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
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