DIY 10MHz real square wave generator - Page 1
Forum section: electronics / projects

author: Karel
author_rank: Super Contributor
timestamp: February 11, 2016, 12:56:44 pm
content: Is there a cheap circuit/component to build yourself a 10MHz squarewave generator?Requirements:- output impedance close to 50 Ohm- frequency 10MHz, doesn't need to be very accurate- output voltage, anything between 1 to 5 Voltpp, reasonably stable- most important requirement: real square shape, including all odd harmonics up to 1GHz with right phase and amplitude (or as accurate as possible).

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 11, 2016, 01:00:05 pm
content: Just a 10MHz crystal oscillator wired to a Schmidt trigger?

author: Karel
author_rank: Super Contributor
timestamp: February 11, 2016, 01:27:11 pm
content: Quote from: HAL-42b on February 11, 2016, 01:00:05 pmExactly. Lets say I connect a 50 Ohm resistor in series with the output.Which part (schmidt trigger) gives the best result up to 1GHz?(fastest risetime in combination with less ringing, overshoot,etc.)

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 11, 2016, 01:36:15 pm
content: snap ad9850 and arduino togother, you get sine signal generator, and the corresponding 4Vpp square wave, next time you will want the not 10MHz freq i'm sure. i cant say about the rise time, if you want to be sure, feed the square output to fast rising comparator. but...Quote from: Karel on February 11, 2016, 12:56:44 pmnow thats the devil in the details stuff, you dont want ringing and reflection come into play. once you get over it, you need to transfer it properly along the transmission line to your measurement device such as scope or SA to be sure. and make sure greater than 1GHz BW T&M equipments. great stuffs to learn nevertheless.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 11, 2016, 01:37:11 pm
content: Also,Jim Williams App Note 13 - High Speed Comparator Techniqueshttp://www.linear.com/docs/4109

author: Fungus
author_rank: Super Contributor
timestamp: February 11, 2016, 01:54:42 pm
content: Quote from: Karel on February 11, 2016, 01:27:11 pm74HC14 ?Risetime will depend on the capacitance of whatever you connect to it.Ringing and overshoot will depend on the inductance of whatever you connect to it.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 11, 2016, 04:28:00 pm
content: 20 Mhz clock in Can, divide by two for ensuring 50/50 symmetry, Schmidt Trigger...You'd be amazed how many clocks do not offer a clean waveform.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2016, 10:37:37 pm
content: Use an oscillator module (or a three-inverter RC oscillator), and a 74LVC04 with several gates in parallel to get a bit less than 1ns rise time into 50 ohms.Tim

author: Kalvin
author_rank: Super Contributor
timestamp: February 12, 2016, 12:46:43 am
content: PECL-family logic gates have pretty fast rise/fall-times (150ps or less) and they are capable of driving 50 ohm transmission lines, but they may need some clever biasing techniques.

author: MickM
author_rank: Regular Contributor
timestamp: February 12, 2016, 01:24:31 am
content: The square wave out of the ad9850 is max 1MHz, sine to 30MHz.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 12, 2016, 01:36:36 am
content: Quote from: MickM on February 12, 2016, 01:24:31 amno, there must be something wrong with your setup, try trimming the trimpot if you have one on the module board. this is my AD9850 square wave pushed to 30MHz. risetime is within the limit of the scope BW somewhere around 2 - 3ns.

author: uncle_bob
author_rank: Unknown
timestamp: February 12, 2016, 02:47:15 am
content: HiOk, so let's reset things a bit.Stuff like ECL and LVDS has edge rates that are deliberately made to be ... wait for it ... slow. They get their speed by switching when the two lines are equal. They get good EMI performance by having low edge rates and balanced signals. They are not what you want in this case.What you drive your gizmo with pretty much does not matter. An edge is an edge. The logic involved will have just as fast an output after it buffers it up, regardless of drive. You may need to cascade a couple of gates to make that happen, but it will.Now some basic square wave stuff. A 50 / 50 duty cycle 10 MHz signal will have a 100th harmonic at 1 GHz. That's an even number so in theory there should be no harmonic. Taking the 101st harmonic, we get something that is roughly 20 log (101) = 40 db. That's a pretty low level signal.Next step, if we are going to have microwave signals, we need microwave board layout techniques and good microwave modeling for our traces. It's amazingly easy to get a 10 db (or more) droop simply by doing something a bit off with a trace. Assume for a moment that's all taken care of.Next, what sort of load do you want to drive. If it's 50 ohms, you will need several gates in parallel. You do not want to loose bandwidth because you ran out of current. How many gates and how to combine them ... that's the question.Oddly enough:http://www.potatosemi.com/potatosemiweb/datasheet/PO74G04A.pdfHas the part for you. Nice and fast, looks wonderful. It even says "1.xx GHz" right up in the big print. Hmmmm ... output load ... hmmm.We're going to need a bunch of them in parallel to drive 50 ohms.Are you *sure* you want to do this this way? A SRD would be a much more conventional approach if harmonics are all you are after. If it is a real signal generator, a proper output amp is likely a higher priority than a specific logic gate.Bob

