A new push-pull switch/potentiometer for the Heathkit HM-102 VSWR/Wattmeter

Figure 1:
The Heathkit HM-102 HF VSWR/Power meter.  This meter -
often found at swap meets and hamfests for low cost - is still
as useful as it was when new - and is easily repaired today,
decades after its introduction.
Click on the image for a larger version.

A few weeks ago I got my hands on a(nother) Heathkit HM-102 VSWR HF Wattmeter/SWR bridge.  This device was available from the early 1970s and into the early 1980s and was a common sight in ham shacks in the U.S.¹

This unit consists of two parts:  The front panel meter/switch and the "bridge" section and it included a fairly long cable, giving the user the ability to use it as a single, stand-alone unit with the bridge mounted in the enclosure with provided screws or to have the meter and the bridge separated by several feet - often more convenient in a busy ham shack with lots of cables that cannot be conveniently routed to where the meter resides.

Repair needed!

The unit depicted in Figure 1 had belonged to a (silent key) friend of mine and due to an unfortunate set of circumstances, it was submerged for some time when the basement of his house flooded after his passing.  Amazingly, the meter movement survived unscathed - aside from a few small particles of dust that were easy to blow/brush out after carefully popping off the front  - but the rest of it needed a bit more attention.

The case itself and the knobs - once worked over with a bit of "Formula 409" cleaner and old toothbrush - were in excellent condition, as was the "SWR/2000/200" switch that selected between displaying the VSWR and forward power after it had been cleaned.  The circuit board comprising the bridge and the SO-239 coaxial connectors looked a bit "crusty" and would require a few parts, but a few minutes with an old toothbrush, some dish soap and an ultrasonic cleaner did wonders in making it look better.

What did not fare well was the combination front/panel SWR "Set" switch and potentiometer.

SWR measurement by the HM-102 and other single-meter bridges

Using a single-meter VSWR bridge is a multi-step process:

• Transmit at a relatively low power.  If the VSWR of the load is uncertain, using a low power setting very good idea as is prevents possible damage to the radio and it simplifies measurements by minimizing interaction with the radio's protection circuit. ²
• Set selector switch to "SWR" and put the potentiometer in "set" mode.  This bypasses the wattmeter's calibration, allowing the user to set the meter and is accomplished on the HM-102 by pulling on the "Set" switch.
  
• Set the meter to full scale.  This sets the meter so that the full scale is relative to the current forward power to allow a direct comparison of the reflected power and thus the VSWR.
• Take the meter out of "set" or "calibrate" mode.  With the forward power calibrated, the meter will now directly read SWR on the appropriate scale.  On the HM-102, this is done by pushing on the "Set" switch and the VSWR can be directly read from the meter's scale.
  

For the HM-102, the "set/calibrate" function uses a "push-pull" switch on a potentiometer, and as Figure 2 shows, this assembly did not survive.  This type of switch was ubiquitous in the 50s through the 70s, found on radios and televisions as well as many other consumer electronic devices, but it has all but disappeared in recent decades and I thought that finding a suitable replacement might be difficult.

Figure 2:
The remains of the original push-pull switch/potentiometer.
The aluminum shaft had seized/galled to the aluminum
bushing and efforts to free it resulted in its eventual
destruction - although it was likely already a lost cause.
Click on the image for a larger version.

While I figured that it was a long shot, I initially tried several things:  Some "PB Blaster" as well as some heating/cooling cycles, but it became apparent that even if I had been able to free the potentiometer shaft that had seized to the bushing, both the potentiometer and the switch itself were likely to have been destroyed by my attempts to free it, if not having been submerged in water.

For a few days I wasn't sure what I might do to find a replacement:  I rummaged around in my collection of potentiometers and while I found one or two "push/pull" units with an integrated switch, none of them were anywhere near the needed 200k-Ohm value.

The Internet to the rescue

On an impulse, I decided to check the parts suppliers like Mouser and DigiKey and was surprised to find out that these devices were still readily available.  On a whim, I also checked Amazon for a "push-pull potentiometer switch" and was greeted with dozens of options:  I'd managed to overlook the possibility that such switches would still be sold as replacement in other gear - namely certain brands of guitar amplifiers.

