[time-nuts] Question for my new GPSDO

[Tobias Pluess]

>> I would advice against using a CMOS switch. They are relatively slow and
>> their timing is not well defined.

> There are many variants of these tailored to different applications.
> You will need to check some more characteristics like charge injection
> if you go that route.

I also don't think that a CMOS switch is wrong. OSA did the same on their GPSDO which is a successful commercial product, so they can't be that wrong ;-) the switch-on and switch-off time is 20ns and 6ns. So, when a PWM frequency around 1 kHz is used, this should not matter too much, should it?


________________________________________
From: time-nuts [time-nuts-bounces at lists.febo.com] on behalf of Achim Gratz [Stromeko at nexgo.de]
Sent: Sunday, November 03, 2019 17:45
To: time-nuts at lists.febo.com
Subject: Re: [time-nuts] Question for my new GPSDO

Attila Kinali writes:
> There is, as far as I know, only two types of DA converters that
> are inherently linear: single-bit delta-sigma modulators and
> Kelvin-Varley dividers.

No, the Kelvin-Varley is not guaranteed to be linear, same story as a
string DAC actually.  Whether a delta-sigma modulator (for D/A
conversion) is theoretically linear depends on a few things that are
hard to ascertain, so in the end it is better than some other
architectures but will still have nonlinearity if you look closely
enough.  For the application discussed here the two characteristics that
matter are that sigma-delta modulators have no large spikes or lumps in
the DNL curve and you can realize much higher resolution than the noise
floor at DC.

> If you are using used OCXOs from ebay and similar sources, they might
> be quite a bit off in frequncy, so that you might need the whole tuning
> range.

These days it should be easy enough to use two or more DAC channels and
effectively provide a small tuning window within the full EFC range.  If
you want to slide that window during long continous operation runs
you'll have bit more of a challenge on the control side.

> I would advice against using a CMOS switch. They are relatively slow and
> their timing is not well defined.

There are many variants of these tailored to different applications.
You will need to check some more characteristics like charge injection
if you go that route.


Regards,
Achim.
--
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

SD adaptation for Waldorf Blofeld V1.15B11:
http://Synth.Stromeko.net/Downloads.html#WaldorfSDada

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