# FFT Measurement on High Impedance IC's with LFRX

**URL:** <https://www.ruby-forum.com/t/fft-measurement-on-high-impedance-ics-with-lfrx/219338>\
**Category:** GNU Radio\
**Created:** [April 29, 2012, 8:47pm UTC](https://www.ruby-forum.com/t/fft-measurement-on-high-impedance-ics-with-lfrx/219338 "2012-04-29T20:47:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![dubstep](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/cdc98d/32.png) [@dubstep](https://www.ruby-forum.com/u/dubstep)\
**Post date:** [April 29, 2012, 8:47pm UTC](https://www.ruby-forum.com/t/fft-measurement-on-high-impedance-ics-with-lfrx/219338/1 "2012-04-29T20:47:49Z")

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Hi,

I am trying to do a FFT measurement on a Switched Capacitor Filter  
(MAX297)  
with the LFRX.

However the LFRX’s 50ohm impedance puts a high load on the filters  
output  
which causes all sorts of problems.

Anyone with knowledge in electronics knows how to solve this??

Typical load on the IC’s output should be 10kilo ohms…

Greets,

Jasper

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**Author:** ![jjkooij](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ecd19e/32.png) [@jjkooij](https://www.ruby-forum.com/u/jjkooij)\
**Post date:** [April 29, 2012, 8:59pm UTC](https://www.ruby-forum.com/t/fft-measurement-on-high-impedance-ics-with-lfrx/219338/2 "2012-04-29T20:59:37Z")

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On Sun, Apr 29, 2012 at 11:46, Jasper K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> Typical load on the ICs output should be 10kilo ohms..  
> You could build an opamp or transistor buffer amplifier:

> **[Buffer amplifier](https://en.wikipedia.org/wiki/Buffer_amplifier)**
>
> In electronics, a buffer amplifier is a unity gain amplifier that copies a signal from one circuit to another while transforming its electrical impedance to provide a more ideal source (with a lower output impedance for a voltage buffer or a higher output impedance for a current buffer). This "buffers" the signal source in the first circuit against being affected by currents from the electrical load of the second circuit and may simply be called a buffer or follower when context is clear. A volta...

Johnathan