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# XR-8038A function generator IC datasheet question.

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#### boylesg

What is Vsup? There is no explanation of it any where in the datasheet.
If Vcc is +9V and Vee is -9V then what voltages should Vin be between?

Solution
Hi,

Klaus
Hi,

Klaus

Is Vsup = Vcc - Vee?
In my case that would be about 16.8V since I am using 2 x 9V NiMH batteries in series with GND in the middle. Which output about 8.4V or so.
--- Updated ---

And another question re this....
Would 100nF and 10k be OK for that cap and resistor on pins 7 and 8?

Is Vsup = Vcc - Vee?
In my case that would be about 16.8V since I am using 2 x 9V NiMH batteries in series with GND in the middle. Which output about 8.4V or so.
--- Updated ---

And another question re this....
Would 100nF and 10k be OK for that cap and resistor on pins 7 and 8?

View attachment 178230

Yes Vsup = Vcc - Vee

The R on FMB1 allowed to go to 1 Mohm. C, not sure, as there is no
data on Zin of the FMS1 pin, but try your val out,m should be fine I
think.

Regards, Dana.

Hi,

I´d say the C (in combination with the R) determines the lower cutoff frequency for the modulation.
Or the other way round: calculate C by using R and fc

Klaus

I would say its more compicated than just simple RC calculation of
externals based on whats connected to pins 7 and 8 :

However looks like we can "swamp" out the effective internal Z's with large R. assuming
Rextb is small relative to the R on pin 8.

Regards, Dana.

Last edited:

Hi,

I´d say the C (in combination with the R) determines the lower cutoff frequency for the modulation.
Or the other way round: calculate C by using R and fc

Klaus
So I take it that capacitor and resistor forms a low pass filter then? Which I would have to refresh my memory on.

Hi,

Exernal R has to be added to internal R of pin 7 (8k) to get the total R for fc calculation.
(ignoring Pin8 current)

Klaus

A LPF would help remove noise and unwanted changes to your control V input
signal. So it would aid in making your modulation input, your control signal, free
of any unwanted deviations from what you would like.

Regards, Dana.

Hi,

I agree that a low pass filter attenuates noise (and jitter)

But the R-C at the FM input of post#1 form a HIGH pass filter.

Klaus

Or does it do nothing other than AC couple modulation signal, given that
cap directly connects to a voltage control bias point in internal circuit ? The R
is for Zin control.

Regards, Dana.

Hi
Or does it do nothing other than AC couple modulation signal,
Yes, AC coupling usually is done by using an HPF.

Klaus

Practically speaking are we DC coupled when we use a bulk cap, like electrolytic, or
any cap for that matter, with their leakage issues ? Eg. we cant actually build a true

Regards, Dana.

Hi,

Yes, if you are focussed on the leakage current of a capacitor ... rather than it´s function as a capacitor.

Klaus

Of course we care about non perfect components, like caps. Their ESR,
for some (ceramics) wild dependency on V, or T dependence, mechanical
microphonics.....to name a few. Should we be worried about muon and
neutrino interaction in dielectrics ? Gravitational waves, all should be
focused on.

Buts its fascinating, is there such a thing as a realizable HPF ? I am having
my doubts.

Regards, Dana.

Hi,

I guess you´re getting off topic.
You are free to start your new thread for discussions of non perfect parts and whether a HPF realizable.

Please here post topic related content only.

Klaus

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