Advertisements

60 kHz Preamp: Tuning Fork Resonator Protection

Limiting the resonator drive to about 1 μW in the face of wildly varying RF from the antenna (or the occasional finger fumble) requires brute force. A nose-to-tail pair of Schottky diodes seems to do the trick:

Tuning Fork Resonator Filter - protection and biasing

Tuning Fork Resonator Filter – protection and biasing

The 100 Ω resistor blunts the drive from the LM353 op amp (implementing a bandpass filter) when the signal peaks exceed 200-ish mV in either direction from the Vcc/2 bias stored in the 10 μF cap.

The 11.5 kΩ resistor downstream of the resonator isolates it from the Vcc/2 bias, with the 100 nF cap sinkholing the signal and the 4.7 kΩ resistor preventing feedback into the bias supply. The cap looks like 26 Ω at 60 kHz, so the feedback runs -52 dB from the output and the bias supply knocks it down a bit more. The preceding amps apply 40-ish dB of gain from the antenna terminals, so the loop gain looks OK.

It’s another few components on the board:

LF Crystal Tester - resonator protection

LF Crystal Tester – resonator protection

The blue twiddlecap should allow pulling the tuning fork’s series resonance upward to exactly 60 kHz.

Applying way too much signal to the antenna terminals in order to get 1 Vpp from the LM353 shows the limiter in action:

BP and Xtal filter out - 10.0 v sine 10 Meg xfmr

BP and Xtal filter out – 10.0 v sine 10 Meg xfmr

The resonator sees no more than 200 mV in either direction from the bias level, so it’s all good.

On the low end, the diodes have no effect:

BP and Xtal filter out - 1.1 v sine 10 Meg xfmr

BP and Xtal filter out – 1.1 v sine 10 Meg xfmr

Pay no attention to all that noise.

My first thought was to put the diodes across the resonator, a Bad Idea: straight up, doesn’t work. The 1N5819 datasheet shows they have about 300 pF of junction capacitance at zero bias and a pair of ’em will swamp the resonator’s internal 0.8 pF parallel capacitance and punch it out of the circuit.

Advertisements

,

  1. LF DDS Sine Generator With 0.1 Hz Steps | The Smell of Molten Projects in the Morning
  2. 60 kHz Preamp: Filtering and Rebiasing | The Smell of Molten Projects in the Morning

Spam comments vanish. Comment moderation may cause a delay.

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s