RF test technician basic Tests, RF gain, Gain Flatness, Phase Tracking, Log Linearity

[Billy Mayo]

RF Test Technician should know how to do these basic tests , how are they done please?

How do you test for these when testing an RF circuit?

Please give examples on how to test for these requirements:
1.) RF Gain
2.) Gain Flatness
3.) Gain Matching
4.) Return Loss P1 dB Compression
5.) Phase tracking
6.) Log linearity

Do you use an oscilloscope or scalar and network analyzers?

And How do you do these please?

 

[JimB]

1.) RF Gain
To measure the gain (or loss), inject a signal, usually from a signal generator, and then measure the output using either a voltmeter or a power meter.

2.) Gain Flatness
Measure the gain at various frequencies.
The difference in gain at the different frequencies is an indication of the "Gain Flatness".
A better term may be "frequency response"

3.) Gain Matching
Never heared the term before.
Sounds like verifying that two channels have the same gain.
Where did you find this term?

4.) Return Loss P1 dB Compression
Uh, what?
Sounds like two different things mixed up and stuck together.

Return loss is a measure of impedance matching.
Effectively another way of expressing VSWR.
To measure it, use a "Return Loss Bridge" or maybe a Network Analyser if you are rich.

Gain Compression.
As the signal through a (linear) amplifier is is increased, at some point the output will be less than the input signal multiplied by the gain.
To measure this, plot the input and output voltage or power of the amplifier and you will find that the gain drops off at some power level.
The 1dB Gain compression point is where the output power is 1dB lower than expected from the input times the small signal gain.

5.) Phase tracking
No idea. Where did you hear this?

6.) Log linearity
Not sure what you mean here.
It sounds like a measure of the linearity of a logarithmic amplifier.
By plotting the output of the amplifier on a logarithmic scale against the input on a linear scale, we should get a straight line.

Does this help?


JimB

 

[Billy Mayo]

Thanks for the info.
I got these terms from a job ad.

The difference in gain at the different frequencies is an indication of the "Gain Flatness".

Why is it called Gain Flatness?

Return loss is a measure of impedance matching.
Effectively another way of expressing VSWR.
To measure it, use a "Return Loss Bridge" or maybe a Network Analyser if you are rich.

Yes how do you measure the Return loss? using a network analyzer?

Return loss is a measure of impedance matching of the inputs and outputs?

The 1dB Gain compression point is where the output power is 1dB lower than expected from the input times the small signal gain

How do you know what output voltage is 1db lower than the input? is there a simple formula for this?

 

[davenn]

Thanks for the info.
I got these terms from a job ad.

Why is it called Gain Flatness?

it means exactly what it says ... if the gain is flat over a particular bandwidth say 20MHz ... then the gain doesn vary over that bandwidth. A flatness tolerance may be provided ... eg plus or minus 1 dB

Yes how do you measure the Return loss? using a network analyzer?
Return loss is a measure of impedance matching of the inputs and outputs?

you are measuring the transmitted/injected power level Vs the reflected power level. As JimB said, VSWR is another description of this
you could use a network analyser or a spectrum analyser to sweep a receiver/transmitter or individual filters or even an antenna

How do you know what output voltage is 1db lower than the input? is there a simple formula for this?

Jim didnt say voltage, read his comment again ... its referring to POWER = P1 dB output = P1 dB input + (Gain - 1) dBm

to follow on from Jim's comments
here's my usual response to people who ask about 1dB compression.....
When operating within the linear region of a component, gain through that component is constant for a given frequency. As the input signal is increased in power, a point is reached where the power of the signal at the output is not amplified by the same amount as the smaller signal. At the point where the input signal is amplified by an amount 1 dB less than the small signal gain, the 1 dB Compression Point has been reached. A rapid decrease in gain will be experienced after the 1 dB compression point is reached. If the input power is increased to an extreme value, the component will be destroyed.

cheers
Dave

 

[Billy Mayo]

you could use a network analyser or a spectrum analyser to sweep a receiver/transmitter or individual filters or even an antenna

Don't you have to use a RF signal generator to SWEEP a receiver/transmitter?

When Sweeping a Filter or antenna or receiver / transmitter , What is a test technician looking for?

If the input power is increased to an extreme value, the component will be destroyed.

What component will be destroyed?

 

[JimB]

Thanks for the info.
I got these terms from a job ad.

Beware of HR Department waffle!

Why is it called Gain Flatness?

Have a look at the first graph which I have knocked-up. (UK meaning, not USA meaning!)

How do you know what output voltage is 1db lower than the input? is there a simple formula for this?

No formulae that I know of.
You have to make some plots of input and output power and look at the graphs.
So, look at my second graph.

Return loss is a measure of impedance matching of the inputs and outputs?

Sort of.
This is a big subject which can be confusing.
Let us try some simple generalisations.
In RF work, inputs and outputs of "equipment" is specified to have a certain impedance, usually 50 Ohms.
Connecting cables, usually coax, and their connectors have a characteristic impedance. Again usually 50 Ohms.
(Note, you CANNOT measure that 50 Ohms with a multimeter !).

When a perfect system is connected with perfect cables, all the energy which comes out of a source is absorbed in the load at the far end of the cable.

When the load is not perfect, some of the energy is reflected back from the load to the source.
These reflections give rise to standing waves in the connecting cable. Hence we get the expression VSWR (Voltage Standing Wave Ratio).
If we were to measure the voltage at points along the cable from the source to the load we would see that the voltage varies. The ratio of the maximum and minimum voltages is the VSWR.
The VSWR depends on the characteristics of the load (and the connecting cable, but lets just keep is simple).

So, one way of specifying how well a "load" is matched to the ideal 50 Ohms would be to specify its VSWR.
A better way is to specify its "Return Loss", how much of the incident power from the source is reflected back.
The return loss is usually measured in dB.

A Return Loss of 10dB would mean that one tenth of the incident power is reflected. Not a very good match.

A Return Loss of 20dB would mean that one hundredth of the incident power is reflected. A reasonable match for most common applications.

A Return Loss of 30dB would mean that one thousandth of the incident power is reflected. Good for all but the most demanding applications.

Yes how do you measure the Return loss? using a network analyzer?

Return Loss can be measured using a "Return Loss Bridge" - Google it.
I dont know of it will show up in Google, but a company I worked for many years ago made a bit of kit called a Rho-Tector" which was basically an RLB adapter for use with existing signal generators and power meters.
As for the network analyser, I suggest that you Google it.

My brain is aching a bit now, I hope that this helps.

JimB

 

[Billy Mayo]

tune standard catalog amplifiers up to 18GHz up to 40 GHz

What does this mean? how do you tune the amplifier? is it using a RF signal generator and how would you tune it from 18GHz to 40Ghz?