Measuring RF signal with high impedance probes?

[clk]

Hi all,

I would like to know if it is possible to measure a RF signal (lets say at 5 GHz) at the output of a VCO, which is driving a PLL, without the need to design a buffer/driver/matching network at 50 ohm.

To have the circuit working I need a probe with around 1 kohm. I asked to some equipment sellers and I got a reply from one of them that the fundamental limit for impedance probe is around 700 ohm?!?

Did any one already faced the same problem?

Thanks in advance,

clk

 

[flatulent]

HP once made probes for their spectrum analyzers that had built in amplifiers. Your real problem will be the capacitive impedance which will change the frequency.

Have you tried an inductive loop? Put one turn of a few mm diameter on the end of a coax and use a spectrum analyzer. This will be able to measure the frequency exactly and the harmonic and spur content approximately.

Another option is to make a resistive voltage divider with surface mount parts soldered into the circuit. 1k and 10 ohms will give you your 1k input impedance and 40 dB of attenuation which you can take into account in your measurements.

 

[clk]

Thank you for your idea about the divisor, flatulent.

But I still have a problem. I need to do on-chip measurements and I would like to skip the job of adding a pcb. Also, when adding a pcb I need transmission lines that will load the output of my circuit.

About your idea of inductive loop, I guess that you are talking about inductive coupling between the loop and the circuit output. Or not?


clk

 

[flatulent]

1. Try to solder two surface mount resistors in series and between your VCO output pin and ground pin. This will eliminate the need for the PC card.

2. Yes the loop is a very crude but effective method of searching for signals. You hold it over different parts of your circuit a mm or two away and it picks up enough to display on a spectrum analyzer. Amplitude measurements are not to be believed, but frequency and the presence of spurious oscillations and harmonics and reference spurs on your PLL signal will be indicated. The reference spurs will be accurate because they are measured with respect to the main signal.