Additions and Corrections to "The AN/PRC-68 Legacy" by Alan Tasker
© Brooke Clarke, N6GCE
Revised 14 Jan 2002

The following information comes mainly from analyzing the radios and reading the documentation about them.  This discussion is from a engineering viewpoint.


The PRC-68 is the first military radio to use battery saver mode?

14 Jan 2002 - It turns out that the 68HC05 is not fast enough to do the frequency synthesizer function, so the chip is a customized synthesizer made specifically for Magnavox.  The idea below about the chip being a microcontroller is not correct.

The chip that is shown in the schematics for both the PRC-68 and the TS-3354 is a 40 pin CMOS IC with a 10 Volt power supply.  It does not have a crystal but does have an input clock of either 10 or 12.5 MHz selected by a frequency switch.  In addition to the frequency control output it has outputs for the 150 Hz Tx PL tone and it generates the battery saver waveform.  This is a case of it walks like a duck, quacks like a duck.  Motorola will not confirm or deny that it is a microcontroller.  The early literature for microcontrollers promoted the idea that they could be used to replace a number of MSI ICs, that is to say that they could be treated as a programmable logic device and this application seems to meet that description, so it could be called a custom chip or it could be called a microcontroller programmed to do this task.
Three of the frequency programming switches are connected directly to the 40 pin IC and one just selects either the 10 or 12.5 MHz oscilator.  The 40 pin IC also generates the 150 Hz PL tone on transmit and maybe another first, battery saver mode.  Battery saver turns the receiver on and off at a fast rate and removes power from the audio amplifier when there is no signal being received.

Microprocessor vs. Microcontroller

The first CPU chips where microprocessors like the Intel 4004, 8008, 8080, 80286 or Motorola 6800, 6809, 68000.  These chips have a data bus and address bus plus control lines like Read/Write, interrupt, etc.

Later microcontroller were developed.  They do not have data and address buses but rather I/O pins that can be used to Input or Output data under control of a program that's stored inside the microcontroller.  The Motorola 68HC05 was one of the first microcontroller and I think a customized version is what's used in the PRC-68 series radios.  This was also the first chip to be used in the "brain box" in GM cars.

Microcontrollers can have such built-in features as counters, A/D converters, D/A converters, serial data UART, etc.


The problem of Squelch Capture was known when the PRC-68 was designed so although it transmits with a 150 Hz tone to be compatible with radios like the PRC-25 and PRC-77, on receive the squelch operates based on noise quieting not 150 Hz tone.  To check for Squelch Capture jamming all the radios in the PRC-68 series (and many others) have some provision to disable the squelch.  When this is done you should hear noise under normal conditions.  If when the squelch is disabled the radio remains quiet you may be the victim of Squelch Capture jamming.  To check this the antenna can be removed, and if this causes the radio to produce audio noise, then Squelch Capture jamming is present.

Squelch Capture

On those radios that must see 150 Hz to open the squelch (PRC-25, PRC-77, AN/VRC-xx) there is the possibility that  an enemy can transmit a carrier on the frequency of the radio but with no 150 tone.  This can lock up the receiver and not allow normal communications to get through.  This is why the newer radios no longer require 150 Hz to open the squelch, but send it for compatibility with the older radios.

To test for Squelch Capture, press the SQ DSBL button on the radio.  You should hear static.  If there is no static you are being jammed.

PRC-68 Frequency Assignment Determination

If you get a PRC-68 radio the channel assignments can be determined by removing the module cover and looking at the switch settings.  The early batteries for the PRC-68 had a table that related switch settings the frequency.


Independent Channel Simplex Frequencies by adding a EEPROM

I think that once the engineers at Magnavox saw that the microcontroller had much more program memory than they were using in the PRC-68, ideas for new features started coming out.  One of the problems with the PRC-68 was that once the switches were set all 10 sequential channel frequencies were determined.    By adding a EEPROM (Electrically Erasable Programmable Read Only Memory) chip to the PRC-68A a different simplex frequency could be remembered for each channel.  Note that a EEPROM will remember it's data with no power supplied so that the channel assignments will survive even though the radio battery is removed.  This chip is programmed/read using a serial data stream in a way similar to how the synthesizer chip is programmed.

Antenna Tuning for Each Channel

Because the PRC-68 had all it's channels within 1 MHz of the center channel frequency a single setting of the L-C antenna matching components worked for all 10 channels.  This required the use of the TS-3354/PRC-68, TS-3951/PRM-34, or some other Field Strength Meter to properly tune the antenna matching L-C network.

But now a way to match the antenna for each channel was needed as well as a set of R/T filters.  I think the first try was to eliminate the wasted space caused by using eight modules and instead go to two modules, but this did not provide enough space for the antenna matching network so the frame length was extended from the 2.5" inches used on the PRC-68 to 3.5" for the PRC-68A (and all the rest of the series).  Now as part of the channel programming there was a switch to tell the microcontroller the proper antenna matching network (4 bands) and another switch for the R/T filter (4 bands) in addition to the A, B and D frequency programming switches (the C frequency programming switch is the Channel switch on the radio panel).  Also there now is a LOAD - NORM switch that tells the microcontroller to remember a channel setting.

Low Battery Warning Tone

The other feature incorporated into the microcontroller was Low Battery Warning Tone.  This turned out to be not such a good thing.  It's kind of like when the clip of an M-1 Grand rifle pops out when the gun is empty, it's a convenient thing for the rifleman, but it also tells a nearby enemy that you have an empty gun.  In the case of the PRC-68A (and later radios) it would tell a nearby enemy that you were there.  The PRC-68A has an adjustment for the threshold voltage and for this radio it can be set to zero, defeating the tone for combat conditions.

