Is there anything more fun than listening to E Skip? Or staying up all night logging long haul tropo, or going to bed at 4am and getting up for work at 8?
 

 

 

Over the years, this article has been stolen and posted on Usenet and other places by individuals who claimed to have written it themselves. I never realized it was that good!  Usually I find out about it within hours. 

 

 

How to Greatly Increase Your Receiver's Selectivity

Originally written in 1999

 
Over the past year and a half I have had about half a dozen requests from DXers and audiophiles alike for information on this subject. These people all want to know what new tuners/receivers are on the market, and which of them has the best selectivity. One or two wanted me to recommend a tuner for them to buy. A couple told me about their particular reception problems and wondered that new tuner would solve their problem for them.

 

Sad to say, but I had to tell them that buying a new receiver would not help them solve their problem. But, I was glad to tell them that they might be able to solve their problem themselves, quickly and easily, and without paying huge amounts of money for a tuner that probably wouldn't be any more selective than what they already have.

 

What I am talking about is direct replacement of the 10.7khz ceramic IF filters in their tuners with much narrower filters, resulting in a much more selective piece of gear that could rival the selectivity of the still much sought after McIntosh MR-78, the tuner that serves as the standard by which all other tuners are measured ...at least for adjacent channel selectivity. In super narrow mode, this fabled tuner measures around 100db of adjacent channel selectivity. Almost everything else on the market measures around 10db, probably much less.

 

There are two brands of ceramic IF filters, Toko and Murata. Toko filters are readily available and cost less than a dollar each, even less in quantities of 10 or 100. Murata filters are more difficult to find, but are more in demand by FM DXers. At the end of this article I will show you where to buy the filters.

 

The first thing people want to know is this: what do these filters look like? It's a bit difficult to explain the size and the shape of a ceramic filter, but looking at the picture above should make the job easy. The picture above is a collection of various IF filters. The red filter on the upper left corner is a Toko filter. All of the rest are Murata filters of various types. These are the components you will look for in your own tuner or receiver.

 

The next thing people want to know is where to find these filters in their own unit. The best guide I can give you is this. In an analog unit look in the area in the vicinity of the tuning capacitor. You should notice them. In a digital tuner look in the area of the antenna input. You should be able to find them. Notice in the picture that the filters come in different colors. The filters you have will probably be one of the three colors (red, brown, blue) shown in the photo.

Next, count how many filters you have in your unit. A portable radio like the GE Superadio will have only one. An AM/FM stereo receiver or tuner should have at least two (my Sony ST-5130 analog AM/FM tuner has 8!). Most high end tuners will have around 4 to 5 filters. Generally speaking, the more stages of filtering a unit has, the better the quality of the unit. However, I have an old Sherwood AM/FM receiver which has only two filters, but with two 150khz filters installed instead of the stock filters, the unit shines!

 

How many filters should you change?  If your tuner has two filters, change them both. If your receiver has more than two filters, change only two. Although some DXers will disagree with me, if your unit has five filters I would not change all of them. You might notice some side-effects as the result of changing all of the filters. I will talk about these side-effects later.

 

If you decide to do a filter change, you need some basic equipment. You need a soldering pencil, some solder, some desoldering braid, a magnifier or magnifying glass and good lighting in your work area. You need a screwdriver to take off the bottom cover of your unit. You might need a pair of needlenose pliars to pull out the old filter. If you don't have these basic tools, please find somebody who has them. Take your tuner to a repair shop and have the modification done there.  If you have no mechanical ability do NOT attempt this job. Now, having said that, I personally know one DXer who went out and bought a soldering iron and braid and did the job without a problem, never having done any type of work like that before. You yourself know your limitations.

 

Before you begin your job, I would like to recommend one other thing. I would recommend you visit your local Radio Shack or electronics shop and buy an IC socket of 24 pins or greater. This may cost you a dollar or two. Take a pair of pliars and chop this socket into segments of 3 leads each. When you remove your two filters, you will solder this IC socket segment into your receiver's circuit board, instead of the new filter. Then you will plug your new filter into the IC socket segment. If you decide later to play with filters of varying sharpness, you will only have to plug them into the socket segment. You will never have to solder anything again!

 

If you have some electronic experience, you might want to skip the next paragraph. I will go through step-by step instructions on how to desolder the filters. I am doing this so that anyone who feels he/she can attempt this modification can try it. As I said before...you yourself know your limitatons.

 


CERAMIC FILTER REMOVAL AND INSTALLATION

Before beginning, be certain your unit is UNPLUGGED!!!!

