Alan
>But people, Mobo users in particular, say that reducing op-amp gain reduces op-amp noise thereby >increasing the RF sensitivity/dynamic range with a low noise soundcard.
>Or have I got that wrong?
For a given signal level:
??? The same sensitivity (gain) produces the same output.
??? Increased sensitivity produces increased output.
Reducing the op amp gain reduces the sensitivity but can increase the dynamic range. It is like a ship whose superstructure will not fit under a bridge at high tide. The distance between the water surface and the bridge is the dynamic range. Reducing the op amp gain allows the signal to enter the sound card at low tide, without very strong signals, the superstructure, clipping. If the sound card has enough gain to offset the reduction then the dynamic range is increased. But the "sensitivity" stays the same.
Using a low noise card does nothing to improve the weak signal performance at HF. It may significantly improve the large signal handling capability if used in conjunction with "lowering the tide" by reducing the op amp gain. At HF the signal to noise level is established in the atmosphere and antenna and cannot be improved by anything you do afterwards. It can be degraded by using noisy devices downstream but not improved.
Warren Allgyer
9V1TD
---In softrock40@..., <alan4alan@...> wrote :
Original Message -----
Subject: Re: [softrock40] Re: softrock40
> Sorry Alan but I have to disagree.
>
Warren,
What are you disagreeing with?
> It, of course, depends on your definition of sensitivity. When a given level of RF input produces the same level of IF output
> then, by my definition of sensitivity, the "sensitivity" of the receiver is the same on all bands. And that is the case with the
> Softrocks platform.
Yes
>
> The only gain in the Softrocks receivers is supplied by the op amps. The op amps operate over the same frequency range no matter
> what the operating frequency you choose. The only difference in "sensitivity" for a Softrocks at 28 MHz versus 1.8 MHz is the loss
> in the filters and in the mixer. And, over the range of 2-30 MHz, in a properly built RXII, that difference has been shown to be
> negligible.
>
Yes
> The attenuators are in line in the low ranges of the RXII not because the receiver gain or "sensitivity" is greater at those
> frequencies, but because high power interference is more prevalent at those frequencies. Once a high power signal is applied to
> the op amps to the point that they overload then the overall performance of the receiver is greatly degraded. That can easily
> happen from 1-10 MHz.... and rarely above.
>
No, antenna, atmospheric, noise is far greater on 160m than it is on 20m. So to increase dynamic range anttenuators are required.
And antenna, atmospheric noise is far less on 10m. There can be strong signals on any frequency.
> The fact that Roger's RXII does not produce a noise rise at 24 MHz, when his reference receiver does using the same antenna,
> indicates that the total gain of the Softrocks system is insufficient to overcome the noise of the sound card.
No. It can mean the noise of the Softrock is greater than antenna noise. At least with my Softrocks removing power reduces the
noise floor. The Softrock noise overcomes soundcard noise. Conclusion:- Antenna noise does not overcome Softrock noise.
>This has been demonstrated not to be a design problem
Has it? Where?
>so there is either too much loss in the filter/mixer
Possibly
>or insufficient gain in the op amp/PC input.
>
But people, Mobo users in particular, say that reducing op-amp gain reduces op-amp noise thereby increasing the RF
sensitivity/dynamic range with a low noise soundcard.
Or have I got that wrong?
73 Alan G4ZFQ.
Warren Allgyer