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.

* * * * *

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.

* * * * *

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 1 (of 3)

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

(Be sure to read Part 2 of this article - LINK).

 

Several years ago I bought some Soviet-era neon bar-graph displays - mainly because I thought that they looked cool, but I didn't have any ideas for a specific project.  

 

After mulling over possible uses for these things for a year or so - trying to think of something other than the usual audio VU meter or thermometer - I decided to construct a visual watt/VSWR indicator for amateur radio HF use.

 

* * *

 

I actually bought two different types of these bar-graph tubes:

• The ИН-9 (a.k.a. "IN-9").  This tube is 5.5" (140mm) long and 0.39" (10mm) diameter.  It has two leads and the segments light up sequentially - starting from the end with the wires - as the current increases.

• The ИН-13 (a.k.a "IN-13").  This neon bar-graph tube is about 6.3" (160mm) long and 0.39" (10mm) diameter.  Like the ИН-9 its segments light up sequentially with increasing current but it has a third lead - the "auxiliary cathode" - that is tied to the negative supply lead via a 220k resistor that provides a "sustain" current to make it work more reliably at lower currents.

Note:  It would be improper to refer to these as "Nixies" as that term refers to a specific type of numeric display - which these are not.  Despite this, the term is often applied - likely for "marketing" purposes to get more hits on search engines.

Figure 2:
A pair of ИН-13 neon indicator tubes.  These tubes are
slightly longer than than the ИН-9 tubes and have three leads
Click on the image for a larger version.

For a device that is intended to indicate specific measurements, it's important that it is consistent, and for these neon indicators, that means that we want the bar graph to "deflect" the same amount anytime the same amount of current is applied to it.  In perusing the specifications of both the  ИН-9 and  ИН-13 it appeared that the  ИН-13 would be more suitable for our purposes.

This project would require two tubes:

• Forward power indicator.  This would always indicate the forward RF power as that was that's something that is useful to know at any time during transmitting.
  

• Reverse power/VSWR.  This second tube would switchable between reverse power, using the same scale as the forward power display, and VSWR - a measurement of the ratio between forward and reverse power and a useful indicator of the state of the match to the antenna/feedline.

Driving the tubes

  

"Because physics", gas discharge tubes require quite a bit of voltage to "strike" (e.g. light up) and these particular tubes need for their operation about 140 volts - a "modestly high" voltage at low current - only a few milliamps (less than 5) per tube, peak.

Figure 3:
Test circuit to determine the suitability of various inductors and transistors
and to determine reasonable drive frequencies.  Diode "D" is a high-speed,
high-voltage diode, "R" can be two 10k 1 watt resistors in parallel and
"Q" is a power FET with suitably high voltage ratings (>=200 Volts)
and a gate turn-on threshold in the 2-3 volt range so that it is suitable
to be driven by 5 volt logic.  V+ is from a DC power supply that is
variable from at least 5 volts to 10 volts.  The square wave drive, from a
function generator, was set to output a 0-5 volt waveform to
make certain that the chosen FET could be properly driven by a 5 volt
logic-level signal from the PIC as evidenced by it not getting perceptibly
warm during operation.

Generating high voltage from a low is one of the aspects that I tackled in a previous project on this blog when I built a high voltage power supply for the Zenith Transoceanic:  You can read about that here - A microcontroller-based A/B Battery replacement for the Zenith TransOceanic H-500 radio, with filament regulation - link.

 
The method used for this project and the aforementioned Zenith radio is  boost-type converter as depicted in Figure 3.  The switching frequency must be pretty high -  typically in the 5-30 kHz range if one wishes to keep the inductance and physical size of that inductor reasonably small.

As in the case of the Zenith Transoceanic project, I used the PWM output of the microcontroller - a PIC - to drive the voltage converter with a frequency in the range of 20-50 kHz.  For our needs - generating about 140 volts at, say, 15 milliamps maximum, I knew (from experience) that a 220uH choke would be appropriate.  Figure 4, below, shows the as-built boost circuit.

Figure 4:
The voltage boost converter section showing the transistor/inductor, rectification/filtering and
voltage divider circuitry.

Description:

 

Q301 is a high-voltage (>=200 volt) N-channel MOSFET - this one being pulled from a junked PC power supply (the particular device isn't critical) which is driven by a square wave on the "HV_PWM" line from the microcontroller:  R301, the 10k resistor, keeps the transistor in the "off" state when the controller isn't actively driving it (e.g. start-up).  L301, a 220uH inductor, provides the conversion:  When Q301 is on, the bottom end is shorted to ground causing a magnetic field to build up and when Q301 is turned off, this field collapses, dumping the resulting voltage through D301, which is a "fast" high voltage diode designed for switching supplies - a 1N4000 series diode would not be a good choice in this application as it's quite "slow".

 

R304, a 33k resistor, is used to provide a minimum load of the power supply, pulling about 4.25 mA at 140 volts:  This "ballast" improves the ability of the supply to be regulated as the difference between "no load" (the neon bar-graphs energized, but with no "deflection") and full load (all segments of the tubes illuminated) is less than 4:1.  The resistive divider of R302 and R303 is used to provide a sample of the output voltage to the microcontroller, yielding about 2.93 volts when the output is at 140 volts.  The reader will, by now, likely have realized that I could have used R304 as part of the voltage divider - but since the value of this resistor was determined during testing, I didn't bother removing R302/R303 when I was done:  Anyway, resistors are cheap!

 

Setting the current:

 

Having the 140 volt supply is only the first part of the challenge:  As these tubes use current to set the "deflection" (e.g. number of segments) we need to be able to precisely set this parameter - independent of the voltage - to indicate a value with any reasonable accuracy.  For this we'll use a "current sink".

 

Figure 5:
The precision current sinks that drive the neon tubes precisely based on PWM-derived voltage.
Click on the image for a larger version.

 

Figure 5, above, shows the driving circuits for the two tubes using the "precision current sink".  Taking the top diagram as our example, we see that the inverting input of the op-amp (U401c) is connected to the junction of the emitter of Q401 and resistor R406.  As is the wont of an op amp, the output will be driven high or low as needed to try to make the voltage (from the microcontroller) at pin 10 match that of pin 9 - in this case, based on feedback from the sense resistor, R406.

What this means is that as the transistor (Q401) is turned on, current will flow from the tube, through it and into R406 meaning that the voltage across R406 is proportional to the voltage on pin 10.  It should be noted that current through R406 will include the current into the base - but this can be ignored as it will be only a tiny fraction (a few percent at most) of the total current.  It's worth noting that this circuit is insensitive to the voltage - at least as long as such current can be sunk - making it ideal for driving a device like the ИН-13 (or ИН-9) in which its intended operation is dependent on the current rather than the operating voltage.

At this point it's worth noting that the driving voltages from the microcontroller ("FWD_PWM" and "REV_PWM") are not plain DC voltages, but rather from the 10 bit PWM outputs of the microcontroller.  The use of a 10k resistor and 100nF (0.1uF) capacitors (R405 and C406, respectively) "smooth" the square-ish wave PWM into DC.

 

Q401 and Q402 were, again, random transistors that I found in scrapped power supplies, but since there's at least 70 volts drop across the tube, about any NPN transistor rated to withstand at least 80 volts should suffice.  It's also worth noting the presence of R407 and R409, which provides the "sustain" current on the "auxiliary" cathode.

Figure 6:
An exterior view of the tandem coupler module.
Visible is the top shield and the three feedthrough
capacitors used to pass voltage and block RF.
Click on the image for a larger version.

RF sensing

For sensing forward and reflected power I decided to use an external "sensing head" that was connected inline with the radio, on the "tuner" side of the feedline.  

For sensing power in both directions I chose the so-called "Tandem" coupler which consists of a through-line sampler in which a short length of coaxial cable carrying the transmit power (T1 in the diagram of Figure 7) passes through a toroidal core - using some of the original cable's braid grounded at just one end as a Faraday shield.  An identical transformer (T2) is connected across the first (T1) for symmetry.

When carefully constructed this arrangement has quite good intrinsic directivity and a wide frequency range.  Figure 6 shows the diagram of this section.

Figure 7:
Schematic diagram of the "Tadem" coupler.  A bidirectional coupler sends power to
separate AD8307 logarithmic amplifiers - one for forward and the other for reverse.
The outputs, expressed in "volts/dB" are sent to the microcontroller.
Click on the image for a larger version.

The RF sensing outputs of the second tandem coupler (T2) then goes through resistive voltage dividers (R606/R607 for the reverse sample and R603/604 for the forward sample) to a pair of Analog Devices AD8307 logarithmic amplifiers - one for forward power and the other for reverse - to provide a DC voltage that is logarithmically proportional to the detected RF power.  This voltage is then coupled through series resistors (for both RF and DC protection) R605/R608 and to the outside world using feedthrough capacitors.

The use of a logarithmic amplifier precludes the need to have range switching on power meter as RF energy from well below a watt to well over 2000 watts can be represented with only a few volts swing.  Looking carefully at Figure 6 one can see a label that notes that the response of the AD8307 is about 25 millivolts per dB - and this applies across the entire power range of a few hundred milliwatts to 2000 watts.

All of this circuitry is mounted in a box constructed of circuit board material and connected to the display unit with an umbilical cable that conveys power and ground along with the voltages that indicates forward and reflected power.

Figure 8:
An inside view of the Tandem Match (sense unit) showing
the coupling lines, internal shielding and AD8307 boards.
Click on the image for a larger version.

Figure 8 shows the as-built "sense unit" and the two coaxial sense lines are clearly visible.  As can be seen, the "main line" coupler is physically separated and shielded from the secondary sense line, using PTFE ("Teflon") feedthrough lines to pass the signals.

The AD8307 detectors themselves can be seen at the left and right edges of the lower half of the unit, built on small pieces of perfboard.  All signals - including the 12 volt power and the DC voltages of the output pass through 4000pF feedthrough capacitors to prevent both ingress and egress of RF energy which could find its way into the '8307 detectors and skew readings.

* * * * *

In a future posting (Part 2) we'll talk about the final design and integration of this project.

This page stolen from ka7oei.blogspot.com

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