First, locate the two filters you want to change. Take off the bottom cover of your unit. Find where the two filters are soldered in and mark the place with a marking pen so you won't unsolder the wrong points. Next, plug in your soldering iron and wait until it gets HOT!  I stress that it MUST be HOT! When your soldering pencil is hot, place the desoldering braid against the the solder joint that needs to be unsoldered. Then place the soldering iron on top of the braid and leave it there until the solder on the joint melts and fills up the braid with old solder. Remove the iron and the braid. Cut off the section of the old braid with the old, melted solder on it and throw it away. Repeat this process until you have most of the solder off of the joints. Then, heat up the joint with your soldering iron and at the same time wiggle the filter with your finger until the lead begins to move. Remove your soldering iron but keep wiggling the filter. Repeat this process for all three filter leads. The end result should be a loose filter. Next, with either your fingers or a pair of pliars, gently pry the old filter out of the circuit board. Try not to break the filter. Now, when the filter is removed, check the area where you did your de-soldering and make sure the three holes are clear.

 

Now install your IC segnent. Push the segment through the circuit board and resolder all three leads. Use your magnifier to be certain you have not accidentally shorted out the leads with a blob of solder. Once you are certain that all is well, clean up the area with alcohol on a Q-tip. Double check your work.  Put the bottom cover back on your unit and turn the unit over and plug your new, narrow filters into the IC socket segments. Plug in your unit and listen for audio. If you have audio, you are FINISHED! Congratulations!  If you do not have audio, you have created a short and probably have solder bridged across two of your filter leads. Unplug the unit, turn it over and take off the bottom cover. Use your magnifier, look at your soldering and fix any accidental short. When you are satisfied that your work is correct, put everything back together, plug in your unit and tune to a local station and listen for audio. If you have audio, you are FINISHED! If not, repeat the same process.

 


Toko part numbers: 180khz SK107M3AO-20, 150mhz SK107M4-AO-10, 110,hz SK107M5-AO-10

Now let's discuss what type of filter you want to put in your tuner. Manufacturers generally put the widest filters in their equipment that they can get away with. Distortion seems to be an important spec to equipment manufacturers, so they choose the wider filters. If you have a problem with a very weak station adjacent to a strong local station, your goal is to hear the weaker station, even if it has a slight bit of distortion. A 180khz filter is a good choice for low distortion and good selectivity, provided you are not located in a metropolitan area with dozens of stations packed together on the FM dial.  If you are a DXer and you want those adjacent channels as clear as possible, you must go with the 150khz filters or the 110khz filters. I, myself, use two 150khz filters in the narrowband slots on my Carver TX-11 tuner.

 

Now, this is another judgment call on your part. Is this slight drop of the signal level important to you?

 
Let's talk about distortion. No matter what filter you use, your local stations will NOT have audible distortion. If I use 110khz filters in my Carver, I can notice some distortion on a few of the weakest stations I can receive. Now, since we all live in the real world, would you put up with a bit of distortion on a station that, before the filter mod, you COULD NOT HEAR AT ALL? Only you know the answer to that question.

 

Now let's talk about why I only recommend changing two filters, no matter how many your unit has. There are some who will disagree with me, and I understand their reasoning. If you look at the chart above, you will notice a column marked INSERTION LOSS, MAX. Notice that the insertion loss for the 150khz filter is more than the 180khz filter. And notice the loss for the 110khz filter is greater than the loss for the 150khz filter. If you have a signal meter on your unit, as you change more and more filters you will notice your signal level begin to drop. If you have 5 filters in your unit, and you begin to change them one at a time, by the time you finish changing them all you will see a drop in signal strength on your meter. You will have the same sensitivity, only your meter will not register as high as it was, and some marginal stations that were received in stereo before will not be heard in stereo anymore.

 

When I first starting doing this mod, I tried it on my Carver and changed ALL the filters to 150khz filters. My signal meter, which consists of six stupid LEDs, immediately went beserk. At all times I had a minimum of five LEDs lit, and a maximum of six LEDs lit. I solved that problem by only using narrow filters only in the narrowband slots. I still have the selectivity I need and a normal display on my signal meter.

 

Let's talk about the differences between the Toko and Murata filters. First, the Toko filters are a tad smaller. But there is a difference in the way the filters react in your equipment. The Toko filters tend to "bleed over" on the higher adjacent channel a little. A tuner set to 103.5 will tend to have better selectivity on 103.3 than on 103.7.  This is not a problem for anyone with an analog receiver. I use them in my Sony and I have no problem with them. This effect is more noticeable in a digital tuner. However, the Murata filters do not have this "bleed over" characteristic, which is why DXers tend to like them better.

 

Now you can understand why I told you to solder the IC socket segments instead of the actual filters. I would suggest playing with different types of filters...180s, 150s and 110khz. If you have only two filters in your receiver, you can connect two filters in series, and and plug the two into one of your IC socket segments, which will give you three filters instead of two, and greater selectivity. However, be aware that you will have to put up with some additional signal loss.

__________________________________________________________________________
 
Toko filters are available from Digi-Key Corp in Thief River Falls, MN USA
Part number for the Murata 150khz filter:  SFE10.7MJA10-A
Part number for the Murata 110khz filter:  SFE10.7MHY-A
Murata filters are available from sources on my main page. (1-13-09)
If you can find SNR filters, do not hesitate to use them.
 
This article may be reprinted only if you email me first and ask  for permission.
 

©1999-2012 Mike Bugaj

Rev 01-13-09