Logarithmic detector ICs + inducter as antenna

[Nora]

Has anyone used any of the LT5504 series chips?
If so, what have you used as the antenna input to this chip? They show a 1:1 transformer input in their example circuit, why?

What is a good antenna generally to use to detect a >1 GHz digital signal?
Thanks!
N_N

 

[E Kafeman]

It depends on what you want to achieve. General detection if a rf field exist or not, can a short dipole or monopole be enough. To be able to read and compare signal levels and directions over whole frequency range can a wide-band logarithmic antenna be a simple alternative.

 

[vfone]

The transformer is a BALUN (balance-to-unbalance) used to transform the differential input of the receiver to single-ended mode of the RF input.

 

[Nora]

Thanks for that- I have seen a few circuits using a TF as input.

I do not understand how to design the antenna input part of the circuit.
For example, I am looking at a chip from Analog Devices ADL5511. There is a basic connection diagram on page 18 of the linked datasheet. Page 1 states that the input has a nominal 50 ohm impedance.
One of the pins is RFin.

What goes into that pin? Is it an antenna? What should I do about the 50 ohm impedance? Where can I find or learn about a circuit that goes on the input to the RF input pin?

Thanks!
N_N

 

[biff44]

you need to tell us the frequency and bandwidth you want before that can be answered

 

[Nora]

Good point Biff44.
I am detecting CDMA and Wi-Max. So 800MHZ, 1.9GHz and 3.5GHz.

What do you mean by bandwidth?

 

[tony_lth]

ADL5501 is normallly used in TX chain, after the PA, there is a coupler, about -20dB coupled power for the input of the ADL5501, then from the ADL5501 output RMS voltage, you know what is the transmitted power. The voltage can be used for the ALC circuits.

 

[Nora]

But what about the antenna part of the circuit?
Thanks in advance!

 

[vfone]

You can use any cheap multiband antenna that is already on the market. Just search for them.

**broken link removed**

 

[Nora]

Thank you vfone.
Does the purchased antenna attach directly to RFin?
What do I do about the "nominal 50 ohm impedance" on my desired chip ADL5501?

 

[biff44]

You would probably do best with a logarithmic detector (larger dynamic range) that is already assembled on a connectorized board, such as the ADL5513-EVALZ from analog devices. Then you just hook up your antenna and away you go.

ADL5513 Evaluation Board | Analog Devices

 

[vfone]

You can attach the antenna directly to the 50 ohms input of the chip, but to increase the sensitivity of the system you can place in the front of the detector/receiver a MMIC wideband amplifier, which have also 50 ohms input/output.

**broken link removed**

 

[jiripolivka]

From the above discussion I think Nora wants to use a log detector to monitor signals over 0.9 to 2.2 GHz, and is not sure how to couple a suitable antenna to a MMIC with a differential input.
From MMIC data sheet I can understand that the 1:1 balun transformer is needed if a well matched response is expected over the wide bandwidth. If Nora wants to use the log detector with an antenna as an accurate instrument, she will need to get a commercial wideband antenna like a logarithmic one which is matched to 50 Ohms, to guarantee system flat response over the full band.

If Nora needs a signal indicator only, then any monopole whip or wire antenna, approximately quarter-wavelength long at band center will work. Log detectors like ADL5513 have a dynamic range > 60 dB, so signal levels from ~+10 dBm down to -50 dBm can be indicated. I have used AD8313 for the same purpose, with a monopole antenna; I can detect signals down to -50 dBm, and even point direction to their source by that simple monopole.