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CRO DISPLAY on Microwave Testbench

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vinodquilon

Member
In the Microwave Testbench shown,
I am replacing VSWR Meter with another CRO.
Then, What would be the waveforms on two CROs ?

According to my prediction:
CRO at Detector Mount will indicate SQUARE WAVES( AM modulation set on MW source supply)
CRO at slotted line will indicate STANDING SINE WAVES tracked by the slotted line

Is it correct ?




 

JimB

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Most Helpful Member
How is the microwave source modulated?

If it is modulated with a square wave, you will see a squarewave at the detector mount, and at the slotted line detector.

If it is modulated with a sinewave you will see a sinewave.

JimB
 

vinodquilon

Member
How is the microwave source modulated?



If it is modulated with a sinewave you will see a sinewave.








JimB
Hi JimB,
There is an option for choosing between SQUARE WAVE AM & SWEEP WAVE FM MODULATION in Microwave power supply.
I have set it on AM.
Then modulated signals will be in SQUARE shape.What would be the shape of STANDING WAVES ?
Can I display these Standing Waves on CRO connected at Slotted end ?

Can I connect VSWR meter at the end of Detector Mount XD-451 to measure VSWR reading ?
 
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JimB

Super Moderator
Most Helpful Member
You seem to be all confused in your head about standing waves.

You do not display standing waves on an oscilloscope, a standing wave is not a time-varying voltage.

The standing wave is a voltage which varies with the position along the line.
When you move the sliding carriage of the slotted line, with an ideal match (vswr = 1:1) the voltage will be the same at all points on the line. (OK so the voltage will vary at 10Ghz or whatever frequency, but the RMS value will be the same all the way along the line).

When the VSWR is 2:1, as you slide the carriage along the slotted line you will find that the voltage peaks and troughs, and the peak voltage is twice the trough voltage, because the VSWR is 2:1 !!

Note, whilst I have been refering to the VOLTAGE, if your slotted line is a co-ax, that is fine, but it is a bit of a streatch of the imagination to think of a voltage in a waveguide which is effectively a brass tube!

JimB
 

vinodquilon

Member
Thanks JimB,
I got the idea.
One more doubt----- for 'peak' points of standing waves, SWR needle moves to 'low' positive db gain values(towards right-max.needle deflection). Similarly, for trough points of standing waves ,SWR needle moves to peak positive db values(towards left).

Why this contradiction exists ?
 
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JimB

Super Moderator
Most Helpful Member
DeeBees!

0dBm is the reference level, dBm the m indicates that the reference level is 1mW.

As power increases from 1mW to say 2mW the change is +2dB, when the power is 10mW the change is +10dB, the power is now +10dBm (plus 10dB with respect to 1mW).

When the power is reduced to 0.01mW, we have -20dBm a reduction of 20dB, and for 0.001mW a reduction of 30dB, the power is -30dBm.

I hope that you can follow this waffle!

So, 0 is the reference level, +ve numbers represent more power, negative numbers represent less power.
-30dBm is less power than -20dBm.

OK?

JimB
 
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