| example MuRata filter (below): | 3 MuRata's, a Toko, and an SNR Filter: (view close-up) | |
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BACKGROUND
Ceramic IF filters are perhaps the easiest,
cheapest, and best radio modication you can perform! Most tuners
and receivers are sold with "wide" filters by default. Manufacturers
chose to do this because the wide filters offer the best fidelity without
having to worry about quality control issues.
Without getting into the depths of the technical reasons, receivers and tuners require at least enough bandwidth to allow for all of the signal to get through. If you start cutting off the sidebands of a signal, you lose information, and could have distortion, loss of high audio frequencies, loss of SCA audio, and loss of RDS data.
Ceramic IF filters are centered at 10.7 MHz for FM radios (because the IF - Intermediate Frequency - is 10.7 MHz). These filters are measured based on their 3dB bandwidth, and is best explained by example. A 280 KHz filter means that you will have 3 dB loss at 280 KHz from the center frequency. If you go beyond 280 KHz, the loss is much more than 3 dB. Essentially, it means within 280 KHz of the tuned signal, you will have minimal loss. Beyond 280 MHz, loss increases quickly and substantially. View MuRata's documents for more information:
Murata
Filter Guide - pre-2001 version
Murata
Filter Guide - current version
Most low end tuners and receivers, and high-end tuner's first IF stage, consist of 280 KHz, 250 KHz, or 230 KHz filters. Only the high end tuners use narrower filters by default, and usually only in the last stage or two of the IF section (usually the "narrow" setting on the tuner). These high end tuners may come with a few 180 KHz or 150 KHz filters installed.
The difficulty in using narrower filters (150 KHz and 110 KHz) is that you can encounter quality control problems. While filters are supposed to be centered for a 10.7 MHz IF, often you will find some that are slightly off frequency. Like all components, ceramic IF filters are manufactured to meet certain tolerance constraints, so you will see varying results. If you use 2 filters skewed to one side, or even different sides of 10.7 MHz, you will not get an optimal bandwidth filter curve, and might lose some fidelity, especially on low level signals.
MODIFYING YOUR TUNER
A simple replacement of your wide filters
with narrower ones can often radically improve your selectivity - the ability
to reject the signal on an adjacent frequency to allow you to hear signals
on the current frequency. DX'ers primarily peform these modifications,
as their interests are to hear distant signals - signals which are often
adjacent to locals. Some audiophiles like to perform a limited modification
to help them receive favorite fringe area stations.
Before you consider modifying your tuner, you have to consider the risks:
I recently ordered 8 110 KHz SNR filters from a DX'er to test out. In a head-to-head comparison with some MuRata 150 KHz filters, the SNR's all seemed close to comparable, but a few were a little off frequency on the low side. There was a little more loss in SNR's, but that is to be expected since they are narrower filters. I suspect that MuRata 110's would have at least as much insertion loss as the SNR's, but I didn't have MuRata 110's at the time of the test.
If you can find a supplier of small orders
of filters, expect to pay about $1 per filter. When ordering in bulk,
you can get much better prices (less than
$0.50).
When you remove your old filters, rather than installing the new filter onto the circuit board, install a gold plated three prong socket. [Mouser sells sockets that are suitable if cut into pieces]. This will allow you to experiment with a variety of filter combinations without having to resolder a bunch of times. Gold plated sockets have negligible loss, too.
You usually want to install your filters
in a cascaded fashion...larger sized on the first stages, and the 150's
or 110's on the last stage. Remember,
narrower filters have more insertion loss,
and too many can reduce your audio highs, RDS, etc. Unless you have a lot
of time to experiment, only plan on replacing a few of your filters with
super narrow 150's or 110's.
A primary consideration for which filters you replace with which sizes should be based upon your listening requirements. Audiophiles might just replace all of the filters with newer 230 KHz filters that are properly centered at 10.7 MHz. DX'ers might replace all of the filters cascading from 180 KHz down to 110 KHz. A happy medium would be to replace the wide-mode filters with new 230's, and the narrow mode filters with cascading 150's and 110's.
Other Considerations
Narrow filters, while having steeper slopes
and better adjacent channel selectivity, often have worse alternate-channel
selectivity. This is another fact to support using a wider filter
or two. Experiment to find the best combination
Case Study
I recently purchased a Pioneer TX-9800.
This was a top of the line Pioneer tuner from the late 1970's. I
opened up the cover and saw that it had 5 filters (see
photo).
These filters were primarily wide filters (I'm not 100% sure the exact sizes, but they were at least 230 KHz). I had a stock of MuRata 150 KHz filters and a few newer 230 KHz filters to replace the old stock filters with.
I decided to replace the first two filters with the new 230's. I immediately noticed a GAIN of a couple of dB of sensitivity. These older filters apparently had worse insertion loss. Additionally, the Pioneer used GDT filters (aka Flat Group Delay - filters that have better audio characteristics, but more insertion loss). [Note: only audiophiles need to use GDT filters, in my opinion. Most DX'ers won't notice the fidelity differences of non-GDT filters].
I also replaced the final 3 filters with the new MuRata 150's. Not only did selectivity improve, but I gained a little more sensitivity. Even new narrow non-GDT filters have less loss than older wide GDT filters.
The result: Filter 1 and 2 were 230
KHz, and 3, 4, and 5 were 150 KHz. If selectivity was even more important
to me, I would consider replacing filter 5, and perhaps even filter 4,
with 110 KHz filters.
Reading MuRata Filters
If you're like me, after awhile you'll
end up with a bunch of these small filters in your electronics box.
How can you differentiate the bandwidth of each filter? MuRata has
followed a standard scheme over the years. You can identify if your
filter is a MuRata one by looking for a symbol that resembles this: Cm.
It's
a small "m" with a semi-circle around it - MuRata's symbol.
An FM ceramic filter will have E10.7 or SFE10.7 labled on it somewhere, with a letter
after it. This letter, combined with the color and markings, indicates the 3dB bandwidth. Some of
these letters that are known are listed here:
10.7HY = 110 KHz if tan w/ red dot upper right (non-GDT?)
E10.7J = 150 KHz (GDT unknown)
E10.7JA = tan w/ red dot in upper left = 150 KHz non-GDT
10.7JA = tan w/ red dot in upper left = 150 KHz non-GDT
E10.7S = either 180 or 230 depending
on color/markings.
SFE10.7MS2-A (230 KHz) has a red dot in the upper left
SFE10.7MS3-A (180 KHz) has a red dot in the upper right
10.7SA = 180 KHz if tan w/ red dot in upper right
230 KHz if tan w/ red dot in upper left
E10.7Z = either 180 or 150 depending
on color/
markings. Not sure which is which. GDT.
E10.7X = either 250 or 220 depending
on markings/
color. Not sure which is which. GDT.
These are Tan w/ red dot in upper right.
SFE10.7MX = see E10.7X [these
are blue w/ gray stripe]
SFE10.7MP = 250 KHz (Blue w/ red stripe)
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