--- In BITX20@..., Rahul Srivastava <vu3wjm@y...> wrote:
Hi!issue arises only due to varying characteristics of same devices
Even within the USA there are poor spec parts problems and
Where noise figure is concerned under ideal components there is alimitation to what we can get in a negative feedback config. BITX
nature and paralleling of the amps further effects the feedback and
consequently affects the noise figure on the whole.
Very true, for the IF stages the is not a big problem unless poor
devices are used. The Q1/Q13 stage has more than adaquate noise
figure for 20m or lower HF bands. For 15/17/12/10M bands I'd
want a better device than the 2n3904/2n2222 in the recieve path
a 2sc2570 or 2n5179 would do well there. At VHF we want better
as the band noise is much lower.
Another device that needs some attention is mixer driver Q7. At
above 25mhz the 2n3904 is getting near it's limits though a
2n4124 worked well at 41mhz.
Where BJT preamps are concerned a grounded base cofig providesbetter performance though at a lesser gain. A Norton noiseless preamp
provides very low NF probably the best in BJT. Even the Icom IC7800
Grounded base BJT is stable but low gain and generally requires
a propper input match for good noise performance. I've looked
at a Norton amp as they are quieter for the same BJT
though they tend to run lower gain. I can get a U310 common gate
fet to a below 1db noise figure and 12db gain with less effort.
However I'd need two stages to overcome the DBM and image filter
losses. Based on earlier work with DBMs at 6m if I want a good NF
and MDS a dual gate MOSFET with around 20db gain is successful.
The idea is enough RF gain to overcome the image filter and DBM
losses (typically -10 to -12DB) with a little extra. The only
problem is that is a more difficult amplifier due to high gain.
The positive is the IP for a dual gate mosfet is very good for the
Maybe we ought to look at the unique switching arrangement used inRU design Desna already under files for improved NF.
I am considering splitting the path at the mixer with a optimized RX
path to mirror the TX. It's a trivial thing to use a diode switch
at that point. Building a second filter for the RX is trivial.
Other considerations is balancing power consumption as this is for
portable ops and making fit the intended package. That and keeping
it simple to reproduce at least in part. The PLL I use may be a
parts procurement problem for some.