To see this working, head to your live site.

Simple and stable triode BFO

I have not seen this before, maybe unique, thort to pass it on.

Developed this to use in my Deltahet.

Any triode will work, The .01 anode cap is to isolate the HT from the resonator, they hate voltage gradients. One caveat, the resonator will oscillate about 5kC above the nominal, use a 450 kC to get 455, a 455 will resonate at 460. Small freq changes , as in very small can be coaxed by fiddling either resistor values or capacitor values.

I developed this to use the triode section of the ECH81 in the product detector of the Deltahet.

3 comments

3 Comments

w5jag

May 03

I used CSB455E in a JFET oscillator as the carrier oscillator in my first scratch built SSB transceiver. I recall it being pretty close on frequency and once set with a trimmer, never had to be touched again.

I used matched USB/LSB mechanical filters, so I never had to change the frequency.

My CSB455E only have two leads, and not three as your device does.

73,

Win W5JAG

Triode

May 03

Specifically, what to do to correct the frequency ? Playing with the 470p capacitors ?

73, Cris

dayleedwards

May 04

Replying to

Yes,, its pretty much all there is to change, but the change is very small, less than 1kc. The issue I think is the impedance mismatch. These ceramic resonators expect in/out impedances of a few hundred ohms, hence the large value capacitors, these effectively load the thing down with capacitive reactance. Increasing the capacitance values will lower the frequency, but a point is reached where the thing will stop oscillating, and thats not very useful. There is also quite a large variation in the center frequency of any one sample of these resonators, they are cheap to make, and cheap to buy, much cheaper than dedicated 2 pin ceramic resonators normally used for this application, explaining the spread.

It was just an easy way to create a stable BFO cheaply and using "junkbox" parts. If for instance this BFO was installed in an AM only radio, then the actual IF frequency can be moved to suit the BFO oscillator center frequency, its easier to do this, and will not effect the radio performance to any great degree.. Just why each and every resonator/filter is 5 KC too high, I have no clear idea, I have not investigated it. I assume it oscillates in some sporadic mode, although the frequency is and does remain very stable.