Author Topic: RF amplifier gain  (Read 2779 times)

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Offline Rémi

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RF amplifier gain
« on: November 15, 2021, 05:11:11 pm »
Hello,

I'm a beginner in the RF field and I have a question regarding RF amplifiers.

I'm trying to calculate the output power level of a HMC414, based on the datasheet the gain is 20dB and input return loss 8dB.

If I apply a 10dBm signal at 2.4Ghz to the input, how can I calculate the output level of the amplifier?

Is it 10dBm-8dB+20dB = 22dBm ?

Thanks for your help.

Rémi
 

Offline rfclown

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Re: RF amplifier gain
« Reply #1 on: November 15, 2021, 05:29:48 pm »
The input return loss isn't part of the output level equation in this amplifier. So the output it is just input+gain which would be 10dBm +20dB = +30 dBm BUT... this amplifier can only go up to +30 dBm maximum, so the output will actually be less than that. Look at the Pout vs. Input Power graph on the datasheet. At a low level input like -9 dBm, the output is +11 dBm (20 dB gain). But at +10 dBm input the output looks to be about +28 dBm (18dB gain). This is called gain compression. Input return loss is a measure of how close the input impedance is to 50 ohms. Since the manufacturer specifies the gain with a 50 ohm input connected, it wouldn't even matter if the input return loss was horrible, the gain would be what is specified on the datasheet.

 
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Offline hcglitte

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Re: RF amplifier gain
« Reply #2 on: November 15, 2021, 10:34:14 pm »
You also need to consider if you want to use the PA in its linear operating range, ie below the 1dB compression point. If your signal has variable input amplitude, it should operate in its linear range. Otherwise the output amplitude will not vary in correspondence to the input.

If it is FSK or similar, it can operate in compression. However, in compression you will generate a lot of harmonics, and these needs to be filtered in order to comply with most regulations.

Operating near compression will also ensure that the maximum efficiency is achieved. Otherwise a lot of power is wasted as heat and not pushed out to the load.

A lot of research tries to make RFPA operate in compression at varying input signals by changing the the bias current relative to the input signal. Just to achieve better efficiency.
 
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Offline Rémi

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Re: RF amplifier gain
« Reply #3 on: November 16, 2021, 08:57:04 am »
Thanks for your answer so far.

Actually I'm trying to amplify the signal coming from a DJI Lightroom 2. The modulation used is based on OFDM from what I could find online.

The signal I apply to the amplifier is around 0dBm:
[attach=1]

The signal I get on the output is around 10dBm, I get an amplification of ~10dB:
[attach=2]

I have also measured the S21 with a VNA:
[attach=3]
At 2.44GHz I have a S21 of 9,23 -> Gain=20*log(S21)=20*log(9,23)=19dB

Why is my input signal not amplified by 20 ?

Thanks

 

Offline rfclown

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Re: RF amplifier gain
« Reply #4 on: November 16, 2021, 03:31:03 pm »
For a device with 50 ohm input and output, the VNA S21 reading is gain (no math needed). So you are measuring a gain of 9.23 dB, which according to the datasheet is not right, so something is wrong. Could be many things:
1. power supply is low voltage, current limiting
2. control voltage is not correct (Vctl on datasheet)
3. cables are bad
4. overdriving the amplifier (gain is compressing)
5. overdriving the instrument (display shows 0 dBm in, which would give +20 dBm out. Can the VNA handle that? if not you have to have an attenuator on the amplifier output. In general, when measuring an amplifier with a large output you ALWAYS want to put an attenuator at the instrument input in order to avoid damage.
6. more than one of the above (my guess)

With the spectrum analyzer you can't measure power this way. This is a very wide bandwidth signal (looks like 75 MHz). You have to add up the power in the entire spectrum, not just measure at one point. Some analyzers will have a band power measurement function. Another way is to read the spectrum into a computer, convert the points to a linear format (from dBm to watts), add up all the points.

OFDM has a very high peak to average ratio, so for a given average power the peaks are much greater. In order not to compress the gain your output has to stay in the linear range of the amplifier. With Vs=5v the P1dB is +27 dBm. With Vs=3.6 the P1dB is +25 dBm (datasheet numbers). The P1dB is the point where the gain is compressed 1 dB. You can often operate up to that point. Say your signal has a 10 dB peak-to-average ratio (I don't know, and this is hopefully a worse case guess) the maximum average input with a 5V supply would be P1dB-gain-PkAvg = +27-20-10 = -3 dBm.
 

Offline Rémi

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Re: RF amplifier gain
« Reply #5 on: November 17, 2021, 09:11:12 am »
Hello,

1) The amplifier is powered with 3V3
2) The control voltage is also set to 3V3

I don't know what to expect when the amplifier is powered with 3V3 instead of 3V6. In the datasheet we can see that the Psat is lowered when powered with 3V3 but the gain should remain around 20dB (see attached screenshot).

3) I will test other cables
4) Possible
5) The VNA can cope with 26dBm maximum

I will repeat the power measurement with the OCBW mode used on the spectrum analyzer.

Thanks for your help.




