KBa wrote in :

On 4.7.2011 7:31, clifford wright wrote:
Good day!
Can anyone point me towards a good design for a good RF stage for my
home brew HF receiver. I have no less than 6 others hams in my small
town and one within 400 metres, so I need good strong signal
performance. 10 to 15 dB gain would be ample, but I would like to
have 50 Ohm input and output impeadances.
The receiver is an upconverter to 45MHz with a 15 kHz bandwidth xtal
filter followed by a downconverting mixer to either 9MHz or 10.7 MHz
with separate IF strips.
Tuning is by a PA0KLT synthesiser.
I have been a bit out of touch lately and am not very up to date with
the latest MOSFETS etc. The mixers use the 1992 Ulrich Rohde FET
double balanced mixer circuit.
The rest of the sytem was built back in the early 1990's but put to
one side until a better frequency sythesiser was available.
Now it looks like the time to get things moving again!
Regards Cliff Wright ZL1BDA ex G3NIA.

Hello

Some late thoughts about amplifier candidate...
One simple but low cost one could be push-pull Jfet amplifier using
two J309 in parallel, gate grounded. Input can matched to 50 ohm using
1:0.7 input transformer ( fets do have lower input impedance than 50
ohm ) and output at drains can be 4:1 balanced transformer.
Spice analysis shows around 9dB gain and very good wide-band input
matching, probably noise figure is also not much greater than one fet
alone, actually it should be lower. One thing needed is quite equal
fets, Idss and Vp, four matched fets should be still easy to find.
Sorry don't have more exact figures to give, but this could be one
candidate to evaluate.

br Kari B
Oh6io




Just a quick follow up on the RF amplifier question. Yes I do need a bit
of gain (say 12 dB) before the mixer. One reason is that for many years I
have experimented with low angle radiation on 14 MHz cw. Signals are
often quite weak from the antipodes of NZ (the western mediterranean
area) and I often work them when the band appears dead around local noon.
Fortunately my neighbouring ham is not very active these days.
Anyway for now I am using a cascode JFET amplifier with a relatively high
supply current and this seems adequate. However applying AGC is a bit of
a problem.
Cliff Wright ZL1BDA