HIGH PERFORMANCE S-METER FOR THE
and possibly other scanners with the 108-136 MHz Aero Band
Revised November 3, 1997
This article/file is for the public benefit.
This article/file is for personal use only and may NOT be placed on a CD-ROM nor any other media that conveys, stores, or transports for any monetary cost, without expressed permission of the author. This article/file can only be given away, absolutely free of charge.
PRELIMINARY NOTE: This article is self-contained and you really don't need any other supporting materials other than the service manual for your scanner. Bear in mind, however, that only so much detail can be in an ASCII text file like this. If you want more detail than offered here, a hard-copy laser printed version of this article, complete with professionally rendered graphics is available for US$5.00, ppd. See my signature box at the end of this article for addresses, phone
What is an "S-Meter"? Well, it stands for "signal meter" and is used to provide an "RSL" (received signal level) idication of the strength of the incoming signal. S-Meters are all calibrated pretty much in the same fashion with two scales. The first 2/3 of the metering range is calibrated in "S-units", a fairly linear scale of 0-9. The units are read as S1, S5, S9, etc. S9 is the calibration point, generally meaning a signal at that strength is 100-microvolts, but mnufacturers sometimes use 25-uV or 50-uV as the reference for S9 signals.
The relative signal difference between S-units is typically 6 dB per S-unit, between S0 and S9, but in practice this can vary widely, from as little as 3 dB per S-unit to 7 or 8 dB per S-unit. Unless you know different, it's okay to figure 6 dB of RF change between S-units 0-9.
The last 1/3 of the scale is calibrated in decibels (dB) over S9 and is, of course, a
logarithmic scale, typically in three 10dB sections though some meters go to 60 dB over
Up to this point, you may think that S-Meters are useful only for measuring the relative strength of incoming signals, but think about it for a minute. What determines the strength of signals going into your receiver? More specifically, what factors are under YOUR control that determine received signal levels? I can think of at least four things right off: antenna type; antenna height; coaxial cable quality and length; and the correctness or precision of the receiver's electronic alignment (internal adjustments).
Therefore, there are a number of things you can do around your monitoring post that will affect the strength of received signals. Wouldn't it be nice to know if something you did had a beneficial or harmful effect? Sure! That's one powerful use for an S-Meter.
As an example, say you want to change out your coax cable and want to be sure the new stuff is better than the old. So first tune to a known consistent signal such as a NOAA weather station or perhaps an 800 MHz control data signal. Write down the S-meter reading. Then change out the coax (or elevate the antenna, etc). Then write down the new
S-meter reading and compare the two. You'll know if you did right or wrong.
Many scanners are capable of supporting an easy to build and install S-Meter! Most Realistic scanners made by GRE-Japan with the civilian aero band, 108-136 MHz, can support this low cost, high performance S-Meter modification.
It takes special circuitry for a scanner to receive Amplitude Modulated (AM) signals because AM is an uncommon mode in the VHF/UHF spectrum. In fact, AM is used pretty much exclusively for civilian aircraft comms on 108-136 MHz and military aero comms on 225-400 MHz. Most scanner signals are Narrow FM (NFM). Wide FM (WFM) is used in
the FM Broadcast band, 88-108 MHz, TV audio signals; and studio-transmitter-links. Everything else is pretty much NFM.
Therefore, most scanners are designed strictly with NFM in mind, and will not have either AM or WFM capability except as a costlier extra feature. That's why you pay more for scanners that have the civilian aero band, and more still for those with the military aero band.
If your Realistic scanner has the AM mode, and it will, if it has the civilian aero
band, then chances are good that we can take advantage of its extra circuitry to add the
simple, low cost S-metering function.
Scanners known or strongly thought to be capable of this easy S-Meter mod include the following listed in Table 1:
TABLE 1: COMPATIBLE SCANNERS FOR THIS S-METER MOD
CAVEAT: Many of Radio Shack's later scanners are made by Uniden, the design of which leaves so much to be desired that this version of the S-Meter is probably not feasible. There may be other alternatives but are beyond the scope of this document.
If your scanner is not listed in Table 1, the best way to determine applicability of this procedure to your scanner is to review the main schematic diagram for your receiver section, and compare it to the circuit of IC-2, Q-12, Q-13, and D-33 of the PRO-2006. This circuit is used in all the above compatible scanners, and is required for this version of my S-Meter mod. Specifically, you will look for the NFM chip and AM detector diode shown in Table 2 further below.
The AM section of the PRO-2006 is almost identical to the AM circuits for the scanners listed in Table 1. Each of these scanners takes itsAM signal from the output of the 455 kHz IF filter that's attached to the NFM Discriminator chip. Part of the IF signal goes back into the NFM chip for NFM signals as needed. The other leg of this "Y" is that part of the signal that goes on to a separate AM processor. The key thing here is that both circuits are used full time. Special switch
circuits do the controlling of which mode is actually in use, but the circuits are still on full-time. That's why we are able to use the AM circuit to power our S-Meter
The key point here is that you must identify and use the cathode of the AM Detector Diode as the pickoff point for your S-Meter signal. If your scanner is capable of this mod, it will have an AM circuit almost identical to that of the PRO-2006. Find it, and then locate the AM detector diode. The diode's banded end is the cathode where the S-Meter signal will be tapped.
For base scanners, build the simple circuit in Fig-1 on a bit of perfboard. For handheld scanners where space is at a premium, it may be best to not use a board, and just build the circuit of Fig-1 directly into the scanner, part by part. Start by soldering C1 to the cathode of the AM detector diode, and the anode of D1 to a handy ground. Then build the rest of the circuit around those two components. Following is Fig-1, the schematic of the S-Meter circuit.