author: Kalvin
author_rank: Super Contributor
timestamp: February 12, 2016, 09:40:10 am
content: Quote from: uncle_bob on February 12, 2016, 02:47:15 amTrue, sort of. Although PECL (ECL) and LVDS circuits are deliberately made to switch nicely, still PECL switching in150ps range is fast no matter how gently it switches. The main advantage is that PECL and LVDS use differential signalling which will reduce EMI considerably. The PECL will switch about 800mV which is less than OP's desire to get 1V - 5V output amplitude, though. There may be also some other issues with signal levels when interfacing the existing logic devices to the PECL logic devices, so I am not pushing the PECL to be the solution to the OP's problem. I just wanted to bring it out as something to consider if really fast rise/fall times in 150ps - 500ps range are required.

author: tggzzz
author_rank: Super Contributor
timestamp: February 12, 2016, 09:52:30 am
content: Quote from: Karel on February 11, 2016, 12:56:44 pmBy "square" are you meaning a 50:50 duty cycle or the shape of the edges?If the latter, then use a parallel combination of three 74LVC1G14 gates each with its own 120? series termination resistor, with a 5V supply. The output impedance of the combination is approximately 50?. Measuring the spectrum when that is driven by a noise source indicates the edges are around 650ps rise/fall time.FFI, seehttps://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/

author: Karel
author_rank: Super Contributor
timestamp: February 12, 2016, 10:08:10 am
content: Quote from: tggzzz on February 12, 2016, 09:52:30 amI mean the shape of the edges.

author: Karel
author_rank: Super Contributor
timestamp: February 12, 2016, 10:15:32 am
content: Thanks everybody for the input.I'll start to order some components and build a small circuit in order to do some measurements.When I'm done, I'll report the results here.The main reason I want to build this little circuit is to check my setup for measuring/displaying fast digital signals.I have some 500 Ohm 10:1 probes which are specified for 1GHz which I would like to check.So, it would be nice to have some sort of "reference" that can be compared with real measured waveforms in digital circuitsto check if my setup is correct.

author: Fungus
author_rank: Super Contributor
timestamp: February 12, 2016, 10:33:01 am
content: Quote from: Karel on February 12, 2016, 10:15:32 amYou don't need a 10MHz signal for that, a push-button fed into a 74HC14 will do.

author: Karel
author_rank: Super Contributor
timestamp: February 12, 2016, 10:49:13 am
content: Quote from: Fungus on February 12, 2016, 10:33:01 amYou'r right. But I think it's more practical/comfortable to have a repetitive signal to avoid pushing the button when making changes in the setup.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 12, 2016, 05:17:04 pm
content: How about the Jim Williams pulse generator? It is well proven and is kinda de-facto standard for measuring scope bandwidth.It has repetitive signal albeit not 10MHz and a very fast rise time.

author: bson
author_rank: Unknown
timestamp: February 12, 2016, 07:16:10 pm
content: What tesla said. I've had good luck with Crystek 575 and when the 957 series is available(maybe now, haven't checked yet)and I have timeI plan to try them out as well. I don't think either of these is available in 10MHz though (they really target audio DAC master clock applications so are multiples of 44.1k and 48k with high-Z standby mode for easy clock switching without a discrete 2:1 mux in the 957). But they're inexpensive (~$15 IIRC for the -575 in single units from Mouser, and I just checked - the -957 is priced at ~$25) and other product series target other applications (ethernet, USB, 10M, 100M, 50M, etc). These have very respectable close-in phase noise specs (~-100dBc/rtHz @ 10Hz, ~-130dBc/rtHz @ 100Hz) for an easy-to-use drop-in component.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2016, 07:22:53 pm
content: If you need a 50% duty cycle, why not the 74HC74? One section can be configured as an astable and the other as a divide by 2 counter.AstableOf course the values of R & C need to be changed.Divide by two counter