Fwd: LF: Re: WA1ZMS Loop design

[Andre Kesteloot]

Sent from my iPhone

Begin forwarded message:

> From: "James Moritz" <james.moritz at btopenworld.com>
> Date: December 3, 2008 8:34:45 EST
> To: <rsgb_lf_group at blacksheep.org>
> Subject: LF: Re: WA1ZMS Loop design
> Reply-To: rsgb_lf_group at blacksheep.org
>

> Dear Mike, Chris, LF group,
>
> I would agree with Chris' comments regarding Op-amp suitability -  
> the preamp
> circuit appears to be designed for 40dB voltage gain, which it won't  
> achieve
> at MF with an OP-27 or even less a 741. The article gives values of  
> gain
> from 34dB at 25kHz down to 10dB at 500kHz, although it doesnot say  
> under
> what conditions this was measured. I'm sure the circuit "works" in  
> the sense
> that it gives some useful gain, but operating it in this way means  
> linearity
> is poor, input impedance is reduced and output impedance increased,  
> i.e. you
> lose all the advantages of using an op-amp in the first place. The  
> 9V supply
> is a bit low for these op-amps also.
>
> With a reasonable receiver sensitivity, you would not need much gain  
> with
> this kind of loop arangement - assuming a loaded Q of 100, I  
> calculate the
> voltage at the loop terminals at resonance will be about 1.3uV for a  
> 1uV/m
> field strength. The noise in a 500Hz bandwidth at the loop terminals  
> would
> be about 0.4uV just due to the loop itself, with a similar level due  
> to
> external band noise on a quiet band. The internal noise of better  
> receivers
> would be swamped even with 0dB preamp gain, i.e. just a buffer,  
> although you
> might want 10 - 20 dB if your RX is a bit deaf at 500k.
>
> The remarks about the loop bandwidth in the article are a bit  
> mysterious -
> it implies the bandwidth is wide and that tuning is not critical,  
> but my
> experience with this type of loop is that Q of 100 - 200 is normal,  
> giving a
> bandwidth of a few kHz at 500k, so tuning is critical.
>
> My experience with op-amps as RF preamplifiers is that they can be  
> made to
> work quite well, but are not any simpler or better performing than  
> discrete
> component circuits in this frequency range. Check out the variable- 
> gain loop
> preamp on G3YXM's "circuits" page for example. Video op-amps can be  
> used
> well into the HF range, but like all high-frequency, high gain  
> devices with
> feedback, care is needed to avoid instability!
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
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