
Thank you for your replies. I struggled the whole of today to implement the PLL in Quartus but I couldnt get it right. one of my problems was that I have no control over the output bit width of some of the megacores (like te filter). The NCO also determines the input bit width according to my specs & this causes there to be a mismatch between the subsystems. Would the NCO be able to output a 5MHz & 20MHz singal from a locked 31,250KHz signal anyway?Honored ContributorIf these are separate NCO output signals, they won't be phase locked, I think. The design is obviously different from a classical PLL, where the VCO output is divided down and phase locked to the reference input.
I'm not clear about the specific implementation problems you reported. I never used a Megacore to implement a PLL, but this should work as well.Honored ContributorI am trying to implement a classic PLL actually; I've just never done it in software before. What I basically need to do is lock onto an incoming 31.250KHz Signal with the PLL. The outputs should be 2 frequncies, one at 5MHz (incoming times 160) and the other should be 20MHz. Whats the simplest way to achieve this? (this is my challenge in a nutshell)
Honored ContributorI have a rough idea about what you're intending, but not in all details. Particularly:
- What's the reference waveform? - What are the requirements for the 5 and 20 MHz signals? (Waveform, acceptable phase jitter, phase locking needed) - Dynamic parameters of reference signal (frequency tolerance, jitter) - Required lock timeHonored ContributorThe incoming signal to be locked onto is a correlator output. Basically two siganls are correlated and the output is a signal with periodic peaks where the two signals are the same. I need to lock onto the frequency of those peaks and multiply that frequency with 160 in order to sample.
Its a frame and symbol synchronisation opearation, where the frames are at the 31.250KHz frequency & the symbols at 5MHz. the output frequencies are therefore clocks with 50% duty ratio & the lock in time/settling time can be anything (obviously, the quiker the acquisition the better, so tradeoffs will have to be made between the settling time and the allowable noise/jitter). The noise/jitter,therefore is not particularly specified.Honored Contributor
