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

[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?:

  * 100 MHz rectangular high slew rate (>2V/ns) output (no sinusoid;
    min. +13 dBm into 50 Ohms)
  * Low phase noise: <-130 dBc/Hz @ 100 Hz; <-140 dBc/Hz @ 1 kHz; <-150
    dBc @ 10 kHz
  * Suitable for mobile use (at least 50 m/s absolute speed) with ADEV <
    1e-10 @ 1 s (better preferred)
  * 1PPS (or preferably fully configurable timepulse signal usable as
    PLL SYSREF) locked to 100 MHz with better than 1 ns relative
    accuracy (between multiple receivers of the same type; not absolute
    accuracy wrt TAI)
  * Fast settling time after power up (preferably 10~30 min) to 1 ns
    synchronization of 100 MHz phase and 1PPS between multiple,
    spatially distributed GNSSDOs
  * Optional: 19" rack mount or desktop enclosure with SMA connectors
  * Optional: Dual-band GNSS receiver for improved GNSS timepulse ADEV
    performance [1]
    (that should enable a shorter time constant for disciplining the
    local oscillator and therefore improve compensation of disturbances
    like acceleration or temperature changes)
  * Optional: Differential timing GNSS receiver via RTCM feed (e.g.,
    u-blox ZED-F9T [2])
  * Optional: Multiple phase matched outputs (preferably at least 4)
  * Optional: 10 MHz outputs (divided from 100 MHz) for legacy
    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: -130	-140	-150	-152
Scaled 10 MHz :	-150	-160	-170	-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