A bit of further searching showed that the two most common values of these potentiometers seemed to be 500k and 250k - the latter being "close enough" to the original 200k part that it shouldn't really matter ³.  While most of these devices have shafts for splined knobs - which would have sufficed even with the setscrew knob of the HM-102 - I did find on Amazon a Bourns unit (P/N:  PDB183-GTR02-254A2 - which I also found at Digi-Key) that had a "smooth" 1/4" shaft and ordered that .  The cost:  About US$13 or so, including shipping - cheaper that the other parts suppliers, and faster delivery, too.

Figure 3:
The new push/pull potentiometer/switch installed in the re-
furbished HM-102.  This is the Bourns P/N
PDB183-GTR02-254A2 - still readily available.
Just visible in the photo is a 1.3 MegOhm resistor across the
terminals to bring the maximum value closer to the 200k-
Ohms of the original device and this may help to make it
slightly less "touchy" during adjustment.
Click on the image for a larger version.

When it arrived a few days later it was a perfect fit with the shim/spacer of the original switch/potentiometer.  As this potentiometer was intended for stereo audio, it has two independent sections - and the switch itself was a DPDT - so only half of the device needed to be wired up.

Getting it working again

The only other casualties were on the bridge's PC board:  The 2-20 pF trimmer capacitor was seized and the 50k board-mounted trimmer potentiometer was too "flaky" to be trusted.  It so-happened that I had an exact replacement for the capacitor and found a very similar trimmer potentiometer in my collection that was 60k - close enough.  Spending a few minutes with a dummy load, I was then able to null the VSWR bridge and calibrate the meter as described in the manual.

A quick test showed that the power meter was within specifications and the VSWR bridge's balance was good from at least 80 through 10 meters when spot-checked.

Conclusion

At first, I thought that this vintage HM-102 might have been parted out and/or consigned to the trash can, but with only a bit of work and a few parts, it was resurrected and cleaned up.  Aside from a few scuffs revealing the fact that it had, in fact, been used for some time, it works well and it looks very good for a piece of electronics that is a half-century old and had been nearly drowned!

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Footnote:

1. The HM-102 - which is designed to work from 1.8 through 30 MHz - appears to have been introduced around 1972 when the kit cost $29.95 while in 1981, the kit cost $49.95 and was apparently discontinued after that.  There was also a VHF version for 6 and 2 meters - the HM-2102 - that looks very similar and in terms of mechanical design and much of the above discussion could be applied to it as well - and it even uses the same potentiometer/switch as the HM-102.
2. Modern radios have "fold-back" circuitry to automatically reduce power if they "see" a high VSWR.  It is best to set the transmitter to low power if the VSWR is unknown (say, 5-10 watts on a 100 watt radio) as even the best protection circuit isn't completely able to protect the radio under all conditions - particularly if one is using an external antenna tuner and forgetting to reduce the power level.  Running a low power output during tuning prevents the fold-back circuit from activating as much:  If the radio had been set to high power, the output power will rise rapidly as one attains a better match (e.g. when using a manual tuner) and the user will have to frequently re-set the SWR to full scale to obtain an accurate reading and possibly confusing the operator - but at low power, this complication is minimized.
   
3. The original potentiometer/switch was 200k, but the modern replacements are only readily available as 250k devices with a logarithmic taper.  This higher value and possibly the logarithmic taper may make this control - which is simply placed in series with the front-panel meter - a bit "touchier" than the original when it is adjusted at some power levels.  Placing a 1.2-1.5 MegOhm fixed resistor (the precise value is unimportant) across the potentiometer connections will reduce its maximum value to about 200k and make this adjustment slightly less "touchy".  If you have a meter like this but can find only the 500k version of the potentiometer, go ahead and use it anyway - just place a 330k fixed resistor across its two connections to limit its maximum value to about 200k:  It, too, may be a bit "touchy", but it will work.
    

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This page stolen from ka7oei.blogspot.com

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Neon bar-graph VSWR/Power meter using the ИН-13 (a.k.a "IN-13") "Nixie" - Part 3 (of 3)

Figure 1:
Power/VSWR meter using ИН-13 neon bar-graph indicators.
This was taken prior to installing the dark plastic to
improve contrast
Click on the image for a larger version

In Part 1 (link)  I talked a bit about the origination of the design - and how the high voltage for the Neon tubes were generated and how the tubes would be driven along with the "Tandem" power detector using the AD8307 logarithmic amplifiers. In part 2 (link) I showed how the tubes were connected and mounted, the laser-cut acrylic backplane and the associated LED-based edge lighting.

