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Re: Steve ugly-build LZ1AQ using MPS2222 sweeps uploaded to Photos

 

If the L-network is single ended - common mode for coax - the balun belongs BEFORE the network!?? The balun converts differential mode to common mode.? That's why it is termed a "balun:? Balanced (to) Unbalanced.

Dave - W?LEV


On Fri, Jan 24, 2025 at 2:11?AM biastee via <biastee=[email protected]> wrote:
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On Sat, Oct 12, 2024 at 11:21 PM, <biastee@...> wrote:
On Mon, Sep 30, 2024 at 01:13 AM, Martin - Southwest UK wrote:
Understood about the switch, but I think the location of the balun is causing problems.
Hi Martin,
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You are right. Placing the balun after the L-network causes problems. ? The simulator showed similar result, regardless of whether the balun is placed BEFORE or AFTER the L-section. This deceptive result arises because the simulator's ideal balun was capable of tolerating high Z, whereas the real balun requires ~50 ohm to work properly.
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When the L-network's impedance is measured without the fabricated balun, it shows the expected trajectory, with a resonance at the modelled 30 MHz (black trace). However, after the balun is added, i.e. AFTER the L-section, the resonance mysteriously shifts down to 13 MHz (red trace).?
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The problem can be rectified by moving the balun L2 to a position before the L-section. With the balun in the new position, the impedance locus is as modelled with a resonant at ~30 MHz.
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Additionally, the previously measured LZ1AQ's gain peak at 22 MHz, also correctly shifts back to the modelled 30 MHz.
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I have posted the corrected results at the link:
Critique welcomed!
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On Sun, Jun 30, 2024 at 08:17 PM, Caaarlo wrote:

  • M0AYF Improved: Input Z is more HF-friendly for a 1m loop without the need for a matching network, but CMR, IP2, and IP3 seem to be a bit worse than the LZ1AQ.

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@Caarlo
My measurement of the LZ1AQ (fig. 11 in above link) shows worse CMRR than M0AYF, especially at the lower HF range. I hypothesize the latter's improved CMRR is due to it using a common emitter resistor in the differential pair. Below link to M0AYF's CMRR graph:
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A variation on the M0AYF's emitter resistor is PA0FRI using an inductor instead. The PA0FRI circuit is virtually identical, but his replacing the emitter resistor with an inductor results in poor CMRR at low frequencies due to the decreasing inductive reactance.

73, Leong, 9M2LCL (ex 9W2LC).



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Dave - W?LEV
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