When the band is open with lots of strong signals you don't want a
preamp, in fact you might want to switch in an attenuator to reduce the
signal strength into the first mixer. The K3 gives the user the option
of using a preamp or an attenuator (or BOTH(!) since the preamp has more
gain than the attenuator has attenuation so using both just gives less
gain). You also have the option of an IF preamp and two gain settings
for it.

In the Picstar there is a preamp using 4 parallel jfets that can be
switched as an RF preamp or an IF preamp. Since the Picastar's first
mixer is used on transmit and receive the preamp is used as an IF preamp
(actually postamp) on transmit. The use of the preamp in the receiver
for RF gives the best NF, while in the IF stage it gives the best IP.
The K3 uses a bipolar transistor for the preamp stage and there can be
some debate as to which type of preamp is actually better (probably the
bipolar gives the best IP and the parallel fet gives the best NF). I
think the fet preamp is preferred on the east side of the 'pond' and the
bipolar is favored on the west side.

Other than saving some parts (but costing something for switches) using
the same mixer bilaterally is probably not the greatest idea.
Personally, I would use the H-Mode mixer for the receiver and provide
the option to switch in a preamp and/or attenuator before the mixer, and
an IF preamp after. The RF preamp would use a 2n3886 or 2n5109 bipolar
transistor running with as high a collector current as possible for the
best IP. The IF preamp might use the parallel fets.
For the transmit mixer I'd use one of the MCL canned diode mixers with a
bipolar preamp. The transmit mixer doesn't need to be a high IP type
mixer, we can control the signal levels here to get the best results and
distribute the power gain in previous and prior stages as required.


On 07/06/2011 07:07 AM, Gian, I7SWX wrote:
On Jul 4, 6:31 am, clifford 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
Hi Cliff,

Piero and Tom gave you good comments.

If you need a strong front end your problem is not in the rf
amplifier. You need a strong 1st and 2nd mixer and for this there is
no better solution than the H-Mode Mixer. Having a una upconversion
1st IF, your critical point will be the 2nd mixer that will receive
strong signal amplified by this stage..

If you look at PA3AKE web page you con find info regarding my
simplified version I7SWX 2T H-Mode Mixer (http://
martein.home.xs4all.nl/pa3ake/hmode/2t-hmode.html). This has only 2
transformers and homebrewed, not purchased. You will see the info on
transformers winding, the one with 5 windings may seem difficult but
is not. In low bands you may avoid to have a preamp. Look at the G3XJP
project Pic-A-Star (RSGB RadCom and also reported in the Handbook).

73

Gian
I7SWX

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