In this - the final installment - we'll see how it all goes together.

* * *

As I'm wont to do, I used a PIC microcontroller for this - chosen because I'm more familiar with it than something like an Arduino - and something more "powerful" (an ESP or similar) would be overkill and arguably more difficult to implement as we'll see.

In the PIC environment I have used - for decades - the PICC compiler by CCS (Custom Computer Services) having started out with a very early compiler of theirs.  Programming in K&R C allows me to get pretty close to the "Bare Metal" of the microcontroller where I tend to write in low-level code and extensively use the interrupts and state machines to get things done.

Before delving right into details about the code, let's first look at the remainder of the schematic - and rather than make you, the reader, go back and look at previous installments, I'll include them all below in their entirety, starting with the controller and power supply.

Figure 2: 
Schematic of the controller and LV and HV power supplies.
Click on the image for a larger version.

The controller that I chose for this is the PIC18F1330 - an device in an 18 pin package that sports a built-in clock to permit operation at 32 MHz with no external crystal, PWM generators and a multiplexed 10 bit A/D converter.

High voltage control

As can be seen, "PWM1" is connected directly to our high voltage transistor, Q301 which is used to do a voltage boost, with R301 - a pull-down resistor - used to turn it off when the processor is in an indeterminate state.  As mentioned before, I tend to use state machines and interrupts heavily in my microcontroller code and with a PWM frequency of 31.25 kHz to generate the high voltage, I also have the interrupts occurring at that same rate, driven by the same clock source as the PWM.

Within the ISR (Interrupt Service Routine) there is a state machine that reads the A/D inputs - namely the high voltage, the forward power and the reverse power - but there's a catch here:  There is only ONE actual A/D converter - and this poses a problem.  Generating a stable high voltage requires a closed loop feedback - and with a processor there's always going to be a bit of delay, but what's worse is that since we have only a single A/D converter we must constantly switch it between the three voltage sources that we must measure - but this has several steps:

• Set the A/D channel.  After doing this we must wait for a time for the A/D MUX switch to settle - but we can't afford to sit and "spin our wheels" in the ISR as we don't have the time:  While we are in our the ISR we really can't do much else.
• Start the conversion.  Once our MUX has settled we can safely start our conversion.  This takes much longer than setting the A/D channel - but much shorter than our ISR period.
• Get the result.  On the next ISR cycle the A/D converter will have finished and we can put the result in memory and set flags to indicate to the rest of our code that it's ready to be processed.
• Note:  At this point we can conserve time a bit and upon getting our result, we can set the A/D channel for the next conversion:  This means that we get a new A/D reading every other ISR cycle.
  

 To minimize "lag" in our closed loop voltage control, I do the following "gets" of analog data:

• Get the HV reading
• Get the Forward power reading
• Get the HV reading
• Get the Reverse power reading

By alternatively grabbing the high voltage reading every fourth ISR cycle we can use this information to "tweak" the PWM duty cycle:  If the voltage is too high, we reduce the duty cycle slightly and if too low, we increase it - and this adjustment, within the ISR, is triggered by a flag that is set every time we get an update of its voltage.

Before we leave the discussion of the high voltage generator I'll note that it's triggered by the detection of RF:

• Immediately on the detection of RF from the transmitter, the high voltage generator is activated.
• About 3 seconds after the last detection of RF from the transmitter, the high voltage generator is turned off. 

According to the specifications, these neon tubes have only a limited lifetime - as is the case with any gas discharge tube that is glowing - so it makes no sense to "wear them out" unless there's information (e.g. a reading of RF power) to be displayed.  Additionally, the high voltage generator in my unit produces a just audible bit of RF interference in the form of weak "birdies" spaced at the 31.25 kHz PWM interval - and shutting off the high voltage generator soon after transmitting has stopped prevents their being heard.

Backlight control

Let's now look at the diagram of the backlight drivers.

Figure 3:
Tube and LED drivers - along with buffering from the power detectors.
Click in the image for a large version.

As can be seen there are three LED backlight drivers:  One for the Forward power using white LEDs, one for the Reverse power power using blue LEDs and another for the VSWR using Green LEDs.  In order to adjust the brightness, these are also driven by a PWM signal that is smoothed and fed to a the same sort of "precision current sink" circuit using an op amp and transistor as the Neon tubes themselves.

