Can the VCTCXO DAC be controlled via the UHD API?
I would rather manually calibrate on demand than see large jumps in frequency.
On Wed, Jan 2, 2019 at 5:16 PM Marcus D. Leech via USRP-users
<usrp-users@lists.ettus.com> wrote:
>
> On 01/02/2019 09:54 AM, Brais Ares via USRP-users wrote:
>
> Hello,
>
> We've just bought an AXIOM90 OCXO [1] (actual configuration: 5 V, ±50 ppb,
> +7.7 dBm) and we are having trouble configuring it as an external clock
> reference on a B200mini.
>
> All we do in code is set the clock source as external:
>
> usrp->set_clock_source("external");
>
> And loop until it gets locked by checking:
>
> uhd::sensor_value_t ref_locked = usrp->get_mboard_sensor("ref_locked", 0);
>
> Once it's locked we capture 60 seconds of a RF sinusoid waveform centered at
> the central frequency (100 MHz) and plot its spectrum using Matlab. We would
> expect:
>
> Frequency drift - Internal oscillator [2] @ ±2 ppm = ±200 Hz
>
> Captured tone shows a freq. offset of +126 Hz, makes sense (see image and
> image zoomed in)
>
> Frequency drift - External oscillator (Agilent E4438C Signal generator,
> output power: +10 dBm)
>
> Captured tone shows a freq. offset of -3 Hz, makes sense, but the tone is
> heavily distorted (see image and image zomeed in).
>
> Frequency drift - External oscillator (AXIOM90) @ ±50 ppb (0.05 ppm) =
> ±5.4 Hz
>
> Freq. offset of +127 Hz, which does not make sense, and the tone is heavily
> distorted as well (see image and image zomeed in).
>
> We've been fighting this for a few days. Any point of view is welcome here.
>
> Regards and happy new year :)
> Brais.
>
>
> The clock servo on the B2xx-mini series is NOT an analog PLL like on the
> other models. It tends to be "jumpy" with fairly high
> close-in phase-noise.
>
> This is probably the cause of your issue.
>
>
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--
Martin K.
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