Simple 10MHz to 25MHz frequency multiplier - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: March 28, 2014, 10:49:27 pm
content: I'm looking for a frequency multiplication solution;Either: 10MHz to 25MHz directly (2.5x frac-N PLL?) or 10MHz to 50MHz with a divide by two option. If possible, it would be also nice to be able to set the multiplication on demand (I2C/SPI), though this is not critical for my application. Also, jitter isn't a major concern.What I've found so far is GHz+ RF ICs. I'm unsure if these will work down to 10~50MHz.

author: dannyf
author_rank: Super Contributor
timestamp: March 28, 2014, 11:46:58 pm
content: The simplest and cheapest is to get one of those STM32F chips and program it to use outside oscillator (from your source), program a 2.5x multiplier, and turn on the mco pin.DDS chips would also work as well.

author: Rerouter
author_rank: Super Contributor
timestamp: March 28, 2014, 11:54:01 pm
content: An AD9850 acting in a PLL would do the job, but its serial loading, not i2c or spi (they even show you how to do it in the datasheet)

author: fcb
author_rank: Super Contributor
timestamp: March 29, 2014, 12:01:02 am
content: Another (old school) way to do it.Make sure you have a square-wave, then filter the 5th harmonic and apply to a divider.

author: dannyf
author_rank: Super Contributor
timestamp: March 29, 2014, 12:06:11 am
content: QuoteHow would you design the cut off frequencies for an input signal of 10M - 50Mhz?

author: jpb
author_rank: Super Contributor
timestamp: March 29, 2014, 12:11:48 am
content: You've not said what you want to do with it, but if you're looking for a 25MHz clock output then this chipON SEMICONDUCTOR - NB3N502DG - PLL CLOCK MULTIPLIER, 8SOIC(which is £2:51 + VAT from Farnell) will do a 2.5 multiply directly (one of the examples they give in the data sheet is 10MHz input with 25MHz output with S0 and S1 both set to 1):http://www.farnell.com/datasheets/699220.pdfhttp://uk.farnell.com/on-semiconductor/nb3n502dg/pll-clock-multiplier-8soic/dp/2101849

author: tom66
author_rank: Super Contributor
timestamp: March 29, 2014, 02:50:59 am
content: What I'm looking to do is to use a 10MHz reference input from a Rubidium source to generate a 25MHz clock for an AD9837.Yes I could use another DDS to generate it, and/or various tricks for picking up on the harmonic. Or a microcontroller purely for the clock. It surprises me though that there's no "drop in" solution for a simple, low frequency multiplier. Is it just not that common of an engineering problem? It's obviously not impossible to do - the fact that every powerful MCU includes one is proof of this.edit: jpb, did not see your post. Looks like exactly what I'd need. Thanks! I take back the "rare" part as above.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: March 29, 2014, 05:23:33 am
content: Quote from: tom66 on March 29, 2014, 02:50:59 amI had a quick look at that part and I couldn't find where the bandwidth of the PLL was mentioned. I don't even see a loop filter in the block diagram. I assume this means that the bandwidth of the PLL is as high as possible, in which case it would be a good thing since with such a low multiplication factor, there is no way the VCO can ever beat the phase noise of your reference.

author: branadic
author_rank: Super Contributor
timestamp: March 29, 2014, 08:05:08 am
content: You could use a CDCE913 for that job.

author: dannyf
author_rank: Super Contributor
timestamp: March 29, 2014, 12:09:30 pm
content: QuoteAnd you can use the spi / i2c module in a mcu to adjust the multiplier ratio, etc.There are various PLL chips (TI for example has a whole family of those chips) that does that. OnSemi has / had some as well (as I recall, they were from Sanyo that On bought, FS63xx). Cirrus also has some chips - those guys are fairly big in timing.What you want are fractional PLL.

