LNA for LORAN

Question

Does anyone know what to use as an amplifier coming from an antenna for a LORAN system? The frequency is only 100KHz. The only RF amplifier that might work is the ADL5530, but I'm not 100% sure. Would an op-amp be better suited here? If so, which one or how do you choose?

ADIApproved · Accepted Answer

Hi Kevin, 
 It''s pretty simple:Pick an op amp with low voltage noise and good IMD performance. The devil is in the details of the matching and the feedback circuit. 
 (The few amateur radio circuits I've seen use resistive networks with 50 ohm terminations. I'm a purist and would try to reactively match the antenna down to something reasonable for the op amp and go from there. Also remember to add a passive pole somewhere to kill out of band signals.) I don't know if any papers on reactively matching op amps exists. 
 Here's some reading on op amp noise calculations: 
 http://www.analog.com/library/analogDialogue/cd/vol3n1.pdf#page=1 
 Regards, 
 Bob)

enash · Answer

AD8353 and AD8354 is specified to operate down to 1 MHz. It can go lower than this. It's just a question of increasing the ac-coupling caps. There is a discussion of this in the apps section of the datasheet along with a plot showing operation down to 300 KHz. 
 A low noise op-amp would also work. However, the addition of 50 ohms resistors at the input and output to properly match the device will drive up the noise figure.

ADIApproved · Answer

At LORAN frequencies manmade and atmospheric noise will dominate so the required noise figure for the system will really depend on the efficiency of the antenna. If it's a poor antenna (most likely scenario), then you'll need a good noise figure. If it's a good antenna, then a 10 dB or so noise figure will suffice. If you use an op amp, then you'll need to reactively match it. The antenna will probably be short electrically and look capacitive so you'll need an inductive match and good luck finding a high Q inductor at 100 kHz (hint: probably not a surface mount part). 
 I'm sorry, to circle back to my point, you don't use 50 ohm resistors to passively match an op amp in RF applications where low noise is a consideration. Use a reactive match and a combination of AC and DC feedback to set the amplifier's gain. 
 Bob

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LNA for LORAN