While there are other PWM channels on the PIC18F1330, I chose to use a "software" PWM as I already had available a rather fast ISR (31.25 kHz) that would be able to provide a smooth enough control voltage:  As can be seen in Figure 3, a 150k resistor and 0.1uF capacitor (e.g. R501 and C501, respectively for the "FWD" channel) are used to smooth the PWM signal - the two components providing a time constant of about 15 milliseconds (67 Hz).  In the ISR the "software PWM" uses 128 steps and at the 31.25 kHz rate this yields a frequency of about 244 Hz - about 4 times that of the R/C filter making it pretty much flicker-free while allowing a fast response time.

The way the "software PWM" works is that in the ISR there's a counter that goes from 0-127, and the value of this counter is lower than or equal to our desired PWM (brightness) value, the corresponding PWM output is turned ON - otherwise if it OFF.

Dimming LEDs in a "believable" way 

Before moving on from the LED brightness control, it's worth mentioning something about the way the human eye perceives brightness.  While the actual LED brightness varies quite linearly in proportion to the PWM setting of 0-127 (off to fully "on"), if we wanted to slowly dim the LED from full brightness to off - and we simply decremented the value from 127 down to 0, our eyes would perceive it as dimming slowly at first - and then suddenly going out.  Perhaps it's my OCD kicking in, but I prefer a dimming LED to seem to fade out to nothing - and do so gradually without it perceptibly "snapping" off.

My intent is that when RF power is detected, the relevant backlight LEDs (always the "Forward" LED and the "Reverse" or "VSWR" as selected) are immediately turned on - but 30 seconds after RF is detected they are turned off.  As I found a "sudden turn-off" to be visually jarring, I decided to dim the LEDs slowly - but based on past experience with driving LEDs I knew that to make them visually dim "evenly" would require a bit of extra math.

The trick here - to set the dimming so that it seems to gradually fade out to nothing - is to use a fourth root and a bit of multiplication as follows:

• Start with the brightness value of 0-127
• "Invert" this value by subtracting it from 127 (now it's 127 to 0)
• Multiply by 128 (this can be done by shifting the bits left seven times if using an unsigned integer)
• Take the square root
• Multiply by 128 again
• Take the square root again
• Subtract that value from 127

 The result of the above is a more visually pleasing "dimming" of the LED - one that appears - to the eye - to fade out "evenly" to extinction.

Calculating the RF power and VSWR

One advantage of using the AD8307 logarithmic amplifiers is that it gives us a reading in db per volt to the tune of about 25 millivolts per dB - and we can calculate return loss very easily - simply by subtracting the reverse from the forward readings.

Figure 4:
The ИН-13 neon bar-graph Wattmeter/VSWR bridge showing
the instantaneous forward and reverse powers, with the
neutral-density filter installed.  (Very difficult to photograph!)
Click on the image for a larger version.

Return loss - while representing reflected power - is not how most hams think about reflected power - and VSWR is voltage - not power.  What we need to do is to translate our return loss to VSWR and the easiest way to do this is with a table - rather trivial to do in a microcontroller.  As we don't really need a lot of resolution on our VSWR meter (it's enough to represent "tenths" of a VSWR reading between 1.0 to 3.0 - and subsequently lower resolution than that at higher VSWR) the table need only consists of a couple dozen entries in the form of cascading "if-then" statements that spit out the VSWR directly.

We could do it with logarithms and floating point math, of course, but that's overkill!

As for calculating power in watts, there's no need for this:  If you look closely at the Forward Power scale in Figure 1 you'll note that it is already logarithmic:  The A/D values are simply offset and scaled to the PWM values needed to drive the tubes to the designated markings!

Peak and average values

As the power level of an SSB waveform is very "peaky", most analog meters only provide something roughly resembling an RMS value since they cannot move fast enough to capture the peak.  As we get new forward and reverse values every eight ISR cycles our update rate for the power readings is around 3.9 kHz.

As the bandwidth of a standard SSB signal is on the order of 2.5 kHz, we are sampling our power more frequently than its fastest rate-of-change and this means that not only are we likely to be able to capture a reasonably accurate representation, but also be reasonably assured that our forward and reverse power reading samples - which are about 128 microseconds apart - will also represent the same part of the the waveform:  Were this not true the VSWR readings could be "smeared" with the power changing between the instants that the forward and reverse power samples were taken.

