What kind of options are there now for integrated RF receiver front ends for the FM radio broadcast band?
Particularly interested any solutions that would allow use of a small form-factor antenna, maybe a tuned input network with more gain than usual would work, I don't need the whole band's worth of bandwidth just to receive just a MHz or two of it.
Silabs make some nice ones. Si4735 gives you AM/FM/SW in one chip, but there might be more recent ones.
What is the point ??
At 1 MHz ("AM" band), the band noise is so huge that a very bad ferrite rod antenna with -60 dB antenna gain is sufficient. At 100 MHz ("FM" band) the band noise is much lower so a garden variety RF amplifier with 3 dB (300 K) noise figure will only double the input noise. A good preamplifier with 1 dB (100 K) NF, the band noise is stronger than the RF front end noise.
If you are using a small sub optimal antenna with -2 dB gain and 100 K front end it would have similar performance as full size dipole and
300 K front end.You can of course add amplification stages into the front end, but it also amplifies the receiver front end noise, so no net advantage.
Adding too much broadband amplification will increase the intermodulation interference, finally making the receiver unusable.
If you put the amplifier with 1 dB (100 K) NF after the filter, this reduces the interference risk. To get 1 MHz bandwidth, the loaded Q (Ql) should be 100 and to avoid pass band losses (=added noise), the unloaded Q (Qu) should be more than 1000, so no LC filter will do, but a helical resonator or a 1/4 wavelength (0.75 m) long resonator will do.
pretty compact, lotsa options, controllable via 2 wire interface, I like it already. Also there don't seem to be a lot of other options on the fully integrated one, the Si473x seems to be a family that's fairly inexpensive in the ~$2 range. Thanks I'll definitely consider that one
Right, that's the goal, receive a pretty narrow bandwidth somewhere around say 50-100 MHz with a pretty crappy antenna. Maybe 6" of "random wire" at best.
Whatever they do in cheap "walkman" FM players (not that they make them anymore, really, but anyway) I know they generally press the headphone cable into service as an antenna; an off-the-shelf integrated amp and a shorter length of wire might be sufficient for fairly strong signals
Got it. Too big, unfortunately.
I have used one of the SiLabs receivers and liked it. Some have an option for dynamically tuning a loop antenna.
John
There is also the RDA5802 and the TEA5767 You can get small PCB modules with a complete receiver from China for $2 or less.
Yeah I should have figured, via eBay or AliExpress I guess. Do you have a specific recommendation on that front?
I have no specific experience but when I searched for RDA5802 on AliExpress (knowing that this chip is used in one of the devices I have) I got many results for the TEA5767 instead, ranging from very small PCB with only the radio and solder-tabs to complete radio PCBs with connectors.
All of them in the "price that includes shipping but is lower than shipping alone would cost over here" category...
The TEA5767 is from NXP
Such antenna is electrically short, i.e. highly capacitive and the radiation resistance of only a few ohms. In order to effectively connect such antenna to a 50 ohm recitative amplifier input a conjugate match is required.
With a limited frequency range, it is simple to add a loading coil to add the same amount of inductive reactance to cancel the capacitive reactance. An amplifier with very low input impedance, (such as a common base stage) will complete the power match.
With such arrangement, the electrically short antenna is nearly as good as a full sized dipole (within 1-2 dB)..
What is a recitative amplifier?
No, on a receiver a conjugate match is not required although it is beneficial. If You have looked up the SciLabs data sheets mentioned earlier, you will find the input resistance and capacitance.
Please suggest a matching arrangement for a 6" antenna to the SciLabs input specs.
I have one of the demo boards on order, my guess is that a short random wire will pick up many of e.g. the local high-power FM broadcast stations within 10-15 miles well enough for rock-n-roll.
Sorry, resistive amplifier input
Oh no, it is 4 kohm, which is much more than I had expected.
The antenna radiation resistance for electrically short dipoles is proportional to the square of th length. Thus, the 15 cm dipole at 50 MHz (6 m) has a radiation resistance about 0.03 ohms and at 100 MHz (3 m) 0.12 ohms. If the receiver input impedance is 4 kohm, only a very small amount of current (and power) available from the antenna is actually used by the receiver.
To match the impedances, a transformer with 1:400 turns ratio would be required at 50 MHz and 1:200 at 100 MHz, not very practical. .
Since the bandwidth is limited, it might be more practical to use an LC resonator and connect the antenna (after tuning out some of the capacitive reactance) through a single turn primary into the resonator
That is right. The whole issue of antenna matching for best performance really does not apply when receiving broadcast stations within the local coverage area. Of course it would be a bit different when you were trying to make a short private link with micro/milliwatts of power.
Just a random wire will work, or alternatively make a capacitive coupling to some wire that is connected anyway (headpone, low-voltage supply, etc).
The little Silabs chips have app notes showing how to design to use the ear-plug leads as the FM antenna. Lots of good info in those.
Some also have switched capacitor banks for tuning AM on an external coil as well, also nice.
Clifford Heath.
Actually it is 25-j315 at 100 MHz.
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