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A Field Strength Meter Using A Biased Schottky Detector.
A temperature compensated Schottky diode is used in an amplified, untuned
field strength indicator that is powered by two AA cells idicates the relative field strength of
RF fields from a few kHz into the microwave region.

Download the Download FreePC files in and the detector board layout png file in zipped forma: schottkydedtector080309

Find updates at www.projects.cappels.org

Starting to do a little work at 330 MHz, I decided that my existing field strength meters were not adequate for for my needs. The 330 MHz project came to a stop, and I set out to put together a field strength meter with a moving needle indicator, with higher sensitivity than the others I have built, and the ability to indicate RF in the 2.4 GHz band.

Before getting too far into the description, I would like to apologize to those who design real field strength meters and acknowledge that this is device indicates relative field strength from moment to moment. It is not calibrated and I have certainly not gone to any pains to assure a flat frequency response, or even a characterized frequency response for the detector. Having said that, this is a pretty sensitive indicator, and there are provision to add a tuned circuit to the input so as to give the device some selectivity. I will continue to make loose use of the term "field strength meter" in this web page to aid web searches for this kind of instrument. Such are the conditions of our time. I kindly request the reader's kind indulgence in this matter.


    External Connections and controls

When an antenna is attached, (or even if not, in the presence of a strong RF field) the field strength meter has a moving coil meter to indicate relative field strength. The box holds a Schottky diode detector, a DC amplifier, and a meter display.  If the parts used in construction are the same values shown in the schematic, the meter shows full scale ("+3db" on the surplus store VU meter movement) when the output of the Schottky diode detector is between 1.6 millivolts and 160 millivolts, depending up the setting of the gain control. Since opamps are used as amplifiers, the range of the gain control can be modified easily. The actual sensitivity in terms of field strength is primarily a function of the antenna system, and that is well beyond the scope of this web page.

An RCA connector with which you can attach an antenna (as in the photograph near the top of the page),  of if desired signal preconditioning circuits such as a filter and/or preamp. I expect that for most of my low power uses, a short whip antenna or no antenna at all will be sufficient.

A stereo headphone jack that provides a way to connect the amplified detector output to a digital voltmeter, chart recorder, oscilloscope, or other instrument, and also allows the application of an offset voltage from an external device. I had imagined adding an external logger with an automatic offset adjust function.

An offset knob allows correction for drift in the Schottky detector as well as offsets created by interfering signals.  A coarse offset control on the circuit board allows for compensation for the mismatch between components.

A power switch and pilot light complete the set of user controls.

The meter operates from the power of two AA cells. With new Zinc Chloride cells installed, it draws only about 3 milliamps, including the LED when the meter reads full scale, and about 3.5 milliamps when the meter reads zero, so it seems that the use of an external power supply would not be worthwhile.

    The Circuit

The field strength meter is a broad band temperature compensated biased Schottky diode detector with an offset adjustment, followed by a buffer (U1A), amplified by a single gain block (U1B), which drives the meter driver (U1C). A integrated circuit voltage reference is used to generate stable +1.25 and -1.25 volt reference supplies and an opamp (U1D) provides a ground reference which is at half the voltage of the 3 volt battery. The circuit blocks will be discussed in turn below.

Temperature Compensated Biased Schottky Diode Detector

The detector board takes its input from short wires soldered directly to an RCA audio connector.

The Schottky diode detector is built on a single sided fiberglass circuit board that is separate from the amplifier board. This was mainly because the detector board needed to be close to the RCA input connector, which would be on the front panel. With the presence of the other connector and the controls on the front panel,  there was no room for a larger circuit board to accommodate the DC amplifier.

Whip antennae require a ground structure in order to work.  The large copper areas on the detector board, as show in the picture above, are intended to improve the efficiency of a short whip antenna at GHz frequencies.  Being in a mostly unshielded enclosure, the meter can detect strong GHz range signals picked up on its internal leads, without the need for an external antenna.

Advantages of using a printed circuit board for the detector over using other construction methods, are that it allows the use of surface mount parts, which not only provides for small physical dimensions which minimize parasitic reactances, but also provide for good thermal coupling of both of the diodes to the circuit board. Keeping the diodes at the same temperature is important in keeping temperature caused drift as small as possible.