As it happens, we really don't need the ultimate in temporal resolution for VSWR, so the calculation of "return loss" can be averaged a bit with no ill effects - and, in fact, the VSWR reading is quite stable at the widely disparate power levels intrinsic to SSB, anyway.

The code itself does have "peak" and "average" modes and the former uses a "sliding peak" detection - that is, it has a bit of a "hang time" on the output sent to the forward and reverse power displays to better-indicate the peak value and visually hold it.  The "average" power is more of a "sliding average" over the past hundred milliseconds or so and results in a "busier" display, with more movement.

Tube calibration

A quick look at the data sheets for the ИН-13 tubes will reveal that they are not well calibrated in terms of "milliamps-per-millimeter" with a fair bit of variation being allowed.  The ИН-13 tubes have lines painted on them at the factory indicating the "low" end (near the wire pinch) and the "high" end (near the tip) and these represent the useful and linear range over which the current can be represented.

The firmware thus includes - for each tube - a set of calibrations, accessible by the "mode" buttons in Figure 2 - that can be used to set the bottom and top of the scale.  This process will yield the needed PWM values - and thus the current - for the low and high end of the display and in so-doing, our microcontroller will "know" how to scale each tube to indicate with the best-possible accuracy.

Having been using this device for several years, now, I have observed that the tube sensitivity changes more with temperature than aging - but as it's not intended to be a "precision" indicator of power:  I only check the bottom/top scale calibration every year or two and have resisted the temptation of including temperature compensation.

Final comments

The only change that I made to this device after its construction was to add a sheet of 70% (dark) theater gel in front of the display to improve contrast.  While the neon tubes themselves and the laser-etched plastic look cool on their own, the display looks a bit "cluttered" unless a somewhat dark piece of plastic covers everything:  While this does dim the display a bit, the dark plastic - since it affects both the incoming and reflected light - offers the illusion that the display is brighter as the contrast is improved.

If you have any questions about this project (including underlying code and/or hardware) please let me know via a comment, below.

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This page stolen from ka7oei.blogspot.com

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Neon bar-graph VSWR/Power meter using the ИН-13 (a.k.a "IN-13") "Nixie" - Part 2 (of 3)

Figure 1:
Power/VSWR meter using ИН-13 neon bar-graph
indicators.
Click on the image for a larger version

In Part 1 (link) I laid out the requirements of the ИН-13-based neon bar-graph VSWR/power meter.  Admittedly, this is a "buy cool, old tech and figure out what project might use it" scenario - but having one tube always showing the forward power and the other tube showing either reverse power of calculated VSWR was the goal.

In the previous installment we talked about how to generate the high voltage (130 volts or so) for the bar-graph neons, the means to drive precise amounts of current through the tubes using precision current sink circuits, and the "Tandem" coupler to detect forward and reflected power.

 

Mounting the tubes

 

Figure 2:
ИН-13 tubes in the raw.
It is up to the constructor to determine how best to mount
these tubes - and how to connect them to the circuit.
Figure 3 shows how flexible wires were attached as the
wires on the tubes themselves are very easily broken!
Click on the image for a larger version.

In looking at Figure 1 you can see that the ИН-13 tubes are mounted to pieces of clear acrylic, but a quick look at Figure 2 shows that they don't really have a means of mounting, leaving the method to the imagination of the user.

In preparing the tubes for mounting I trimmed the wire leads and soldered flexible wires to them, covering them with "hot melt" (thermoset) adhesive to passivate the connection, making them relatively durable:  The original wires will NOT tolerate much flexing at all and are likely to break off right at the glass "pinch" - which would make the tube useless.   Figure 3 shows how the leads were encapsulated - the thermoset adhesive being tinted with a permanent marker - mainly to add a bit of color.

Laser-cut sheets and markings

Figure 3:
Close-up of the "hot-glue" covered wire
attachments for the ИН-13 tubes.  Also visible
are the black wire loops holding them in place
and the laser-edged markings on the acrylic.
Click on the image for a larger version.

In looking at Figure 1 and 3 you will also notice that there are scales indicating the function and showing scale graduations and the associated numerical values.  I'm fortunate to have a friend (also an amateur radio operator) who has a high-power laser cutter and it was easy to lay out the precise dimensions of the acrylic sheets and also have it cut the holes for the mounting screws in the corners as well.

