Using a Lecher Line To Measure High Frequency
How do you test the oscillator circuit you just made that runs between 200MHz and 380MHz if all you have is a 100MHz oscilloscope, a few multimeters and a DC power supply? One answer is to put away the oscilloscope and use the rest along with a length of wire instead. Form the wire into a Lecher line .
That’s just what I did when I wanted to test my oscillator circuit based around the Mini-Circuits POS-400+ voltage controlled oscillator chip (PDF). I wasn’t going for precision, just verification that the chip works and that my circuit can adjust the frequency. And as you’ll see below, I got a fairly linear graph relating the control voltages to different frequencies.
What follows is a bit about Lecher lines, how I did it, and the results.What’s A Lecher Line? Plain Lecher line and with test equipment in use The end with a loop
A Lecher line consists of two parallel wires or rods that form a balanced transmission line . The technique results in a way tophysically measure wavelengths. This methodhasbeen around for a long time. Its namesake is Ernst Lecher, a physicist from Austria who perfected the practice in 1888.
The length should be some multiple of the oscillator’s output wavelength. The oscillator’s waves are applied to one end of the Lecher line where the two wires are connected together, forming a loop in my setup. Also in my setup the other end of the line is open, the wires are not connected together there.
A metal bar, or a screwdriver in my case, is put across the width of the two parallel wires, shorting them. As I slide the bar along the wires, it influences the waves. When the barreaches the wave’s nodes, positions along the wires that are at a half or full wavelength (the zero crossing locations), it can be detected in various ways. One of those ways is to have the bar be two terminals of a neon bulb. The bulb goes out at the nodes, where the voltage is zero. But I didn’t have a neon bulb so I’ll show another way below.
In the photos you can see a measuring tape running the length of the wires for measuring the distance from the end of the Lecher line to the nodes. By doing so, you’re measuring the wavelength of the waves and can use that to calculate the frequency.
The distance between the wires should be even and should besignificantly smaller than the length of the wavelength being measured.My Setup
Since my oscillator can produce between 200MHz and 380MHz, I neededa Lecher line that was long enough to accommodate that range.The formula for converting frequency to wavelength where electromagneticwaves are concerned is:wavelength = speed_of_light / frequency
which gives:300,000,000 m/s / 200,000,000 cycles/second = 1.5 m
300,000,000 m/s / 380,000,000 cycles/second = 0.79 m
I made mine 1.5 meters long.
The end with the oscillator but pulled back from the Lecher line
Another angle showing the diode
To test the oscillator circuit, I formed a loop at the oscillator’soutput that matched the loop at the end of the Lecher line. Part of that loop is a 6.8 Kohm resistor, there so that the oscillator circuitwouldn’t see a dead short. To apply the circuit’s waveform to the Lecher line, I simply put the oscillator’s loop very near to that of the Lecher line. i.e. The oscillator puts electromagnetic waves on the Lecher line using induction. In the first photo above, the oscillator is pulled back a bit to make things clearer.
NTE583 diode loop
Lecher line, oscillator and other equipment
The next step was to detect when the screwdriver was at a node onthe Lecher line. For that I used a high frequency diode, anNTE583 silicon diode whose packaging says “Schottky Switching for High Level UHF/VHF Detection and Pulse Application”. I solderedwires to either end and formed a loop. As shown in the above photos, I added the diode loop to the collection of loops at the end of the Lecher line.
The diode’s two wires go to an analog meter set to the 1-volt scale. When the screwdriver is still less than half a meter from the oscillator end, and at the node for the first half-wavelength, the voltage across the diode is above 0.5 volts. When the screwdriver is between nodes, the voltage is less than 0.1 volts. But the further the nodes are from theoscillator, the lower the voltage is on the meter, until the meter’sneedle barely deflects at all.Making a measurement Results
The node closest to the oscillator end of the Lecher line is ahalf wavelength. The next one further away is a full wavelength.When the screwdriver is at a location that causes a voltage peakon the analog meter, you’ve found a node and the distance from theoscillator end to that node is recorded.
The oscillator is a voltage controlled oscillator. My circuit includes a voltage divider and potentiometer for adjusting that voltage, which controls the oscillator’s output frequency. The yellow Fluke digitalmeter is there to show the control voltage. That’s recorded along with the distance measurement.
The oscillator’s control voltage is then turned up, increasing thefrequency and decreasing the wavelength. The new distance and controlvoltage measurements are then recorded.
That’s repeated for the oscillator’s full 0 to 12 volt range ofcontrol voltages. The resulting data and graph are shown below.
The frequency is derived from the data using this formula:frequency = 300,000,000 m/s / (2 * half-wavelength measurement)
The half-wavelength measurement (the 1/2 wavelength column in the table) is multiplied by 2 rather than using the full-wavelength measurement (the 1 wavelength column). That’s because, as was mentioned above, the further the screwdriver was from the oscillator’s end of the Lecher line, the lower the voltage was on the analog meter to the point where there was hardly any deflection at all.
Lecher line data
Graph of the dataLooking for more high frequency measuring projects here on Hackaday? There’s one using 74-logic for a DIY 100MHz frequency counter , an inexpensive timebase add-on for HP 53131 10MHz frequency counter that [Gerry] built instead of buying the stock $1000 one, and a project that started as a sub-project, the Nanocounter, built with an FPGA, STM32F072 and an Android front-end . We’re always looking for unique mechanisms and