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cyrstal frequency and H1/H3 frequency

Started by wenq...@sina.com May 24, 2007
hello friends here,I want to know what error with my new broad.:20MHz crystal feed to Xin and Xout pins,but the H1/H3 is a sine wave of 100MHz(I thought it should be 50MHz) with a DC offset,H1=1.2sin(2*3.14*100MHz*t),please tell me why,thanks.
The 100Mhz should be obvious. The x5 PLL is enabled (use a 10Mhz input
for 50 mips). The H1/H3 outputs will be square wave unless they are
severely loaded (Ios approaches 100mA on the output pins of this
device). More than likely your scope probes are not properly adjusted,
or you are not grounding them properly (no HF getting into the scope probe).

Hope this helps
Keith Larson
DSP and Analog Consultant
Lincoln, Ma 01773
w...@sina.com wrote:

> hello friends here,I want to know what error with my new broad.:20MHz
> crystal feed to Xin and Xout pins,but the H1/H3 is a sine wave of
> 100MHz(I thought it should be 50MHz) with a DC
> offset,H1=1.2sin(2*3.14*100MHz*t),please tell me why,thanks.
>
>
Thank Keith, when H1/H3 have no load,they still give me the the same wave,I am in confusion.

The 100Mhz should be obvious. The x5 PLL is enabled (use a 10Mhz input
>for 50 mips). The H1/H3 outputs will be square wave unless they are
>severely loaded (Ios approaches 100mA on the output pins of this
>device). More than likely your scope probes are not properly adjusted,
>or you are not grounding them properly (no HF getting into the scope probe).
>
>Hope this helps
>Keith Larson
>DSP and Analog Consultant
>Lincoln, Ma 01773
>w...@sina.com wrote:
>
>> hello friends here,I want to know what error with my new broad.:20MHz
>> crystal feed to Xin and Xout pins,but the H1/H3 is a sine wave of
>> 100MHz(I thought it should be 50MHz) with a DC
>> offset,H1=1.2sin(2*3.14*100MHz*t),please tell me why,thanks.
>>
>>
>
Hi

Even the third harmonic of 50Mhz is at 150Mhz, and this already above
the range of more modest oscilloscopes. To properly resolve rise and
fall times of this device the oscilloscope bandwidth needs to be in the
>=1Ghz class. On top of this you need to be using probe connection
techniques that are valid at these frequencies.

Another snag that sometimes comes up is probe types. A 10x problem will
commonly have ~11pF of what appears to be tip capacitance while a 1x
probes is more like 100pF. An active FET probe (~2pF) would be even
better. You may also want to check and see if you have the 50 Ohm
termination enabled inside your oscilloscope.

Best regards,
Keith
--------------------------
w...@sina.com wrote:

Thank Keith, when H1/H3 have no load,they still give me the the same
wave,I am in confusion.

The 100Mhz should be obvious. The x5 PLL is enabled (use a 10Mhz input
for 50 mips). The H1/H3 outputs will be square wave unless they are
severely loaded (Ios approaches 100mA on the output pins of this
device). More than likely your scope probes are not properly adjusted,
or you are not grounding them properly (no HF getting into the scope probe).

Hope this helps
Keith Larson
DSP and Analog Consultant
Lincoln, Ma 01773

w...@sina.com
hello friends here,I want to know what error with my new broad.:20MHz
crystal feed to Xin and Xout pins,but the H1/H3 is a
sine wave of 100MHz(I thought it should be 50MHz) with a DC
offset,H1=1.2sin(2*3.14*100MHz*t),please tell me why,thanks.