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# Frequency measurement in a standing wave detector

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#### fiquran

##### Full Member level 1
frequency measurement

this is related to the standing wave detector. i am supposed to design a two port transmission line system with a standing wave detector. let's say i have an oscillator connected to port 1 (and 1') and i am supposed to find the frequency of this oscillator. the problem is which element should i need to connect port 2 (and 2') of the transmission line so that i can get more acuurate results? would it be just a short circuit? or a matched load? or another component?

thanks,

frequency measurement

hello,
i am really not sure , but as far as i could remember u need : frequency meter "which is a cavity with variable lenght and known freq. for each length", u will also need a T-junction "to be apple to add the frq. meter", and a power meter.
connect the T.L to the T-junction , then connect both
the freq. meter "Cavity" and and a power meter to the other ports of the junction.
adjust the freq. meter till min. power reading ,then u see the lenght and look up for the freq.
i am not sure that is what u want , but this is what i could remember.
regards,
a.safwat

### fiquran

Points: 2
Re: frequency measurement

thanks for the reply. although the method you suggested is not applicable for the system that i am dealing with, it is good to know. I have made some research and what i am gonna do is just short port 2, and by finding the distances between the minimas of standing wave, i'll try to find the frequency. thanks for the info again.

Re: frequency measurement

hi
what you need is:
power generator whose f is going to be calculated.
slotted line(a WR90 with a nonradiating slot in the middle so that it can send a probe to be sent to oscilloscope to see the pattern of standing wave)
connect the generator to one port of wr90 and a nonmatching load to the other.
(the load should not be dummy(full absorber) and no difference a short or open)
move the probe along the slot. what you see on oscope is a dc voltage which is sinusoidal. the distance between two min is half of wavelength = λg/2
use this formula:
(1/λ0)^2=(1/λg)^2+(1/λc)^2
λc=2*a (a=bigger side of WG)
you find: λ0
then
f0=(3*10^8)/λ0

ok?

### fiquran

Points: 2
Re: frequency measurement

thanks hamid! this is exactly what i need! so what you are saying is that rather than making port 2 a short circuit, i need to put a nonmatching load. do you have any suggestion for the value of this load?

frequency measurement

hello,
i think if u want to use this methode a short will be OK , since the reflection is max (|reflection coeff.|=1,note open circuit has the same refl. and can be used too) which will give u max. reflection (i.e. max standing wave), also i think "but not sure" that it was more accurate to measure the distance between two min. rather than two max. as the sin becomes flat around the max and sharp around the min. ,
finallly u could use this methode with power meter to sense the min. power if u dont have CRO

Re: frequency measurement

i did not know that sin becomes flat around the maximum points, so the way to go is find the minimum points. i'll measure more than 2 minimum points so that i can get a better average value for the lambda/2 distance. it makes sense to me. thanks for your reply.

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