While it takes a bit of laser power to cut the sheets, a far lower power setting will ablate the surface, yielding a result not unlike surface engraving and when lit from the edges, these ablations will light up with the rest of the sheet remaining pretty dark:  A total of four sheets were cut and "engraved" in this way:  The front sheet for "VSWR" and its markings, the middle sheet for "Reverse Power" and the rear acrylic sheet for "Forward Power".  It was possible to arrange the lettering so that only "VSWR" and "Reverse Power" were atop each other but in subdued light - and with a bit of darkened plastic in front of the display - the markings on the un-lit sheet are practically invisible.  The fourth sheet mentioned was left blank, being the protective cover. 

Edge lighting

Edge-lit displays go back decades - and the idea likely goes back centuries where it was observed that imperfections in glass (later, plastic) would be visible if the substrate was illuminated from the edge.  Since the early-mid 20th century, one could find a number of edge-lit indicators - usually in some sort of test equipment of industrial displays - but they occasionally showed up in the consumer market - usually acrylic or similar with the markings engraved with a rotary tool or - as may be done nowadays, a laser.

While incandescent lamps would have been used in the past, LEDs are the obvious choice these days and for this I selected some "high brightness" LEDs to light the edges of the engraved acrylic sheets.  For the "Forward Power" sheet - which would be that which was always illuminated in use - I chose white while using Green for VSWR and Blue for Reverse Power.  I'd considered Yellow and Red, but discarded the former as it might appear too much light the white under some conditions and past experience has reminded me that - particularly in a dark room - the human eye can't see or focus on fine detail on red objects very easily.

Figure 4:
Six LEDs are epoxied to the edge to evenly light the laser-
etched markings in the acrylic sheet.  The faces of the LEDs
were filed flat to facilitate bonding and improve efficiency.
Click on the image for a larger version.

Figure 4 shows some details as to how the edge lighting is accomplished.  Six equally-spaced LEDs were epoxied to the bottom edge of the display, arranged to be nearly the width of the engraved text.  In writing this entry I observed that photographing edge-lit displays such as this is nearly impossible owing to the variations in illumination (e.g. it's difficult to take pictures of very bright objects in the dark!) but the effect is very even as viewed by the human eye.

The six LEDs were connected as two series strings of three LEDs:  As each LED requires about three volts - and I have only a 12 volt power source - doing so requires only a bit more than nine volts to power the LED arrays.  As the green and white LEDs are also silicon nitride based as well, they take similar voltages.

Not readily apparent from Figure 4 is the fact that the LEDs were modified slightly.  As we are trying to interface a standard T1-3/4 LED to the flat edge of a plastic sheet, it's apparent that the rounded, focused lens makes this physically difficult.  To mitigate this, the top of the LED was flattened with a file and the clear epoxy was removed to just above the light emitting die.  The result of this is that a flat surface is mated to another flat surface for a physically stronger bond and a more efficient coupling of light and a bit of the LED's original directivity in the form of the "lens" is removed from the equation. 

Just prior to mounting the acrylic sheets in the "stack up" some black electrical tape was applied.  This tape was put on both sides of the sheet, extending just above the bottom edge, to reduce the glare from the LEDs and to minimize the possibility of this light coupling into the adjacent sheet.

Mounting the tubes and sheets

As can be seen from Figure 3, the tubes are held in place with loop of solid-core insulated wire - the holes mounting them also "drilled" with the laser.  The "stack-up" of acrylic sheets and the tubes - both of which were mounted on "VSWR" acrylic layer - is held together using 6-32 brass machine screws and spacers with a piece of 1/4" (5.2mm) plywood covered with black felt for the back to provide contrast.

The box and base

As can be seen from figure 1, the entire unit is in a wooden base:  The same friend with the laser cutter also had some scraps of red oak and a simple base was made, decorated with an ogee cut around the perimeter with the router while atop it a simple box with mitered corners - facing at a slight upward angle - in which the display and electronics reside.  On the base itself are two buttons:  One switches between VSWR and Reverse Power and the other between peak and average readings.  These switches have other functions as well, which will be discussed in the third installment when the final circuit and internal workings of the software is discussed.

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This page stolen from ka7oei.blogspot.com

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