Change from Square to Sine wave - Page 1
Forum section: electronics / beginners

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 07, 2014, 10:48:15 am
content: Hi Everyone, I need a bit of help ,not sure which way to go I have a 10 MHz oxco which has a Square wave output, and I want to change it to sine wave before the output buffers, has anyone got any ideas how best to achieve this, I was playing with the idea of using some kind of filtering, to achieve this, or have I got it wrong , any help please.Paul

author: amyk
author_rank: Super Contributor
timestamp: June 07, 2014, 11:03:00 am
content: Yes,a filter tuned to the right frequency will work.

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 07, 2014, 11:18:14 am
content: Quote from: amyk on June 07, 2014, 11:03:00 amThank you, that's great gives me something to work with, "bread board time"

author: Marco
author_rank: Super Contributor
timestamp: June 07, 2014, 11:32:47 am
content: At these frequencies you want LC filters ... resonant band pass filters are probably not a great idea (amplitude shift as they go out of tune). So LC pi or t filter low pass ... or ladders thereof.http://edeca.net/wp/2012/09/generating-a-simple-sine-wave/PS. should really be designed correctly for source/load impedance and if load impedance is not constant output needs to be buffered.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 07, 2014, 11:40:15 am
content: I agree - Q of 10 active filter at 10 MHz is going to require world class high speed op amps with attendant need for world class high frequency layout...a passive LC filter could be easierbut the standard low cost sine shaper circuit integrates the square to a triangle and then diode/transistor nonlinear shapes the triangle corners to give fractional % THD sine when levels are set wellagain 10 MHz may be a bit much for the standard chips, but you could roll your own designed for the frequencyyou can also PLL lock a sine wave VCO to your 10 MHz

author: larry42
author_rank: Regular Contributor
timestamp: June 07, 2014, 11:42:36 am
content: As the question is fairly basic, I would ask the OP why he wants to have a sine wave...Sent from my One S using Tapatalk

author: rob77
author_rank: Super Contributor
timestamp: June 07, 2014, 12:18:35 pm
content: i would definitely go for a PLL + sinewave VCO , as it was suggested above. it's much simpler and easier than building and tuning a filter.

author: jpb
author_rank: Super Contributor
timestamp: June 07, 2014, 12:28:26 pm
content: Since square waves involve odd harmonics, if it was a pure square wave (which it won't be) the first harmonic to be filtered out is 3 times the fundamental so I would have thought a fairly simple low pass filter would work well.With my OCXO I ended up with a rather distorted sine but this was down to using a cheap video amplifier to get the amplitude up and this turned out to be rather non-linear.

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 07, 2014, 12:30:42 pm
content: Quote from: larry42 on June 07, 2014, 11:42:36 amOk this is a fair question, What I'm making or trying to make is a constant 10 MHz Frequency standard , hence the OXCO ,oven crystal, which I intended to buffer to the output. my problem is the OXCO has a square wave output, I wanted to add a sine wave output as well, so this is ware i've become stuck on how to do it. so hope this helps you, to help me. one of the reasons for wanting to add sinewave is I just prefer it, and of course just seeing if it can be done.

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 07, 2014, 12:36:06 pm
content: Quote from: jpb on June 07, 2014, 12:28:26 pmSome who has done what I'm trying to do, this is good, i was going to put the project into "I'll come back to it box", 10MHz standard would be very useful to me in my Radio amateur station. So it's back on the bench now, to have another look.Here's what I'm thinking of trying a low pass pi filter the top section inductor I made is 1.4uf either side a 280pf capacitors, to form the pi, that's as far as I got

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 12:56:04 pm
content: The cheap and cheerful solution would be to connect a series 390R resistor and a 10nF series cap out of the logic gate, then have a shunt 100pF. Then have a series 2.2uH inductor. This forms a basic L match at 10MHz and it also keeps the current peaks from the logic gate at sensible levels of about 10-12mA.If fed from 5V logic at 10MHz, it will deliver a few milliwatts at 10MHz into a 50R load and the harmonics should be about 30dB down. So you won't get a perfect sinewave but it will look like fairly reasonable sinewave at 10MHz.But it all depends on your design requirements so the above solution might not be suitable.

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 07, 2014, 12:59:41 pm
content: Quote from: G0HZU on June 07, 2014, 12:56:04 pm,Hello I have the breadboard at the ready so I'll give this one ago to thanks.

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 01:01:35 pm
content: Note: it's designed to be terminated in 50 ohms so make sure you test it with a 50R load...Also, I forgot to include the series 10nF blocking cap in the previous description. See the circuit below

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 07, 2014, 01:12:29 pm
content: Quote from: G0HZU on June 07, 2014, 01:01:35 pmOK will do.

author: retrolefty
author_rank: Super Contributor
timestamp: June 07, 2014, 01:24:17 pm
content: Would be curious how it would work just passing the square wave through a cheap series 10MHz crystal?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 07, 2014, 01:42:34 pm
content: Quote from: retrolefty on June 07, 2014, 01:24:17 pmBeat me to it!

author: G0HZU
author_rank: Super Contributor
timestamp: June 07, 2014, 01:48:01 pm
content: QuoteI've seen plenty of circuits where a 10MHz reference is fed through a specially designed crystal filter. However, this is usually done to clean up the close to carrier phase noise. Often the square wave is fed through the filter(s) to clean it up and then it gets squared back up again to use as a clock signal. The benefit is that you get a square wave with lower phase noise a few kHz away from the carrier.But I suppose you could use the same approach and not bother squaring it up again at the output

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 07, 2014, 03:19:26 pm
content: the oscillator itself must have a pretty good sine internally, often both sine and square output options are offered on standards grade oscI don't know much about xtal filters - but osc xtals can operate at overtones so may not give good harmonic rejection