Zener noise

[brandon7861]

Out of boredom, I wanted to see if my radio could detect the noise from a zener diode alone, so I attached a set of clip leads to the coax jack of my washington and applied DC directly to the zener via 1k resistor using my variable bench supply with the radio across only the zener (DC blocked inside the radios filter network, discounting the 10k bleeder).

The zener noise starts, and is strongest, when the diode just begins to conduct, and then decreases in strength as the voltage is brought up further. I found it very easy using any of the zeners I have above 8v to adjust the supply to get S3 through S9+10 on my meter.

At first, I thought this would make for a good poor man's meter adjustment if one had a radio available with an accurate meter. Set zener voltage for S9 on good radio and then attach it to the radio that needs it's meter adjusted. Unfortunately, radios have different bandwidths, so I don't think this will work unless both radios are very similar in design/bandwidth.

Edit: ran some numbers, and even radios with a couple kHz difference in BW would still be within about 1/6 of an S-unit, so I guess it may be a practical approach after all. I don't know, sometimes my calculator effs up this late at night..

 

[nomadradio]

Very cool! I suspect that connecting a receiver input directly to it might cause the noise voltage to change from one radio to the next. All receivers have a "SWR" they call Return Loss. Receivers with different input impedance would affect the amplitude of the noise. A buffer amplifier would isolate the zener from the receiver.

I think.

73

 

[brandon7861]

I have some of those MAR4 amps left. If I assume the diode is a low resistance (since it is conducting), I would think 47-50ohm series resistance should be added to match the Mar4 input. If that sounds right, I will try that.

Edit: or do I decouple with a capacitor and use a high-z amp? Whats the best way to load that diode? Does the noise have a typical impedance? I guess its doing the job staring into 50ish ohm, so maybe I shouldn't worry.

I think my washington meter is off a little. I tried this on other radios and got a max of S9 (not +10). But still, thats where I wanted it anyway, and adding an amp would ensure I could land right on S9.

One thing I did notice was a bit of bounce in the needle, but it isn't too hard to visually average. Close enough I think.

 

[nomadradio]

A zener that's just beginning to conduct won't be a low impedance. A 6.8-Volt zener with 1 mA through it should be close to 6800 ohms.

I think.

73