Build the following simple circuit left of Points A and B on a piece of perfboard. The circuit to the right of A and B is optional, explained further below.
FIGURE 1: SCHEMATIC DIAGRAMS
|<-------Build on perfboard------->| |<--Project Box-->| in scanner o-------| Connect | | to Y .01-uF 1N34A A C v | Cathode>----||-----o--->|------o-----o----< (+) <--o-/\/\/\----o <+> detector | | | 5-k | diode --- --- \ Trimpot ( CB ) in 1N34A ^ --- / (Jack) ( type ) scanner | .01-uF | 68k\ (Plug) (S-Meter) | | | | Ground >-----------o-----------o-----o----< (-) <--------------o <-> Z B D
|<--------keep leads short-------->| |<--Any length ok-->|
THE PARTS: Radio Shack stocks the parts shown in Fig-1 except for the S-Meter. Diodes must be germanium, 1N34A or 1N60. Capacitors should be ceramic disks. Resistor (56k-100k) and trim-pot (3k-10k) are not critical. The circuit of Fig-1 must be as shown. If you use an external CB-type S-meter, so much the better....just build the meter of choice into an attractive or functional project box with a cable of any length from the box to Points C and D which should be wired to a male phono plug of some sort.
Most any analog CB-type S-Meter is ideal for this project, but the larger the face, the more readable it will be. There are thousands of junked CB rigs all over the planet, and most any CB or radio shop will have a bin full of them. The cost of a salvaged S-Meter should be negligible.
The scanner should have a mating female jack connected to Points A & B, installed in an out-of-the-way place for the plug from the external meter.
Keep the Point Y lead length very short, not more than an inch.
Connect Point Z to scanner PCB or chassis ground and keep that lead short, not more than 1 inch.
Connect Point Y of the S-Meter circuit via a very short wire to the cathode of:
TABLE 2: CONNECTION POINTS IN VARIOUS SCANNERS
AM -near- SCANNER DIODE NFM CHIP ========= ====== =========== PRO-2042: D-34 TK-10420 PRO-2035: D-34 TK-10420 PRO-2032: D-23 TK-10420 PRO-2006: D-33 TK-10420 PRO-2005: D-33 TK-10420 PRO-2004: D-31 TK-10420 PRO-2003: D-122 MC-3357P PRO-2002: D-119 MC-3357P PRO-2027: D-24 MC-3361N PRO-2024: D-24 MC-3361N PRO-2022: D-29 MC-3361N PRO-2021: D-24 TK-10420 PRO-2020: D-120 MC-3357P PRO-62: D-302 MC-3361N PRO-44: D-202 MC-3361N PRO-43: D-302 TK-10420 or 10427 PRO-39: D-202 MC-3361N PRO-37: D-122 TK-10420 PRO-36: D-114 TK-10420 PRO-34: D-117 TK-10420 PRO-32: D-122 TK-10420
Other Scanners: This mod may work with other scanners that have the civilian aero band of 108-136 MHz. If your scanner has 108-136 MHz and is not listed above, then find the last 455 kHz IF section, and locate the AM detector. Connect Point Y to the cathode or IF-side of the AM detector diode.
NOTE: While this mod probably won't work with Uniden scanners nor Radio Shack scanners made by Uniden, a little R&D and experimenting on a Uniden aero-band scanner could prove successful. I don't own any Uniden scanners, but can assist you in your R&D if and only if you and I both have a copy of the service manual for your scanner. Contact me
by e-mail if interested.
WRAP-UP DETAILS: Points A and B are the metering points where any DC voltmeter can be used. S-Meter signal range will be about 0-1v. The circuit of C and D is an optional CB type S-Meter, and should be used external to the scanner. Therefore, it is ideal to wire A and B (length not critical) to a phono jack on the rear panel of the scanner for easy connection of the external meter. Use 1/8" or RCA phone jacks for base scanners and 3/32" submini phone jacks for handhelds.
METER CALIBRATION: Any DC voltmeter can be used directly at A and B including a bargraph LED display. If an external S-meter circuit is desired, then wire it as shown. Use a walkie talkie or other transmitter to send a strong signal into the scanner, and adjust the trimpot so that the meter reads exactly full scale. All other signals will then read proportionally lower. Too cool for words!
METER LIGHTING: You can embellish your S-Meter with a small pilot lamp behind it to illuminate the face. Many CB meters have such a lamp built into the body of the meter. You may have to tap 12-volts or other power from the scanner for the lamp, or provide external power, like for handhelds.
SUPPORT: I do not supply S-meters. I can provide other parts including printed circuit boards, assembled or as a kit. I will provide technical support of this project on the public network forums and/or by e-mail; never by phone. See the beginning of this article if you need a hard copy version of this article.
MOD BOARD INSTALLATION HINTS: Most modification circuit boards really need no special effort in the mechanical installation. I used to use fancy standoffs, bushings, and machine screws, nuts, and bolts, but no more, considering all my mods!
Now I loop a stiff #18-ga copper wire (RS# 278-1217) through a pair of ground holes in my perfboard or printed circuit; solder it securely to the board; and then solder it to a ground on the main board.
I solder the #18-ga wire to a chassis sidewall, or in some cases, to a ground foil on the main receiver board. This mount is rigid, safe, and easy, not only to install, but also to remove if needed. A "glop" of hot-glue on the bottom of the mod board prevents accidental shorts below it.
Enjoy S-metering at its finest!
(c) 1997 Bill Cheek
| Bill Cheek ~ COMMtronics Engineering ~ World Scanner Report |
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