Original Message -----
Subject: Re: [softrock40] Re: softrock40
The 10 ohm resistors have been discussed before. From memory I think you have covered all points:-
1) Prime purpose to swamp switch variations.
2) Forms part of the op-amp gain setting.
3) Does attenuate the RF signal. I wonder how much? I see it as being in series with the op-amp input impedance. Is that likely to be very low?
Interesting re. changing gain of the op amps. If the Rx II MDS is
limited by the noise floor of the op amps, then it is not clear to me
that changing the gain will affect this in a beneficial way. The spec
sheet for the op amp suggests to me that the circuit is well designed
and the noise is limited by the op amp itself, not by the gain resistors.
Warren and I discussed this earlier, I think without any agreement.
Afterwards I did a few practical tests on 10m after. MDS figures mean nothing to me, I do not know the noise level at the antenna:)
Softrock on dummy load raised soundcard noise floor, maybe 10dB. (EMU Asio 24 bit although lesser cards gave similar results, just a lower noise rise)
Disabling mixer and/or LO made no difference to the noise level.
On 10m connecting antenna showed no increase in noise.
I setup a battery powered generator at the bottom of the garden underneath the antenna producing a weak signal about 10dB above the noise..
I checked the SNR.
Then agsin with a 8dB preamp. This confirmed my antenna noise was low, 6dB increase in SNR.
Back to no preamp.
Reducung op-amp gain reduced SNR.
Increasing op-amp gain, no increase in SNR.
Getting the balance of gains correct is difficult. I have been told that reducing the gain of the op-amps will increase dynamic range.
On the lower bands this could be true, but it seems to me the same can be achieved by using antenna attenuators.
The more I look and read discussions I feel that the basic Softrock is not easy to improve.
73 Alan G4ZFQ
