Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com

A question came up on the Elecraft reflector recently concerning the relative merits of different diode types for a simple RF probe. The simple RF probe considered here has a DC output proportional (but not linearly proportional) to the RF voltage at the input.

The ARRL Handbook has had a RF probe design for many years, as illustrated to the right. (The phone plug goes to a microammeter or a high sensitivity voltmeter.) The Handbook design calls for a 1N34A Germanium diode, as it has reasonable sensitivity at low RF voltages.
 

I wrote a simple computer program to step the signal generator output from -50 dBm to +15 dBm, in 1 dB increments at 2, 10 and 21 MHz, and to capture the diode's DC output voltage. The data is saved to a disk file for post-processing with Origin, a scientific and engineering data analysis and plotting program. Both the VP8191A signal generator and 34410A digital voltmeter are controlled over an IEEE-488, or GPIB, control bus, via a model 4.2 Prologix USB-GPIB adapter.

The 1 Mohm shunting resistor represents the input impedance of typical DC meter. The Agilent 34410A has a considerably higher input impedance and the additional 1 Mohm shunt makes the  test more representative of commonly available equipment. The 50 ohm through is necessary as the VP8191A signal generator output is calibrated only into a 50 ohm load. For this setup, we may regard the diode and ancillary components are presenting a sufficiently high impedance when compared with the 50 ohm through to provide negligible loading.
 

• 1N5711 Schottky signal diode
• 1N270 Germanium signal diode
• 1N4148 Silicon signal diode

I've written about these three diodes before, in the context of their forward voltage versus current behavior, at Diode Vf vs If which you may wish to also review.

The plot below shows the DC output voltage versus RF stimulus voltage for one sample of each of these three diodes, at 2 MHz. The data at higher frequencies is similar.

We expect the 1N4148 silicon diode to be the worst performer, and we are not disappointed. I did not have a 1N34A germanium diode on hand, but the 1N270 is similar and it shows the greatest sensitivity.

The 1N5711 Schottky diode falls between the germanium and silicon diode performance. Schottky diodes use a metal-semiconductor junction, not the semiconductor-semiconductor junction more commonly found, such as in the 1N4148. Schottky diodes exhibit a faster turn on/off time (higher frequency response), lower capacitance and lower voltage drop for lower current levels when compared with semiconductor-semiconductor diodes. (But not necessarily lower when current levels increase as demonstrated in my Diode Vf vs If measurements.)

In any particular operation, the sensitivity differences amongst these three diode types may or may not be important. Operated into a sensitive digital voltmeter, all three diode types would likely be usable. Operating into a 50 microampere moving coil meter, when measuring lower level signals, in contrast, a germanium diode may be the only usable alternative.

If I were designing a diode-based wattmeter, however, intended for reasonably power levels and voltages, where flat frequency response is essential, a Schottky diode is the best choice.