Better Selectivity and Sensitivity

From Virtually any FM Radio!

by Bruce Carter

I am assuming that you have read my basic article that covers equipment basics, antennas, transmission lines, and grounding. If not, your answer may be there! If you have already done all you can on the outside of your receiver, this article describes a simple modifications you can make to the INSIDE of your receiver.

A MANDATORY SAFETY NOTE!

This article is designed for serious electronics hobbyists only. If you don't know how to solder and don't know what you are doing, don't bother reading the rest of this article. And a mandatory safety note - unplug the thing before you work on it! And those big filter capacitors on the power supply are an energy hazard - if you short them EVEN WITH THE RECEIVER UNPLUGGED - you could have some big sparks and melt your tool! If you happen to be holding onto the tool - hands don't melt, they BURN. BADLY! Do not wear rings or watches or any other type of metal jewelry while working. They are potential shorts and snags. I assume no responsibility if you electrocute or burn yourself, or if you hopelessly destroy your radio. Enough said?

BACKGROUND

There is one area where even good audio gear skimps - and it is called "test-selecting" parts. They don't do it because it is expensive and labor intensive. Which parts am I talking about? I am talking about little parts the size of your fingernail, called "IF ceramic filters". For those familiar with electronics, they look like little ceramic disk capacitors, except they have three leads on them instead of two. Here is an enlarged picture:

ceramic filter

These ceramic filters are relatively new innovations in electronics, and have only been in use about 15 years. Early ceramic filters had much looser specs and wider bandwidths - Delco car radios are notorious for this! Vast improvements can be made by swapping these older filters for newer ones. Even older receivers may not use them at all, having several "IF transformers" instead. If you really know what you are doing, you can modify these radios by finding an interstage coupling capacitor or two, and swapping it out with a ceramic filter. You will need to hook the center conductor of the filter to ground. Watch the DC voltages in tube equipment, however!

The majority of these filters are manufactured by two manufacturers - Murata and Toko. They come in several varieties, or bandwidths. Almost without exception, what is installed in your tuner is a 280 kHz variety. Since FM stations are spaced every 200 kHz, this means that they are wider than they have to be for each station. The reason for this is that they are not precise, they can be a little high or low. If you use 280 kHz, you are guaranteed of covering the whole station.

So - by test selecting filters that are all centered the same, you would only need 200 kHz. Standard filter values, or bandwidths are: 280, 230, 180, 150, 100, and 90 kHz. So you should use 230, right? WRONG!!! The way FM is actually transmitted, you only need a little over half of the 200 kHz bandwidth for stereo reception. The rest of the channel is used for elevator music, paging, telemetry, and other things that you don't need at all. A good compromise is to use the 150 kHz ceramic filters, matching them carefully for center frequency. This will yield superb selectivity, and you will notice a big increase in sensitivity as well. The reason for this is a thing called the "gain/bandwidth product". As the bandwidth goes down, the gain goes UP! It isn't hard to understand the conceptual reason - instead of amplifying parts of adjacent channels and all the sub-carrier stuff you don't need, you are focusing right on just the important portions of the station you are interested in. You aren't wasting energy and resources on noise. It is VERY important to match the filters. If they are not matched, your stereo receptions will suffer BADLY! The best way for an amateur without the necessary equipment is trial and error. It is pretty easy if your stereo only has two of these filters. If it has three, the best thing to do is leave the first one alone and just change out the second two. Then you can go back and change the first one to match the other two if you want. It won't make that much difference.

PROCEDURE

(1) Before doing anything else, you need to order the parts you need. The quickest way is to order them from Digi-Key (1-800-344-4539). They can accept credit card orders, so have your credit card handy. Think ahead - most radios have 2 FM ceramic filters, so order 2 for each radio you want to modify. They don't cost that much. The part number you want is: TK2308-ND Single pieces are 72 cents, it is $6.27 for 10. The description is "FM ceramic filters" (with 150 kHz bandwidth). The Toko part number is: SK107M4-AO-10. Here is the catalog page:

Digi-Key catalog page

If you are modifying a portable radio that does NOT have stereo reception, you can substitute a TK2309-ND / SK107M5-A0-10. It is slightly more expensive but will reject adjacent frequencies better.

(2) You need to find the FM ceramic filters in your radio (described above). Some portables may have only one, most other radios 2 or 3. You may have to do a fair amount of disassembly to find them. You will need to be able to get to the underside of the PC board to unsolder them.

(3) Make note of location of the polarity marker on the old ceramic filter(s). It is usually a red dot. Carefully unsolder each ceramic filter in the radio, cleaning the traces and holes after you get it out. Solder wick or a vacuum de-soldering tool is very helpful. Solder in the new ceramic filters with their polarity markers in the same direction as the old. Trim excess lead length and re-assemble.

TRADE OFFS

  1. You are giving up is the portion of the FM station above 57 kHz. If you are interested in the background music transmitted on an SCA subcarrier, or in RDS information, you cannot do this modification.
  2. If you have a portable, or a radio with manual (non-digital) tuning, you may find it harder to tune stations precisely. The radio's AFC function, usually switchable, may help, but it may also lock on a stronger nearby station, usually the one you are trying to get rid of!