Schottky diode D1 is the RF detector. Diode D2 is a diode of the same type, and it provides an offset voltage that tracks the forward voltage drop of D1. R2 and R3 provide 650 nanoamps of bias current. Half the current is drawn through detector D1 and bias resistors R5 and R6, and half the current is drawn through D2. Each diode is lightly biased at about 330 nanoamps to improve sensitivity and linearity for small signals.

Both D1 and D2 are dual diodes, but I only used one diode in each package.

The use of 1k resistor, R2, on the cathode of D2,  with current from offset pots R17 and R18 through their respective series resistors (R7 and R8), allow for an offset adjustment of about plus and minus 10 millivolts. The unadjusted offset of my detector was about 1.5 millivolts. This was without any effort to match the diodes for forward drop. The 2 Megohm bias resistors are physically mounted on the amplifier board, and are shown in the schematic above to improve understanding of circuit operation.  The purpose of R3 is to work with C4 to attenuate any noise that might be picked up in the wiring harness.

There was an unsettling effect just after the meter was assembled. With no external antenna attached, with a strong signal from the right side of the meter (the side on which D1 was mounted), the meter reading increased, but when the signal came from the left side of the meter, the meter reading decreased -went negative. The effect was removed by soldering a 680 pf capacitor directly across D2. The 680 pf capacitor is not shown in the photograph above.

The DC offset corrected output of the detector drives the input of a variable gain amplifier.

Amplifiers And Meter Driver

The amplifier was built on a piece of pre punched phenolic board that has one copper pad around each hole. This results in a circuit that is easy to assemble, and fairly easy to repair or change. A holder for two AA batteries sits on the board. A five pin single row connector, on the left side of the board, makes DC connections to the Schottky diode detector board through a four wire harness and a 14 pin dual row connector on the right side of the board makes DC connections to the indicators, connectors and controls on the front panel through a 12 wire harness.

Although not shown in this version of the schematic, the bias resistors for the diode detector (R3, R4, R5, and R6) are all mounted on the amplifier board with the idea that they will track each other thermally better that way.

In the photograph above, the two blue colored resistors near pin 1 of the quad opamp were removed in order to reduce the gain of the circuit.

The amplifier board sits in the bottom of the enclosure.

The opamp used in this instrument is a Texas Instruments TLC274C. It was chosen because of its ability to operate with low power supply voltages, its fairly wide output voltage range, and its very good offset and drift specifications. There are better opamps around for this application, but this is the best one I had on hand. You can do well with a better opamp, but I do not suggest trying this with a less capable opamp.

The signal from the detector, which we can treat as DC, is buffered by voltage follower U1A. R9 is intended to compensate for input bias drift in U1A and C6 is intended to assure that there would not be excessive lag in the feedback through R9 because of stray capacitance and the input capacitance of U1A's inverting input. Upon reviewing the schematic, I see that if I had a resistor closer to 1.5 Meg Ohms, it would have provided a better approximation of the input resistance seen by the noninverting input of U1A.  C6, The capacitor across R9 is a low leakage polypropylene type.

The second stage is the adjustable gain stage, U1B. With the component values shown in the schematic, this amplifier has a gain that ranges from 1X  to 148X. I went through my collection of surplus store 100k pots and found that they measured between 60k and 87k, so I used the 87k pot, which gave me an actual gain range of 1X to 137X. This meter is not calibrated, so parts tolerance is not much an issue.

The output of this stage is applied to the meter driver and to the stereo headphone jack. When the meter is properly zeroed,  the voltage at this point will go negative with respect to ground.

The meter driver serves two purposes - it limits the maximum amount of current that can pass through the meter movement, and putting the meter movement in the feedback path of an inverting amplifier, instead of to ground, assures that the current to drive the meter movement flows from the battery terminals through the amplifier stages and not through the artificially generated ground. Keeping the meter movement current out of the ground circuit minimizes the chance of incidental feedback through the grounds that could result in oscillation.

The diodes in the meter driver circuit allow separate paths for positive and negative outputs. This is because I want to limit how hard the needle can hit each "pin" that limits its travel. With a signal into the instrument, the output of U1C is positive, and D3 conducts. The maximum current is limited by the voltage output capability of U1C minus the forward drop of D3, and the divided by the series resistances of the meter movement and R14.

