I think you can do a free running oscillator with crystal control based=20 on a logic gate. You will (probably) need a 74F series logic gate to get=20 a clock frequency high enough. The circuit widely circulated on the web=20 shows a Schmitt trigger gate, but when thee frequency is very high, a=20 standard gate works. You can also gate the output if you like. Be sure=20 to use a buffer stage, otherwise you load the oscillator, which can be=20 problematic with starting and with stability. A quartz based free running oscillator built without a Schmitt trigger=20 might be problematic starting when power is applied.....which is why the=20 Schmitt trigger gate is suggested. However, the Schmitt trigger gate=20 just isn't necessary above 10 MHz or so. Be sure to get a fundamental=20 crystal because overtone crystals might lock on the fundamental=20 frequency instead of an odd order mode. Today, fundamental crystals are=20 widely available, unlike 10 years ago when they were new technology. I'm not sure how much stability and power output you need, and your=20 cost/power budget. Does it need to be variable frequency? The logic gate=20 free running oscillator is the cheapest option. The next step up is to build an ac generator and square it with a fast=20 logic gate. It is common practice to bias the logic gate at 1/2 the=20 supply voltage and then feed in the ac signal through a blocking cap. If phase noise or jitter is an issue, DO NOT use a prepackaged=20 inexpensive free running oscillator commonly called a 'clock multiplier'. GL. AG > How about a generic voltage-controlled oscillator? IE: look up "VCO" on > Digikey -- they're fairly monolithic devices and generally require > little more than power, ground and a pot to control the voltage. > > Cheers, > -Neil. > > > On 10/12/2015 2:22 AM, David C Brown wrote: >> The AD9850 sine wave generators are just that - sine wave generators. T= hey >> quote a square wave up to 5MHz which is not good enough. >> >> On 12 October 2015 at 03:33, Bob Blick wrote: >> >>> Hi David, >>> >>> Have you looked the plethora of AD9850 DDS sinewave generators? They >>> have a comparator for squarewave output. I don't know what your spec is= , >>> but you could put a toggling flip-flop on the comparator output if you >>> need better duty cycle accuracy than the comparator itself (if you can >>> lose the upper octave). >>> >>> Best regards, Bob >>> >>> On Sun, Oct 11, 2015, at 11:04 AM, David C Brown wrote: >>>> I am looking for a module to generate logic level square wave up to at >>>> least 20MHz (50 would be better) within a $100 budget. There are a >>>> number >>>> of items on eBay but none have them seem to have a proper spec. Any >>>> recommendations from experience? >>>> >>>> -- >>>> __________________________________________ >>>> David C Brown >>>> 43 Bings Road >>>> Whaley Bridge >>>> High Peak Phone: 01663 733236 >>>> Derbyshire eMail: dcb.home@gmail.com >>>> SK23 7ND web: www.bings-knowle.co.uk/dcb >>>> >>> -- >>> http://www.fastmail.com - Does exactly what it says on the tin >>> >>> -- >>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist >>> >> --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .