[time-nuts] Question about precise frequency / phase measurement
Hello @all, my name is Wolfgang and i'm new to the list. :) I browsed through the list archive, but i didn't find the infos i need, so i decided to join the list and to ask the experts directly. :) I want to measure the frequency difference between a 10 MHz OCXO and a 10 MHz Rubidium. I think that's what many people here have done many times... but i don't want to use expensive equipment like time interval counters with picosecond resolution etc. I would prefer a cheap and easy solution. I also would like to have an update rate of more than 1 measurement per second, or even more. My first approach was to use a simple XOR phase comparator. I tried a 74HCT86 and a 74HCT4046. It works, but it's very noisy, so i don't get better than about 10 mHz frequency resolution. If i look at the lowpass-filtered output i don't see a nice sine or triangular wave, but it looks more than a triangular wave with round tops and some bumps between them. Another problem is that the difference frequency gets very low when the frequencies are very close, so it's not enough to look only for zero crossings of the difference signal. Does anybody know a possibility to get a resolution 1 mHz ? Best regards, Wolfgang ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Question about precise frequency / phase measurement
Hi Bruce, this sounds very good, and seems to fit my requirements quite well. :) I will have a closer look to this concept. I also had the idea to take the reference frequency, divide it, and mix the division result again with the reference to get an offset to the reference frequency which would give a higher beat frequency which allows a reasonable measurement rate. But it didn't turn out as i expected, because there was a lot of 'garbage' in the signals. Best regards, Wolfgang Am 19.04.2012 21:51, schrieb Bruce Griffiths: Using a dual mixer time difference system (either the digital dual mixer time difference (DDMTD) or the analog variant (DMTD)) can easily achieve the required resolution. The DDMTD is relatively cheap to implement however it requires an offset oscillator to beat against the 2 signals being compared. However a DDMTD can use a 5MHz offset oscillator can be used with 5MHz, 10MHz, 15MHz ... input signals whereas a DMTD requires a 10MHz offset oscillator to be used with 10Mhz input signals. The DDMTD uses a pair of shift registers clocked by the offset source where each of the 2 signals being compared is connected to the data inputs of its shift register. The time difference between beat outputs of the 2 shift registers is then measured with relatively low resolution. Some digital filtering of the shift register output transitions is usually required. A pair of 74HC164's will typically have a equivalent input jitter of around 10ps or so, a 74AC164 will be about 4x quieter. With a 5.55MHz offset oscillator and 10MHz inputs the shift register output beat frequency will be 110Hz. It is usually advantageous to use an FPGA to implement the digital filtering, timestamping and even the shift registers (although external shift registers will have less crosstalk). Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Question about precise frequency / phase measurement
Hi Bert I want to monitor the frequency deviation continuously (that means: i don't want to look at a scope ;) and log the data several times per second. The goal is not to make a 'quality test' of the oscillator, but to look for frequency deviations which are caused by external influences of various kind. I have a digital 500 MS scope, so i can use this for circuit development, but i don't want to use it for the actual measurements. Time measurements with microsecond accuracy can be done with a microcontroller, that's not a problem. BTW i'm using the Parallax 'Propeller' controller which has 8 cores running at 80 MHz each, and can measure time intervals with 12.5 ns resolution. It has also some very interesting timing circuitry, and can even generate VGA video signals with user-defined timing without any external special components. May be of interest for one or the other here on the list. :) For those who are interested: http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/Propeller-P8X32A-Datasheet-v1.4.0.pdf I think the DDMTD could be a good solution. The question is, if 74HCxx parts would be good enough to get 1 mHz resolution for a 10 MHz frequency with an update rate of 1 sec. Can i use a 74HCT4046 PLL for the 'helper PLL' or is this one not good enough ? Maybe i will do some tests next weekend to see what i can achieve with these 74HC parts, before looking for better ones... Regards, Wolfgang Am 19.04.2012 23:14, schrieb ewkeh...@aol.com: Wolfgang It would help if you would let us know what equipment other than a scope you have. Also what resolution you would want to achieve. One time set up or want to use repeatedly. Bert Kehren ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