When the output of U1C swings negative, as when the offset is being adjusted, and particularly at first setup when the offset adjustment has a range of  several times full scale, the most of the feedback current is supplied through D4 and the current through the meter is limited to the diode drop of D4 divided by the series resistance of the meter movement and R15. I felt it necessary to allow the meter to operate below zero to make it easier to adjust the offset.

Here (above) is an alternate way to use the first amplfier, U1A. Instead of using it as a unity gain buffer, you can make it into a gain stage. As shown here, it has a gain of abaout 100, which is probaly way too much sensitivity for many applications. For a gain of 10X change the 100k resistor to 10k.

By the way, I used metal film resistors in many parts of the circuit for stability. The gain stability is not particularly important, but thinking that maybe someday, I would calibrate this as an AC voltmeter at the lowest gain, in which case, the gain pots' temperature coeficient would not affect circuit gain). It is a really good idea, however, to use very stable resistors for R4, R5, and R6 to minimize offset drift.

Find updates at www.projects.cappels.org

The Power Supply

Everything is powered by two AA cells. In theory, they need to maintain nearly 3 volts in order for the circuit to work, but in real life, the one I built worked find when I used old batteries that provided only 2.5 volts. A more robust design would use three cells for a nominal 4.5 volt power supply. Changing from two cells to three cells can be done without any circuit changes, but it would be better to increase the values of R20 and R23 to reduce current drain.

The reference diode D6,  LM335Z, is biased by two equal resistors connected to the positive and negative battery terminals. This places a 2.5 volt reference midway between the two potentials. The output of the resistive divider made R21 and R22, is in half way between the negative and positive terminals of the 2.5 volt drop across D6. Voltage follower U1D buffers this voltage, which I call "ground". The voltages cross the anode and cathode of the reference diode are -1,25 volts and +1.25 volts with respect to ground, and these voltages are used to provide bias for the circuitry. The battery voltage provides power directly to the quad opamp.

Construction Consideration

The instrument is housed in a plastic box designed for such purposes. A metal box would have been quite a bit better. I imagine that if the detector circuit were built in a small RF tight box with all the bias and control leads decoupled, a plastic box would have been almost as good as a metal box. Whatever could have been, the reality is that I built it in a plastic box because I don't have any suitable metal boxes available at the time, and I am more interested in using this instrument than building it, so waiting until after my next trip to the big city (Bangkok) to buy a metal box was not a good choice  for me.

In the picture above, you can see  the front panel with the controls, connectors, and indicators mounted on it. A piece of copper clad printed circuit board is held in place with silver colored tape under where the detector board will be mounted.  The piece of copper clad board helps shield the back of the detector, and it is connected in four places to the ground area on the detector board.

Harnesses, one captive to the detector board, and one captive to the front panel connector, indicators,  and controls, are made of #26 stranded wire, antique lacing cord, tied about every 1 cm and at bends and breakouts, holds the harnesses in shape.

The meter movement is held in place with liberal amounts of cyanocarylate super glue backed up with more than enough hot melt glue.

I found that as I put my hand on the back of the instrument and then moved it away, it would change the reading substantially. This effect was greatly reduce by adding a little shielding to to the case under the main circuit board. The wire seen in the photograph above plugs into the amplifier board and connects to the amplifier board ground. The shield, made for a piece of copper clad fiberglass board is held in place by friction between the edges of the boards and the molded-in standoffs in the bottom of the plastic box.

Looking back

Its only been a week since I built this, but I can already see that the sensitivity is about right for my kind of use. I seems to work well at detecting the low amplitude filed around the antenna of a crystal controlled low power FM transmitter that I built a few years ago.  When held next to a piece of wire connected to the output of my MAX038-based function generator, set to 12 kHz, the meter displays the relative amplitude of the filed from that! Taht's pretty low. It also senses the RF field around my WIFI base station. So, it came as no surprise when I was able to moved the around the microwave oven in my kitchen while it was boiling a cup of water and get an idea of where the leakage from the door was maximum -at the edges, and minimum -right in front of the door. So far, I am happy with the instrument.

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Contents ©2009 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in March, 2008. Expanded,  updated April 1, 2009, and April 19 2009.

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