Tunnel Diode Oscillator not Oscillating? - Page 1
Forum section: electronics / beginners

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 10, 2025, 06:58:40 am
content: So I wanted to build an oscillator with a tunnel diode for fun. I had a couple soviet tunnel diodes lying about and decided to use them. I checked them on my Tektronix 5CT1N and they appeared to test fine. There is no clearly defined negative resistance curve, but it's normal from what I've gathered.The tunnel diode I am using is the Gi305B. I made a small circuit and soldered all the parts onto a breadboard, but it appears to not be oscillating? Perhaps I am doing something wrong, although I can't see where I have gone wrong.I did try putting a 100-ohm resistor in parallel with R1 because the anode of the tunnel diode wasn't quite reaching the bias voltage, and that did help. But it only oscillates for a millisecond or so. I don't have a good DSO to capture those series of pulses. I can continuously "retrigger" this oscillation by turning RP1 back and forth, but can never get a continuous oscillation.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: February 10, 2025, 07:10:51 am
content: You shouldn't need to decrease R1 at all. The bias should be the midpoint between the peak and the valley on the I-V curve, which is well under 0.5V.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 10, 2025, 08:09:28 am
content: I replaced the 100 ohm resister with a 200 ohm trimmer and after some adjustments, it works. Surprisingly the oscillation frequency isn't really dependent on the tank circuit. I can vary the oscillation by a good margin depending on were I put the bias point at. The trimmer capacitor does have an effect on the frequency however.I also noticed that the oscillation doesn't seem all that stable. The frequency varies, indicated by the fuzzy vertical slopes near the end of the trace. Perhaps it's because it's too low of a frequency? It oscillates at about 20MHz, -3MHz variable with the bias point.

author: mawyatt
author_rank: Super Contributor
timestamp: February 10, 2025, 12:45:45 pm
content: RF bypass the TD Anode, use a 100pF or 1000pF SMD cap. The resistive loss of the pots as "seen" from the TD Anode is reducing the overall circuit negative dynamic resistance. Also remove RP2, it's not necessary for DC bias, it's shunted by the inductor so ineffective for any bias adjustment.You will achieve better results by using an SMA connector for the output and work in the 50Ω RF environment. Note that anything you connect to the output will lower the resonate frequency due to the parasitic shunt capacitance of the attachment. With a 10X scope probe you'll likely see 30~40MHz Oscillation with proper DC Bias and decoupling.Best

author: iMo
author_rank: Super Contributor
timestamp: February 10, 2025, 03:06:18 pm
content: Btw., those soviet tunnel diodes are extremely sensitive to temperature.Afaik temperature over 70C while soldering them may destroy them easily.Double check that..

author: TimFox
author_rank: Super Contributor
timestamp: February 10, 2025, 03:17:36 pm
content: If I remember correctly, most tunnel diodes use germanium.

author: mawyatt
author_rank: Super Contributor
timestamp: February 10, 2025, 04:12:32 pm
content: Believe they are gold doped also.Best

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 10, 2025, 05:29:16 pm
content: Thanks for the replies.I have omitted RP2 as recommended and have added a bypass capacitor to the anode of the tunnel diode. I do not have any SMD capacitors unfortunately; the only ones I have are on for parts commercial PSU boards, and I'm not willing to desolder them and make an SMD test jig to test each one on my Leader LCR-740. Oh the woes of not having any SMD parts. Reminds me of a time where I made an entire RF amplifier and mixer out of random parts I stole of various boards.I also do not have SMA connectors. Best I have are panel mount BNCs. I'll probably just use some random bits of wire to hook it up to the BNC connectors for a 50 ohm network. Luckily my 2465 also has built-in 50 ohm input termination.As for the tunnel diode receiving heat damage from soldering, would simply using some soldering tweezers and clamping them on the leg being soldered be enough? The tunnel diode I'm using currently has previously been soldered in and out several times out of different experimental circuits. Even after all that a new untouched tunnel diode and the one that has been soldered in and out both have identical curves on the curve tracer.Lastly, how does the board layout look? Any changes I should make before etching it?Edit: Attached a new PCB layout to better accommodate the size of the inductor.