Original Message -----
Subject: Re: [softrock40] Re: softrock40
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.
Warren,
Assuming op-amp noise is the problem then this noise is the limiting factor.
Roger has said "So, the SR output noise level is 10 dB above my sound card¡¯s noise floor." This is not antenna noise, seems to
confirm the op-amp noise is more than the soundcard noise.
Your advice to increase sensitivity by increasing soundcard gain will not work if antenna noise is covered by op-amp noise. Provided the antenna or Softrock noise is seen any increase in soundcard gain just reduces dynamic range.
If reducing the gain of the op-amps reduces their noise and if that noise was more than antenna noise then there is actually an
increase in sensitivity. Up to 10dB might be possible using the spare 10dB but only if the op-amp noise is reduced by 10dB and it is the only factor
involved. .
Or, if quieter opamps were used then there would again be increased sensitivity for the same gain.
I guess this would be true even if there was significant loss in the front end, providing the front end is not contributing noise.
Roger has said that when conditions are quiet he sees no antenna noise. This is what I find. On 12m and 10m a Norton 8dB antenna preamp rectifies this situation, it may not be the best solution but I think it the easiest I do not always use low noise soundcards so I have not fully investigated reducing op-amp gain.
I am not talking about improving on the SNR seen at the antenna terminals.
But if this is degraded by a noisy device or attenuation before the first device this can be rectified.
No doubt there is an ideal noise figure (or MDS) for a receiver on 10m. I do not think my Softrocks reach that figure.
Anyone testing at this time in the sunspot cycle should be wary, I expect there is a high natural noise level there at times.
73 Alan G4ZFQ
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.