RF/IF module & Synth/AF module

All of the RF/IF functionality is in one module and the Synthesizer, Audio Frequency and microcontroller is in another module.  This division will allow future versions of the radio to support a different RF/IF module covering 130 to 170 MHz operation.  Theoretically a 200 to 400 MHz module could have been developed (does anyone know if this was done?).

PRC-68A Frequency Assignment Determination

If you get a PRC-68A it is almost impossible to determine what frequencies are programmed.  In my opinion this is a very major shortcoming.  The radio can be easily programmed in the field without a Field Strength Meter or any other test equipment.  A soldier could set up a radio in a way that made sense to him, but anyone else who got the radio would not know the assignments.


VHF Low or High Band operation with RF/IF module change

The frequency coverage can be changed by simply swapping the RF/IF module.  When in Low band operation (30 - 88 MHz) the LCD reads the operating frequency and in High band operation (130 - 174 MHz) the "1" digit is not displayed.  Pressing SQ DSBL will cause the LCD to tell you LO or HI.

Wide Band or Narrow Band operation

When a channel is programmed the deviation can be set to either 3 or 8 kHz.  All the other radios in this family only have wide band operation on VHF low band and narrow band operation on VHF high band.  The capability of using narrow band operation on VHF low band is unique to this model and is a desirable feature for inter operation with ham band equipment.

2.5 kHz Channel Spacing

This is the only radio in the family that has 2.5 kHz channel spacing, all the others are on 50 kHz channel steps (PRC-68), 25 kHz steps (PRC-68A), or 12.5 kHz steps (PRC-128, PRC-136).

LCD for Channel Frequency and other messages

The PRC-68B made a number of major changes.  The addition of an LCD (Liquid Crystal Display) that shows the frequency of the current channel eliminates the major shortcoming of the PRC-68A.  It also has indications for Low or High band so that you can tell which RF/IF module is installed as well as aiding in the frequency cloning operation.

External Controls

To program the PRC-68A the module cover needed to be removed, exposing the inside of the radio to dirt, mud, sand or whatever else was there.  The PRC-68B programming switches (Increment, Set, 3/8) are located under an O-ring sealed screw cap rather than inside the module housing allowing much easier programming.  The separate Antenna matching and R/T filter select switches have been combined and located on the outside of the PRC-68B.  The number of bands is increased from 4 to 5 for VHF low band and a new "H" (VHF Hi band) position added that in reality is just a 50 Ohm position.  The change from 4 to 5 bands was probably done too improve the spurious frequency problem of the earlier radios that had a large number of frequencies that were either marginal or totally useless.  The microcontroller will sound an antenna warning tone of the ANT switch is not in the correct position.

Cloning & Retransmission

The AUDIO connector the PRC-68B (and all the later radios in the series) has 6 pins instead of the 5 pins on the PRC-68 and -68A.
The additional pin supports cloning and retransmission.  These are two more functions for the microcontroller.

Cloning refers to the ability of one radio (the master) to transfer it's channel frequency assignments to another radio (the salve).  This is done by connecting a special cloning cable between the two radios.

Retransmission is similar to a repeater, but it works in both directions, i.e. radio A and radio B (tuned to different frequencies) are both listening and the first one to hear a signal becomes the receiver and the other radio becomes the transmitter.  This allows two squads, each operating on a different simplex frequency to inter-communicate without any changes to their individual radios in a very transparent way.

Split Tx and Rx Frequencies

Another capability introduced with the PRC-68B is the ability to set completely independent transmit and receive frequencies anywhere in the range of the RF/IF module being used.  The ANT switch needs to be set for the band matching the Tx frequency.

PRC-126, PRC-128, PRC-136

Fully External Controls

While the PRC-68B required something like a quarter or half dollar coin to unscrew the cover to program the radio, the PRC-126, PRC-128 and PRC-136 have the programming controls located directly on the front panel.  These three models all use the exact same frame/panel assembly (Magnavox part number 816173-821.  The thing that separates these three models is the label and the modules that are installed.  The PRC-126 is a VHF low band only radio, the PRC-128 can use either a low or high band module and the PRC-136 is a high band only radio.

Simpler is Better

A lesson learned with the PRC-25 was that a simple radio is much more useful in battle conditions than a complicated radio was relearned with the PRC-68B.  Although it was very capable, it was too complicated so the PRC-126, -128 and -136 are simplified.

PRC-126 Mistake in LCD Backlight

The PRC-68 LCD has no backlight and could not be seen when it was dark without using a flashlight, not a good thing to do in combat conditions.  The PRC-126 introduced a back light that can be activated by pressing the SET button after the radio has been turned for more than 10 seconds (prior to that the programming function is entered).  At the time of this design the military had specification for Night Vision compatibility.  This was done because most of the equipment lighting was much too bright to work with Night Vision Goggles.  The PRC-126 was mistakenly designed with the components of the LCD in this order inside to outside:
backlight -> LCD -> very dark green filter.
This works well for NVG use but is impossible to use under normal daylight conditions.  Later models of the PRC-126 and the PRC-128 and PRC-136 use a different backlight configuration:
backlight -> very dark green filter -> LCD.
This way in daylight you can easily read the display and at night the backlight is NVG compatible.  When you press the SET button on a late model PRC-126, -128 or -136 you should not be able to see the back light with your naked eyes.  If you do see it, then you are looking at an early PRC-126 that has had the very dark green filter replaced with a clear filter.
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