LTC6957-1
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The LTC6957-1/LTC6957-2/LTC6957-3/LTC6957-4 is a family of very low phase noise, dual output AC signal buffer/driver/logic level translators. The input...
LTC6957-1 on Analog.com
I'm currently designing a W-band synthesizer that needs excellent long-term stability, and as such requires exceptionally low close-in phase noise.
To be specific, we need this synthesizer to exhibit a stability as measured in Allan Deviation of 1E-14 at 1 second of averaging time. Converted to phase noise at 10 MHz, this amounts to 1 Hz offset noise lying at -145 dBc/Hz per my latest calculations.
I understand most conventional references will not be able to attain this level of noise, but my application will have access to a resonator with sufficient Q - my main issue is ensuring the synthesizer has lower added noise compared to this super oscillator. However, in order for the rest of the synthesizer to function, I need a method to convert a sine wave to a square wave (for use in referencing HMC699) without contaminating the noise in excess of this very low value of -145 dBc/Hz at 1 Hz offset from a 10 MHz carrier.
As far as I can tell, LTC6957-1 is one of the lowest noise sine wave-square wave converters that is commercially available, but the datasheet only provides additive noise for a 100 MHz carrier. If this additive noise were to be primarily timing jitter, and were to fall by 20*log10(10) = 20 dB for a carrier of 10 MHz, then the part may be usable for my application, but I have no proof that this will occur at present.
Before I setup a test capable of making this measurement, I thought it best to ask - has anyone had reason to make this manner of test and knows the additive noise I can expect for a carrier that is not 100 MHz? Or can one of the ADI engineers provide guidance as to what I should expect?
Hi Michael,
This measurement is really hard to make at 10 MHz. I wrote this post describing the challenges with the test instrumentation at 100 MHz ((+) LTC695x Low Frequency Phase Noise Measurement Issues - Documents - Clock and Timing - EngineerZone (analog.com) ). We also have this design note discussing how to optimize the device for 10 MHz (DN514 - A Robust 10MHz Reference Clock Input Protection Circuit and Distributor for RF Systems (analog.com) ). This design note only shows 100kHz offset. At 10 MHz we also have some limitations with the amplifiers and diodes we selected. The diodes in the DN514 have been discontinued.
In this attached file I used a 10MHz Wenzel OCXO to drive the LTC6957 and then tried to subtract out the OCXO noise and some other known noise sources. This analysis was the best I could do at the time with the instruments I had. I think it gives you a good idea of the performance, but you'll have to ignore the spurs (those were not related to the LTC6957...)
I hope this helps.
Chris
Hi Michael,
This measurement is really hard to make at 10 MHz. I wrote this post describing the challenges with the test instrumentation at 100 MHz ((+) LTC695x Low Frequency Phase Noise Measurement Issues - Documents - Clock and Timing - EngineerZone (analog.com) ). We also have this design note discussing how to optimize the device for 10 MHz (DN514 - A Robust 10MHz Reference Clock Input Protection Circuit and Distributor for RF Systems (analog.com) ). This design note only shows 100kHz offset. At 10 MHz we also have some limitations with the amplifiers and diodes we selected. The diodes in the DN514 have been discontinued.
In this attached file I used a 10MHz Wenzel OCXO to drive the LTC6957 and then tried to subtract out the OCXO noise and some other known noise sources. This analysis was the best I could do at the time with the instruments I had. I think it gives you a good idea of the performance, but you'll have to ignore the spurs (those were not related to the LTC6957...)
I hope this helps.
Chris
