Radio Shack PRO-2042 455 kHz I.F. Filter Modifications

Overview

The "selectivity" parameter of a communications receiver or radio scanner is its ability to reject any adjacent channel interference.  This parameter is mainly determined by the bandwidth of the last Intermediate Frequency (IF) filter in the receiver's RF demodulating chain.  If you have ever tried to tune a scanner to a weak signal, only to have it squashed by nearby higher-powered transmission, then you understand the need for improved selectivity in a receiver.

In Radio Shack scanners, and most other conventional narrowband FM receivers, the last IF is 455 kHz.  The service manual for the Radio Shack PRO-2006 scanner, which is arguably the "best" of the GRE-based Radio Shack scanners, lists the narrowband FM selectivity at -6 dB rejection +/- 9 kHz from the IF's center.  These stock filter specifications are good, but we can easily improve them slightly by replacing the last 455 kHz IF filter with something a little "tighter" to improve the attenuation skirt.

In the Radio Shack PRO-2042 scanner, which we'll be using for this example, the main narrowband FM IF demodulation IC is based around the Toko TK10420, which is a slight improved version of the Motorola MC3357.  The datasheet for the MC3357 goes into much more detail on the technical operations of the IF mixing, amplifying, filtering, and demodulation.  You'll want to study the MC3357's datasheet carefully, but we're only interested in improving the final IF filtering aspect right now.

The PRO-2042's 455 kHz IF filter is most likely a Murata CFUM455D model.  I say "likely" because I don't have the actual part number for that particular scanner, and Murata doesn't fully label their line of miniature IF filters.  It doesn't really matter though, as the only thing we are interested in is the "D" in the part number.  This corresponds to a filter with a -6 dB bandwidth of 10 kHz, which closely matches that of the specification listed in the PRO-2006 service manual, which is very similar to the PRO-2042, circuit-wise.

The filter modification is quite simple, just replace the 10 kHz wide bandwidth "D" model with a slightly narrower "E" or "F" model, which are 7.5 kHz and 6 kHz wide, respectively.  This will help to narrow the final the IF bandwidth slightly, reducing any adjacent channel interference, but can cause overmodulated or wideband FM signals to become "clipped" or distorted.  This may or may not be a drawback, you'll have to experiment for yourself.  You may even wish to toggle the two filters in-and-out of the IF chain using a mechanical relay or PIN diodes.

Sourcing new 455 kHz IF filters is getting to be quite difficult nowadays.  Thankfully, thrift stores a full of useful RF parts in the form of old 49 MHz cordless phones and baby monitors.  Buy everyone you see, take them all apart, and study the receiver's IF chain for a little rectangle or square filter next to a 16-pin DIP with a label similar to "55E" or "55F."  You'll want to study the filter's pin-out carefully, as different models will have different pin-outs.  All the filter's will have the same overall concepts.  They have an input, output, and ground, so tweaking a filter with the wrong pin-out to work is always possible.  You may also have to do some impedance matching in extreme cases, but all the different manufactures seem to use the same gerneral filter impedance of around 1,500 to 3,000 ohms.



Another useful and little-known trick to help improve your receiver's performance is to increase the value of the damping resistor used in the IF receiver's chip external quadrature tank circuit.  This circuit is used to provide a 90° phase shift to the 455 kHz IF to recover the final audio via mixing.  As this resistor's value is lowered, separation and bandwidth are increased but the recovered audio is also decreased.  The stock value is usually around 33,000 ohms and we'll be increasing it to around 47,000 ohms.

Pictures & Construction

Example IF filters in a commercial VHF two-way radio.

The radio's receiver chain is also based around the Toko 10420 and uses two Murata CFU455E 455 kHz IF filters in series to help improve receiver selectivity.

Example IF filter in a 49 MHz narrowband FM baby monitor.

This receiver chain is based around a Motorola MC3359.

The circuit board label above the IF filter says "CFW455E," which is a Murata part number, but the filter is a Kyocerra model with similar specifications and a slightly different pin-out.

Example IF filters in a Radio Shack PRO-2051 "Trunk Tracker" scanner.

Because the PRO-2051 has to continuously monitor the data channel in a trunked radio system, this scanner actually has two receivers in it.  The IF filter on the left is the standard "D" model, and the filter on the right is the "H" model.  The "H" model has a very narrow bandwidth, 3 kHz or so.  This is useful in low-speed data applications, but not very useful in receiving wider bandwidth FM audio transmissions.

Also note that this scanner uses a 450 kHz last IF frequency instead of the common 455 kHz.

Murata CFM455E high-performance mechanical 455 kHz IF filter.

If you want to improve your scanner's selectivity even more, try to track down IF filters like the one shown above.  These basically have the same bandwidth as the Murata "E" model filters, but the filter's "skirt," or attenuation factor, is much greater away from the center operating frequency.  These mechanical filters have at least 60 dB of attenuation only +/- 16 kHz from their center, compared to only 40 dB of attenuation +/- 20 kHz from the center of the stock "D" model.

Internal view of a PRO-2042 scanner showing the last IF strip.

The scanner's second IF is 48.5 MHz and is sent to the TK10420 IF chip to be mixed with a 48.045 crystal-based local oscillator, which produces the final 455 kHz IF.  The crystal on the left marked "XF1" is the 48.5 second IF resolution crystal filter, and the crystal next to the TK10420 is the 48.045 MHz LO crystal.

The large black rectangle marked "55D" is the narrowband FM IF filter, the yellow-orange one is a narrow bandwidth model for further filtering during AM demodulation.

Bottom view of the PRO-2042 main circuit board showing the pin locations of the TK10420.

The red arrow marks the TK10420's pin 1, while the bottom-left red dot marks pin 8.

The upper-left red dot is then pin 9 and the upper-right red dot is pin 16.

The blue arrow points out the stock 33,000 ohm damping resistor in the quadrature tank circuit.

Note the narrowband FM IF filter has been desoldered and removed.  This particular IF filter had only three pins, input, output, and ground.  Some IF systems, such as most Radio Shack scanners, use a "floating" ground in their final IF system, so the IF filter's "ground" pin may not be at actual ground potential.

Installation of a new Murata mechanical 455 kHz IF filter.

The filter is resting on a piece of double-sided tape to isolate it from ground.

The three ground pins on the filter are all connected together and soldered to the IF's floating ground.

The input and output pins on the IF filter are reciprocal, so you can mount the filter however is most convenient.

Changing the quadrature coil's damping resistor from 33,000 ohms to 47,000 ohms.

This helps to increase the "Q" of the tank circuit used to provide the 90° phase shift in the FM demodulator circuit.  This then increases the output audio level from the TK10420 slightly, improving the scanner's overall audio response.




















Old Murata IF Filter Catalog Pages


Radio Shack PRO-2006 NBFM IF Schematic