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I'm designing a radio with the Si4735 chip to use in a car, and I want to use the existing FM antenna for both FM and AM reception. I'm not sure how to go about this, exactly.

How would I go about achieving this? The chip's reference schematic is as follows: enter image description here

I assume I need to somehow match the impedance of the FM antenna to that what the chip expects from an AM antenna.

Any help in this is appreciated.

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2 Answers 2

up vote 5 down vote accepted

The usual technique is to use a pair of filters to connect the common antenna to the two chip inputs. Basically, a low-pass filter for the AM input and a high-pass filter for the FM input, although more-specific bandpass filters for each band would do a better job of rejecting out-of-band signals. Each filter has a high impedance in the other filter's active range, so they don't affect each other significantly.

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+1 Yeah I forgot the filter splitter - it kind of fits somewhere between doing a resistive splitter and a relay splitter (to my mind) but is a decent solution because power is barely wasted. –  Andy aka Jul 9 at 21:26
    
Is there any specific design of LPF/HPF that I should use for this sort of application, or would simple RC-based ones work? –  Tristan Seifert Jul 10 at 1:30
    
I would probably go with L-C filters, either T-network or Pi-network. They'll be less lossy than R-C filters, and the component values will be more reasonable. –  Dave Tweed Jul 10 at 2:13
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The simplest way is to just parallel the two inputs up and hope for the best. It might work adequately but you might get some reflections that cause the FM stations to be partially weakened. It will barely affect AM reception in the same way because the wavelength of AM is hundreds of metres and reflections won't be an issue. However, the wavelength of the 88-108 MHz FM band is about 3 metres and a quarter wave is somewhat less than 1 metre meaning you might hit problems but it's worth a go in the first instant.

The next easiest thing to do is make a resistive splitter that takes in a 50 ohm input from the existing antenna wiring and splits it into two 50 ohm outlets, one for AM and one for FM. The only problem with this is that you lose 6dB of signal for each output but it might easily work.

The last simple option is to buy an RF relay that can be operated as a changeover switch thus you can route it to the AM input or the FM input.

A little more complex is a broadband low noise transistor amplifier that takes the antenna signal in, amplifies it by 6dB (or so) and feeds two outputs which connect to AM and FM inputs respectively.

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Tell me more about this broadband amplifier (or at least point me somewhere useful), please. –  Ignacio Vazquez-Abrams Jul 9 at 21:59
    
@ignacio it's just a wideband common base amplifier based around a single BJT. Use a low noise transistoras per offered by NXP - probably with Ft in order of 1GHz. –  Andy aka Jul 9 at 22:23
    
BFG520 is an awesome choice - 9GHz Ft. –  Andy aka Jul 9 at 22:37
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