I'd suggest the following test:
1. with the current circuitry, measure the background noise floor on a
quiet day with little QRN (ie. not now ;-)
2. temporarily disable the BF981 stage, by disconnecting the supply, or
by pulling down the gate-2 bias. Measure the noise again, which now
comes only from the backend.
If the noise level has gone down by more than 27 dB, you can spare the
~ 17 dB gain stage and still be sure that the antenna noise dominates
10x over that of the backend.
If the difference is too small, you can try to reduce the ADC noise
figure by using a higher gain setting in the soundcard mixer, and
repeat the test.
Von: Rik Strobbe <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Mi, 20 Mai 2015 7:38 am
Betreff: LF: RE: FZ-02-B mechanical filter input matching
Now my question: Is it better to remove a BF981 amp stage (about 16 dB
to 18 dB gain!) in front of the mixer (SBL-3) to get a lower RX noise
or do i better reduce the gain on the LT1028 stage on the AF side? The
AF pass band is 11...18 kHz (LO= 461 kHz)
On MF one must have a very small RX antenna before the internal RX
noise becomes dominant to the external noise.
Using my 475kHz antenna (lazy-L 12m high, 25m long) and a Kenwood TS440
as RX I can attenuate the antenna signal by 40dB or more before the
antenna noise drops below the RX noise.
So I think that in regard with SNR it will not really matter wether you
remove the preamp of reduce gain in the LF stage.
73, Rik ON7YD - OR7T