Hello,

I am a undergraduate electronics engineering student and I recently started working at a company that specializes on GPS antennas. The antenna contains PCB with some low noise amplifiers, RF filters and of course, the antenna itself, soldered directly to that PCB (I am calling the ,,antenna" whole thing with PCB, even though I understand that an antenna in its pure form is nothing more than some metal shape conductor...)

One of my tasks at the job is to test the antennas. For this procedure, I am using a VNA with an external DC supply connected directly to it.

Every antenna has one connector soldered to it. I am plugging one of the VNA ports to that connector to supply the antenna with a signal of defined range frequencies (something like a power supply equivalent) and I am using the second VNA`s port with some custom-striped coax cable to probe various points of the PCB, to see the signal levels etc... Under test, a perfectly working antenna consumes around 60-70 mA at 5V (as I understand, all this current is consumed to power the LNAs and active filters.)

However, when the antenna is fully tested and finally hits the real world application, it has only one port designed to be connected to. The whole PCB and the antenna have only one port. My knowledge in wireless communication is really limited, but I can not understand how is this possible? I understand that a GPS antenna does not send any signals, it only receives them, but how is it possible to both power the LNAs and receive the signal simultaneously via a single coax cable?

I tried asking senior engineers of that company, but they were not very eager to explain anything to me, they just hinted that ,,The process is organized in pulses". However, there are no large electrolytic capacitors on the PCB that could store charge for a defined period of time if the power is removed. Sadly, I can not discover any more information regarding chip codes and the circuit, as it is strictly classified by the company. I may have a completely wrong understanding of how it all works, but any observation on the described situation would really explain how is this possible. Thank you in advance for your time and answers.

 

hi VZ.
Check this link.
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https://www.google.com/search?client=firefox-b-d&q=powering+LNA+via+oax+antenna+cable

https://www.connectortips.com/getting-one-wire-to-do-more-part-3-powering-the-antenna-lna-faq/

Clip from link.

 

RE:""I recently started working at a company that specializes on GPS antennas""
Wow! Does indeed Lithuanians begun to produce gps technics? Congrats!
Yes, gps receivers are receivers only. But when You adjust the antenna, there are no other methods as to adjust is as transmitting antenna what means it automatically is adjusted as receiving antenna.
Because the signal source is settled in 1/2 of geosynchronous orbite or 18 000 km afar, the gps signal is extremely fragile (weak, typically about -125 dBm but in some cases even -158...159 dBm), thus the antenna good adjusting to resonance is important factor for success. For comparizon, when 4G network signal falls beyound 100 dB then many models says - weak signal, but when beyound -115 ...-120 dB then most of modells shows one stripe or zero stripe signal strength.

 

It is a common practice to use a DC offset on an AC signal to power a device. This is done with RF and AF devices alike.

You will find it on electret condenser microphones, LNAs on satellite receivers, and other things of that sort. The key is simply a DC blocking capacitor on before any AC circuitry, which drops the DC component.

I am not completely certain this is the confusion you are having, if not please clarify it.

 

hi VZ.
Check this link.
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https://www.google.com/search?client=firefox-b-d&q=powering+LNA+via+oax+antenna+cable

https://www.connectortips.com/getting-one-wire-to-do-more-part-3-powering-the-antenna-lna-faq/

Clip from link.
View attachment 284743

 

hi VZ,
I checked the links I posted in post #2, they look OK to me.

Please explain your attachment. post #5
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hi VZ,
I checked the links I posted in post #2, they look OK to me.

Please explain your attachment. post #5
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Your text was absolutely fine, the problem was that when I wrote the reply to you, this webpage somehow distorted my text (tabulation problem or something similar) so I just uploaded a screenshot of my reply text, that was dedicated to you Sounds weird, but my reply was completely unreadable...

 

hi VZ,
OK.
I have enlarged your image so that it is easier to read.

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hi VZ,
As you may know, the two inductors are high impedance to the HF signal, so do they do not load the signal between the OPA output and the Receiver.
Consider that the OPA inductor also has a resistance value, this would enable the DC biassing of the OPA output to set the Vout to 2.8V.

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hi VZ,
As you may know, the two inductors are high impedance to the HF signal, so do they do not load the signal between the OPA output and the Receiver.
Consider that the OPA inductor also has a resistance value, this would enable the DC biassing of the OPA output to set the Vout to 2.8V.

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Sadly, I still can not understand... I am attaching a picture, how I see that circuit from the #2 post. If the opamp output is at vcc, the opamp has to be able to output higher voltage than the vcc to produce the signal (with an offset), but, it can not, because vcc is its voltage limitation... This could be my insufficient knowledge problem, but I dont understand your explanation...

 

hi,
The OPA Vout at 2.8V is the steady state, the RF input signal modulates this DC level.
Remember, we are talking of very low level RF signal levels, the GPS receiver also amplifies the signal

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hi,
The OPA Vout at 2.8V is the steady state, the RF input signal modulates this DC level.
Remember, we are talking of very low level RF signal levels, the GPS receiver also amplifies the signal

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So basically you are saying, that circuit could output lets say 2.79-2.81V (Vmin-Vmax) square wave while VCC and the output of OPA is only 2.8V? Sounds like some kind of magic to me...

 

hi @Vilius_Zalenas

A very basic circuit showing how power and signal can share the same connection from Remote to Master.

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hi @Vilius_Zalenas

A very basic circuit showing how power and signal can share the same connection from Remote to Master.

EView attachment 284909

Thank you very much, I will examine this example deeply