author: hlavac
author_rank: Frequent Contributor
timestamp: March 30, 2014, 06:46:28 pm
content: How about some old school 74HC4046A plus a little bit of jelly bean logic (counter for divide by 5, 74hc74 for divide by two...) or 74HC4059 programmable divider?

author: fcb
author_rank: Super Contributor
timestamp: March 30, 2014, 09:58:45 pm
content: Quote from: tom66 on March 29, 2014, 02:50:59 amI just had peak at the d/s for this part. It looks like it max's out at 16MHz.My thought was to change your design so that you could use the 10MHz ref directly.

author: dannyf
author_rank: Super Contributor
timestamp: March 30, 2014, 10:21:43 pm
content: QuoteWould it work at 50Mhz? and hopefully with some headroom too?Probably not.

author: jpb
author_rank: Super Contributor
timestamp: March 31, 2014, 09:55:33 am
content: Quote from: Hugoneus on March 29, 2014, 05:23:33 amI'm not sure if this answers your question but the input bandwidth is 5-27MHz for crystals and 2-50MHz for clocks, there is a divide by P before the phase detector and a divide by M for the VCO but the maximum output frequency is 190MHz so you can't scale 50MHz by 4. Looking at the allowed multipliers they come in pairs that multiply to 10 so 2 and 5 or 3 and 3 1/3 or 4 and 2 1/2 but I'm not sure how that translates to P and M values.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 31, 2014, 10:08:29 am
content: Quote from: dannyfNot at 50MHz - thereis a variantwhich does 38MHz.That would be adequate for 10MHz->25MHz though - divide the 10MHz by 2, and the 25MHz by 5 to get 5MHz and phase compare the two for the drive to the VCO.

author: ConKbot
author_rank: Super Contributor
timestamp: March 31, 2014, 10:28:33 am
content: Quote from: dannyf on March 29, 2014, 12:06:11 am25 MHz crystal and then an amp on the output? Seems goofy to use a the crystal as a filter instead of just a source, but, it would have the stability of the original 10 MHz signal, and as long as the 25 MHz crstal isnt too far off from nominal, then the desired harmonic should pass just fine.

author: fcb
author_rank: Super Contributor
timestamp: March 31, 2014, 11:20:09 am
content: Other options.1. 74LS628 will run at 25MHz, they are a tad better than 4046 running free.2. 25MHz VCXO, then discipline (PLL) this with your 10MHz (someone suggested 10MHz/2 to get 5MHz).3. 50MHz VCXO as above but with /2 to get nicer square wave.All of these (and others) will work - but you need to understand the phase noise they will introduce.Can you tell us more about what your end-goal is, that typically results in other good suggestions.

author: richard.cs
author_rank: Super Contributor
timestamp: March 31, 2014, 11:46:45 am
content: Quote from: ConKbot on March 31, 2014, 10:28:33 amFor some reason this appeals to me. I think I like the simplicity, and if necessary you could deliberately de-Q the crystal a bit to widen the tuning. :-)

author: analogcircuithacker
author_rank: Newbie
timestamp: April 02, 2014, 06:55:52 pm
content: Drive the reference input of an AD9913 DDS IC with your 10 MHz signal.Program the clock multiplier to be its max value of 25 so the internal clock runs at 250 MHz.Then program the output frequency to 25 MHz and you have 10 points per cycle.Enjoy!

author: edavid
author_rank: Super Contributor
timestamp: April 02, 2014, 07:08:17 pm
content: I don't understand why people are still suggesting ideas that are more expensive and more complicated than the NB3N502DG. That just seems perfect.

author: fcb
author_rank: Super Contributor
timestamp: April 03, 2014, 02:54:15 pm
content: +1That's a great little part edavid. I've just ordered a couple from Farnell for my toys draw.

author: macboy
author_rank: Super Contributor
timestamp: April 03, 2014, 05:10:04 pm
content: Quote from: edavid on April 02, 2014, 07:08:17 pmI agree. And there are many equivalent alternatives like ICS511, and others.