[time-nuts] Re: 100 MHz Low Phase Noise Mobile GPSDO/GNSSDO?

[Markus Kleinhenz]

Hello Carsten,

thats quite the set of requirements you got there.

I believe you will have a hard time finding a COTS solution that fits
most of your requirements.

You will likely have to split your signal chain into multiple devices:

1. UBlox ZED-F9T based GPS-Receiver with 1PPS out
2. Something like a JacksonLabs RCM (a ULN Oscillator that locks to a
externally provided 1PPS)
3. Distribution Unit

So you can spread your requirements.

Another big challenge will be the 1ns relative accuracy between multiple
(moving!) devices. Without communication between devices this should not
be possible.

Since your Phasenoise spec seems to only concern itself with values
above 100Hz you could also use a 10MHz GPSDO and a Wenzel PLL Module
with 100MHz or a similar frequency and a PLL bandwith below 100Hz.

Regards,

Markus

Am 17.04.2022 um 21:17 schrieb Carsten Andrich:
> Fellow timing enthusiasts,
>
> for RF up-down-converters up to 40 GHz I require highly stable,
> synchronized reference signals for multiple, spatially distributed RF
> synthesizers like the TI LMX2595.
>
> Do any of you know COTS GNSSDOs that meet (at least some of) the
> following requirements?:
>
> Â * 100 MHz rectangular high slew rate (>2V/ns) output (no sinusoid;
> Â Â  min. +13 dBm into 50 Ohms)
> Â * Low phase noise: <-130 dBc/Hz @ 100 Hz; <-140 dBc/Hz @ 1 kHz; <-150
> Â Â  dBc @ 10 kHz
> Â * Suitable for mobile use (at least 50 m/s absolute speed) with ADEV <
> Â Â  1e-10 @ 1 s (better preferred)
> Â * 1PPS (or preferably fully configurable timepulse signal usable as
> Â Â  PLL SYSREF) locked to 100 MHz with better than 1 ns relative
> Â Â  accuracy (between multiple receivers of the same type; not absolute
> Â Â  accuracy wrt TAI)
> Â * Fast settling time after power up (preferably 10~30 min) to 1 ns
> Â Â  synchronization of 100 MHz phase and 1PPS between multiple,
> Â Â  spatially distributed GNSSDOs
> Â * Optional: 19" rack mount or desktop enclosure with SMA connectors
> Â * Optional: Dual-band GNSS receiver for improved GNSS timepulse ADEV
> Â Â  performance [1]
> Â Â  (that should enable a shorter time constant for disciplining the
> Â Â  local oscillator and therefore improve compensation of disturbances
> Â Â  like acceleration or temperature changes)
> Â * Optional: Differential timing GNSS receiver via RTCM feed (e.g.,
> Â Â  u-blox ZED-F9T [2])
> Â * Optional: Multiple phase matched outputs (preferably at least 4)
> Â * Optional: 10 MHz outputs (divided from 100 MHz) for legacy
> Â Â  measurement equipment
>
> Regarding 100 MHz output GNSSDOs, I've only found the Jackson Labs
> ULN-1100 [3]. Unfortunately, it doesn't meet my phase noise
> requirements and its use of a single-band GNSS receiver is not
> state-of-the-art.
>
> Best regards,
> Carsten
>
> [1]
> https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf#page=25
> [2]
> https://content.u-blox.com/sites/default/files/ZED-F9T-00B_DataSheet_UBX-18053713.pdf#page=6
> [3] https://www.jackson-labs.com/index.php/products/uln_1100
>
>
> ----- APPENDIX -----
>
> Read only if you're interested in my reasoning behind some of above
> requirements.
>
> # Why 100 MHz rectangular output?
>
> ## 1. Phase noise scaling
>
> The primary reason to not use a 10 MHz reference signal is phase noise
> scaling. Taking the LMX 2595 as an example, its typical closed-loop
> phase noise at 10 GHz output [4, p. 15] is (averaged between 9 and 11
> GHz and then coarsely rounded down to add a safety margin):
>
> dBc/Hz @ Off  : 100 Hz    1 kHz    10 kHz    100 kHz
> LMX @ 10 GHz  : -90    -100    -110    -112
> Scaled 100 MHz: -130Â Â Â  -140Â Â Â  -150Â Â Â  -152
> Scaled 10 MHz :Â Â Â  -150Â Â Â  -160Â Â Â  -170Â Â Â  -172
>
> Additionally, I've added the phase noise scaled down to 100/10 MHz to
> determine the theoretical requirements for a reference oscillator
> operating at the respective frequency. Checking out AXTAL (AXIOM*ULN)
> and Wenzel (HF ONYX IV), the lower bound for compact 10 MHz OCXO phase
> noise at both 10 kHz and 100 kHz appears to be -165 dBc/Hz. Therefore,
> using a 10 MHz reference would deteriorate the LMX' RF phase noise by
> ~5 dB. Also, even at +10 dBm OCXO output power, -170 dBc/Hz would be
> barely 14 dB over the thermal noise floor, raising noise and
> interference requirements regarding signal distribution and
> amplification. Admittedly, 10 MHz OCXOs do outperform 100 MHz OCXOs
> regarding the 10 Hz offset phase noise, but for typical RF
> applications, e.g., OFDM communication systems, channel estimation and
> correction typically handle time-variant effects below 100 Hz.
>
> ## 2. Phase noise figure of merit
>
> Most datasheets I've read, particularly TI LMX, show typical
> closed-loop performance for 100 or even 200 MHz reference oscillators.
> The HMC835 -- while certainly not representative -- is a noteworthy
> example I've stumbled over that actually illustrates the effect of
> different reference signals on the phase noise figure of merit [5, pp.
> 9 f.]. Going for anything below 100 MHz for the reference oscillator
> seems not to be encouraged by more or less modern RF synthesizers.
>
> ## 3. Slew rate
>
> Many RF synthesizers, e.g., the LMX2595, generally recommend a "high
> slew rate" for their reference inputs [4, p. 61]. The LMX2820
> datasheet illustrates its phase noise degradation vs. slew rate,
> requiring >0.8 V/ns for optimal performance [6, p. 11]. A 2 Vpp 10 MHz
> sine has only 0.062 V/ns slew rate, so a 100 MHz sine is still
> insufficient. As a safety margin the GNSSDO's output slew rate
> shouldn't be lower than 2 V/ns.
>
> [4] https://www.ti.com/lit/ds/symlink/lmx2595.pdf
> [5]
> https://www.analog.com/media/en/technical-documentation/data-sheets/hmc835.pdf
> [6] https://www.ti.com/lit/ds/symlink/lmx2820.pdf
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