I was told some oscilloscopes have 50 ohm impedance settings for radio frequency measurements for impedance matching (as opposed to one 1M ohm resistor and a 10pF capacitor in parallel for load impedance, which is good for low frequency measurements). I was able to find a circuit schematic of an oscilloscope. for the usual 1M ohm and 10pF capacitor in parallel. But i haven't been able to find a circuit diagram for the 50 ohm impedance oscilloscope. Id lile to know what the oscilloscope circuit looks like for the rf 50ohm impedance settings
Electrical – Circuit diagram of an oscilloscope with 50 ohm load impedance
circuit analysisimpedance-matchingoscilloscope
Related Solutions
Input capacitance is significant for capturing high frequency content signals. Given that you are using discrete parts and not considering RF parasitics, this system will be restricted to the 100's of kHz for 1% accuracy.
The goal is to maintain constant attenuation and input impedance over the entire frequency range. Assume Cp is 100pF, Rp is 941kΩ, Cs is 1000pF and Rs is 49kΩ (ignore adjustment cap for now) The capacitors' impedance will vary as follows:
- ZCp : 1.6GΩ @ 1Hz; 1.6MΩ @ 1kHz; 16kΩ @ 100kHz
- ZCadj. : 16GΩ @ 1Hz; 16MΩ @ 1kHz; 159kΩ @ 100kHz
- ZCs : 159GΩ @ 1Hz; 159kΩ @ 1kHz; 1.6kΩ @ 100kHz
This results in the following range of impedances:
- Zprobe : 940kΩ @ 1Hz; 591kΩ @ 1kHz; 15kΩ @ 100kHz
- Z'scope : 49kΩ @ 1Hz; 37kΩ @ 1kHz; 1.5kΩ @ 100kHz
- Zinput : 989kΩ @ 1Hz; 628kΩ @ 1kHz; 16.5kΩ @ 100kHz
- approximate attenuation: 20X @ 1Hz; 17X @ 1kHz; 11X @ 100kHz
As you can see, adjustment capacitors are required to tune the input. The grander issue is that this calls for wideband matching networks. Another option, and what is done in addition to matching networks in professional equipment, is characterizing the input impedance and compensating for irregularities in software. Standardizing at 50Ω impedances allows the construction of modular probing hardware while maintaining wideband matching and constant attenuation.
Your requirements for switching impedances and an optional 50Ω input will need some more complication. A fantastic project, and I wish I could partake!
The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.
So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.
Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.
If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).
But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.
To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.
(still WIP)
Best Answer
There are many sources on the subject, you need to search for something like "1 GHz PC scope". Many IC manufacturers offer amplifiers that can work only with 50-Ohm sources and loads. In fact, it is very difficult to get GHz-range with 1 MOhm input.
Here is an example, a reference design from Texas Instruments, "50-Ohm Oscilloscope Front-end Design Guide" . It looks like this:
All components are commercially available, and good luck.