RF Checker with LTC5505 - LTC 5505 and Arduino

Hello, I need to check RF level signal present between 300Mhz and  450Mhz, up to 20 or 30 meters from the transmiter device, I have to use a 3V Coin Cell like power supply.
I have selected the LTC5505 device to do that.
I want to connect the Vout to an analog input of Arduino UNO and show the signal level present by display
I have already the prototype pcb like the typical circuit as shown in the pdf datasheet file, to test the system, But, I have a few doubts
1* - The selected device(LTC5505), is the best option for this project?
*2 - What is the right value of the capacitor connected to the RFin pin to check this range of frequencies?
*2 - I have connected a random value capacitor and 16,5cm antenna printed in the pcb, I receive signal, the Vout pin changes the value when the RF signal is present, but the distance between the 1 Watt transmitter and my prototype pcb is 30 cm,  more away this distance I can't receive signal. Why?
*4 - When I receive the RF signal, the Vout pin changes the value, but the value goes from 25mV to 1,5V in a peak of voltage when the transmitter start to send RF signal and falls down to 25mv inmediatally, even when I keep transmiting. why the voltage level does not keep in 1,5V?
Thank You!

[edited by: Eugeniomercol at 12:20 PM (GMT 0) on 4 Feb 2019]
  • +1
    •  Analog Employees 
    on Feb 5, 2019 1:44 AM over 1 year ago

    Hello Eugeniomercol,

    *1 Schottky detectors such as LTC5505-x may not be sensitive enough for your application. See below:

    *2 Normally the input DC blocking capacitor is 1000 pF for this device.

    *3 First check the printed antenna, to validate center frequency, gain, pattern, etc. Next, calculate or measure the received signal level at the required distance. Compare that level to the detector sensitivity graph shown on datasheet page 4. You may find that the received level is too weak for a Schottky detector. In that case, consider a more sensitive detector. For lowest current consumption at this frequency range, consider ADL5506 Log detector, ADL5501 RMS detector (linear in V/V), or LT5581 RMS detector (linear in dB). Other options exist but current consumption is increased.

    *4 Schottky detectors will generally follow the envelope of the RF waveform. What is the modulation on the signal? For long-term storage of a Schottky detector peak envelope output value, an additional peak-hold circuit is needed, or a ADC with firmware to hold the peak value.