rtl-sdr dongle V3: Accessing the 28.8 MHz reference

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papo
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rtl-sdr dongle V3: Accessing the 28.8 MHz reference

Post by papo » Thu Dec 05, 2019 6:48 am

Hello

I am trying to sample the on-board 28.8 MHz reference to feed that clock signal into a counter/divider. I have put a jumper on the 0.05" 2x2 clock selector footprint to route the clock signal to the 2 mm expansion pin row at the edge of the board. This appears to work, but the signal at the CLK pin (expansion pad) does not look like what I would expect to see at the output of a TCXO.

Is there a tap and/or coupling capacitor between the TCXO output and the CLK pin? The signal I get certainly looks like there is. Unfortunately I could find neither a schematic of the dongle, nor the relevant specs of the CLK signal in the V3 datasheet. I'd like to know what signal I should expect on that pin in order to properly design the front-end of my counter/divider.

Best regards and thank you!
Matt

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Re: rtl-sdr dongle V3: Accessing the 28.8 MHz reference

Post by rtlsdrblog » Sat Dec 07, 2019 12:19 am

What does the signal look like? Remember that TCXO's are typically "clipped sinewave" devices. So you shouldn't expect a square wave. It will look like this http://e2e.ti.com/cfs-file/__key/commun ... 1/TCXO.bmp

There is a cap on the CLK pin out, but it shouldn't affect the output.

radiolistener
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Re: rtl-sdr dongle V3: Accessing the 28.8 MHz reference

Post by radiolistener » Sat Dec 07, 2019 1:21 am

is it correct that extension pin CLK (or CIK) is 28.8 MHz oscillator output?

What is the meaning of the middle pin with no marking?
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papo
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Re: rtl-sdr dongle V3: Accessing the 28.8 MHz reference

Post by papo » Sat Dec 07, 2019 8:10 am

The CLK pin and the unmarked one seem to be connected, I guess this is so that you can chain a number of dongles. But my question was about the specs of the signal on that CLK pin.

Regards
Matt

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Re: rtl-sdr dongle V3: Accessing the 28.8 MHz reference

Post by rtlsdrblog » Mon Dec 09, 2019 10:05 am

CLK is connected to the 28.8 MHz clock, and the unmarked pin next to it is too.

What sort of specs are you looking for? It's a 28.8 MHz TCXO, with 3.3V supply, 0.8 - 2.0Vp-p clipped sine wave output.
clippedsine.png
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papo
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Re: rtl-sdr dongle V3: Accessing the 28.8 MHz reference

Post by papo » Mon Dec 09, 2019 11:46 am

Hello and thank you for your response!

I was looking for the circuitry between the TCXO and the CLK pin. (E.g. is it AC coupled, what is the source impedance, how much can I load the output, what waveform should I expect etc. By the way this also goes for the CLK pin as input. What signal do I have to put?) You say
CLK is connected to the 28.8 MHz clock,
but that is most certainly not a direct connection. In the mean time, I think I figured it out .. there appears to be a resistor on the order of 1k and a relatively large capacitor in series. The AC coupling due to that cap causes the voltage to shift from the "0.8 - 2.0Vpp" you quote to pretty much anywhere depending on how you bias the far side of that cap. This was the info I was looking for along with the source impedance of the TCXO-resistor combo, although I did suspect a fairly high impedance there from the beginning. I did not anticipate the coupling cap and my probing did mess up the bias to a point where I really got confused. Of course in hindsight I should have considered a AC coupled output, but then again the datasheet could easily help users not to fall into that trap.

The instructions for the jumper settings in the datasheet are excellent but the wording about how the clock is routed to the CLK pin is somewhat unfortunate as it may suggest to users with little experience that the TCXO output is routed directly to the pin, and apparently can also throw people like me off-course.

Regards and thanks again!

Matt

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