60 kHz Preamp: Simulation

This circuitry descends directly from a QEX article (Nov/Dec 2015, p 13) by John Magliacane, KD2BD: A Frequency Standard for Today’s WWVB. The key part, at least for me, is a 60 kHz preamplifier using a resonant-tuned loop antenna and an instrumentation amplifier to reject common-mode interference from local electrostatic fields.

I tinkered up an LTSpice IV simulation using somewhat more contemporary parts (clicky for more dots):

60 kHz Preamp - LTSpice schematic

60 kHz Preamp – LTSpice schematic

The simulation quickly revealed that the original schematic interchanged the filter amp’s pins 2 and 3; the filter doesn’t work at all when you swap the + and – inputs.

The stuff in the dotted box fakes the loop antenna parameters, with a small differential AC signal that injects roughly the right voltage to simulate a nominal 100 µV/m WWVB field strength. I biased the center tap to the DC virtual ground at + 10 V and bypassed it to circuit common, so the RF should produce a nice differential signal about the virtual ground. The 5 kΩ resistors provide ESD protection and should help tamp down damage from nearby lightning strikes and suchlike.

It works pretty much as you’d expect:

60 kHz Preamp - Frequency Response

60 kHz Preamp – Frequency Response

The LT1920 is mostly flat with 40 dB gain out through 60 kHz, although the actual hardware becomes a nice oscillator with that much gain; my layout and attention to detail evidently leaves a bit to be desired. The LF353 implements a multiple-feedback bandpass filter with about 20 dB of gain; its 4 MHz GBW gives it enough headroom. The LT1010 can drive 150 mA and, with a bit of luck and AC coupling, will feed a 50 Ω SDR input just fine.

This obviously turns into a Circuit Cellar column: March 2017, if you’re waiting by the mailbox.

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  1. #1 by RCPete on 2016-12-16 - 16:48

    doesn’t work at all when you swap the + and – inputs

    What am I missing? Assuming you’re talking about the LT1920, everything looks symmetrical with respect to + and -.

    • #2 by Ed on 2016-12-16 - 16:55

      Good catch: it was the filter op amp. Fixed!

      Thanks…

      • #3 by RCPete on 2016-12-16 - 21:51

        Thanks! My circuit theory is rusty and my vision has semi-major suckage*, but I’m not that bad off.

        Couldn’t handle the QEX schematic; I’m reading one-eyed right now. Some scary surgery will fix it** once Medicare kicks in.

        ** I hope.

        • #4 by Ed on 2016-12-17 - 05:14

          Good luck with the eye!

          Our Chrysalid Engineer learned, early on, that she’d been issued all the eyeballs she’d ever get and, if she didn’t take good care of them, she’d have to make do without. So far, so good …

  2. #5 by RCPete on 2016-12-17 - 11:16

    Luck of the draw for this, mostly (epiretinal membrane). The leaf-rake looking tool they use got my attention. [wince]

    • #6 by Ed on 2016-12-19 - 03:47

      leaf-rake looking tool

      I had to look that up and now I’m sorry I did: “Using highly specialised instruments, your surgeon will remove the vitreous gel from your eye”. Urp!

      Not a home shop procedure, fer shure!

  1. 60 kHz Preamp: First Pass | The Smell of Molten Projects in the Morning