Re: coaxial cable and GHz frequency

[eplanet]

sorry dear Audioguru for my demurral
my LNB drawing 92.6mA in standby mode.
can yo help me for designing circut that starting alarm when this current goes to zero?
thanks

 

[audioguru2]

Hi Eplanet,
For you to measure the current from the power supply that powers your LNB you need its schematic and parts layout.

 

[eplanet]

hi
do you have any schematic with LM358 for this issue?

 

[audioguru2]

An LM358 has inputs that still work properly when their voltage is at the negative supply voltage which could be 0V. So if you place a 1 ohm reistor in series with the 0V DC part of the power supply that feeds the LNB then the LM358 can sense it and amplify it.

 

[eplanet]

hi audioguru
please see this attachment can you show me in this pictures?
1 question
this cable have between 11- 18 voltage. can damage this circuit ?

 

[Hero999]

That won't work, you need to measure the DC current flowing though the co-ax, not just the voltage.

You need to break the shield and wire the resistor in series. The break and the resistor need to be as small as possible to avoid ruining the properties of the transmission line.

I'd recommend a low inductance thin metal film surface mount resistor. I'd also recommend adding a filter capacitor and possibly ferrite beads to the input of the amplifier, op-amps aren't designed to work at these sorts of frequencies and might do funny things when exposed to an 18GHz signal.

 

[audioguru2]

Hi Eplanet,
Maybe you are trying to sense the positive wire. But the LM358 can sense only the 0V wire.
The LM358 must be a non-inverting amplifier unless you add a negative power supply to it.

If the shield of the cable is 0V then I think it should be like this for a positive output from the LM358 when the LNB draws current and close to 0V when the LNB is stolen:

View attachment 40517

 

[Hero999]

What's the point of the 10k resistor?

It's in parallel with 1R so it's not going to make any difference.

He needs something to trigger the alarm, he could configure the other half of the op-amp as a comparator with a small piezo buzzer connected to the output so it makes a noise when the LNB is stolen.

Watch the current consumption of the buzzer though, the LM358 can only source 10mA and loading the power supply too much is probably a bad idea. Luckily you can get <10mA piezos but they're not that loud. If you want a very loud siren to sound when it's stolen, you need an external power supply whether this be a battery or a mains power supply (wall wart).

 

[audioguru2]

OOPs,
You are correct. The 10k resistor is not needed. :-[

 

[eplanet]

hi thanks for replay i have some questoins
1.can i find 1ohm resistor in the market?
2.what about two other resistors value?
3. is output have positive voltage?
4. i want to adding PIC microcontroller after output then i need 5 voltage in output when LNB stolen. can i take it ?(5voltage in output after LNB stolen)
thanks

 

[audioguru2]

1.can i find 1ohm resistor in the market?

Your market is completely different to my electronic parts distributors who have everything.
If you can't get 1 ohms then use 0.22, 0.33, 0.47, 1.5 or 2.2 ohms.

2.what about two other resistors value?

They set the gain of the opamp. It depends on the supply voltage, the sensing resistor value and how high you want the output voltage of the opamp to be when the LNB draws current.

3. is output have positive voltage?

Yes.

4. i want to adding PIC microcontroller after output then i need 5 voltage in output when LNB stolen. can i take it ?(5voltage in output after LNB stolen)

The output high voltage from an LM358 when the load current is small is 1.2V less than the supply voltage. So use a 6.2V power supply and the output will go up to 5.0V.
Use the second opamp in the LM358 to invert the output of the first opamp so that its outrput is +5.0V when the LNB is stolen.