 

Offline Rémi

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Re: RF amplifier gain
« Reply #6 on: November 17, 2021, 09:39:41 am »
I have repeated the power measurements with the OCBW (Occupied bandwidth) measurement of the spectrum analyzer.

So we get 17dBm before the amplifier and 27dBm at the output.

It corresponds to the Psat we see in the datasheet.

Are my measurements correct this time ? I still need to figure out what is wrong with the S21 measurement.

Rémi
 

Offline Emo

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Re: RF amplifier gain
« Reply #7 on: November 17, 2021, 10:12:33 am »
Hi Remi,

Do you have the possibility to test with a CW signal. The problem is that measuring power on OFDM-like signals with a sweeping SA does introduce large errors. Only real-time SA's can be used in that case.
NB. The mentioned +27 dBm input capability is probably the non-damage level and not the level at which measurements can take place.

Eric

 

Offline rfclown

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Re: RF amplifier gain
« Reply #8 on: November 18, 2021, 03:48:37 am »
I see four main problems:

1. You are using 3.3V. Datasheet shows values for 3.6V. I would expect that the gain with 3.3V would be similar to 3.6V (but with lower Psat), but you can't be certain. Datasheet numbers are USUALLY very good guidance of what the part will do. BUT you have to follow ALL the criteria.

2. You say Vpd=3.3V. This is one of your primary problems. The datasheet specifies that with a 3.6V supply (or a 5V supply), Vpd should be 3.6V at the IC pins NOT the demo board Vctrl. The datasheet specifies that the current into the Vpd pins at 3.6V is 7 mA. Therefore resistors R1 and R2 must be chosen to give 7 mA. You have not followed this, therefore your gain is not maximum.

3. Your spectrum analyzer OCBW mode is nice feature, but it is going to show the average power. With 27 dBm average power and an OFDM waveform, your peak power will most certainly be much higher and therefore clipped (saturated). You are overdriving the amp. Put a way lower signal in. Add an attenuator at the input if you can't tone down the input power of your source.

4. This one is also about overdriving your source, but with the VNA. If you can't reduce the source signal of the VNA, you have to put an attenuator at the amp input.

Datasheets are your friend, but you have to read through them and follow everything they talk about doing. You don't have the correct Vpd, and you are overdriving the amp. Anytime I measure a power amplifier with a VNA, I will ALWAYS (ALWAYS ALWAYS) put an attenuator on port 2 to protect my VNA from damage, and I will turn down the source power to a small value. If my amp had 20 dB gain, I'd turn it down to -20 dBm. If I had a VNA that couldn't do that, I'd put a 20 dB attenuator on port 1. Calibrate the VNA with PADs on the ports and the resulting through should measure S21=0dB. For things like this I use a simple THRU cal if the VNA supports that. Insert device under test and read the gain. If my S21 reading then didn't match the datasheet number, I'd assume I was doing something wrong. THEN and ONLY THEN would I start to raise the input level to see where the gain started to compress.
 

Offline Joel_Dunsmore

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Re: RF amplifier gain
« Reply #9 on: November 20, 2021, 08:16:02 pm »
And, it is entirely possible it's not the fault of the measurement...sometimes you get a bad part.
 

Offline rfclown

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Re: RF amplifier gain
« Reply #10 on: November 21, 2021, 01:33:16 am »
And, it is entirely possible it's not the fault of the measurement...sometimes you get a bad part.

This is true. Dr. Joel is my elder and has more experience than I, but in my experience:
how often have I had a power supply that current limited (or had a control voltage wrong) and thought my gain was low?... many times
how often was I compressing the amp and thought the gain was low?... many times
how often have I have bad cables and thought the circuit wasn't functioning correctly?... many times
how often has it been a bad part?... WAY fewer times than the other above mentioned items
 

Offline Joel_Dunsmore

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Re: RF amplifier gain
« Reply #11 on: November 22, 2021, 01:48:23 am »
This is true. Dr. Joel is my elder and has more experience than I, but in my experience:
how often have I had a power supply that current limited (or had a control voltage wrong) and thought my gain was low?... many times
how often was I compressing the amp and thought the gain was low?... many times
how often have I have bad cables and thought the circuit wasn't functioning correctly?... many times
how often has it been a bad part?... WAY fewer times than the other above mentioned items
Well yes. All those are much more common; along with many others.  The advice of "put a pad on the source, put a pad on the receiver, connect a thru and record the number, put in the amplifier and record the change" is about the most fool-proof way to get a gain number. But remember: “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.” ~ Arthur Conan Doyle, The Case-Book of Sherlock Holmes.  or, maybe also relevant: if you make some more fool-proof, they'll just make a better fool.
 

Offline cdev

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Re: RF amplifier gain
« Reply #12 on: November 23, 2021, 01:53:10 pm »
If your receiver is on a drone is there any way you can reduce noise, say from motor or other components without adding too much more weight?

Adding an LNA may give you more possibilities if your receiver is receiving just signal, but it will also amplify any noise that is present at the LNA's input. Any detectable noise may even have the effect of desensitizing a receiver. Fairly small bypass capacitors can often work magic with RF noise. Right at its source, ferrite beads may also help.
"What the large print giveth, the small print taketh away."
 


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