On Jul 3, 9:31*pm, 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.

Ah, one of my favorite subjects...

You don't mention any specific numbers you're trying to reach...

I've been working on something where I need a modest-gain HF amplifier
with good intermod performance. I have a couple of Spectrum Microwave
amplifiers that have about 20dB gain, nice noise figure around 3dB,
and about +55dBm TOI. But...they cost close to $1000 each, and need
400mA at 24V--about ten watts! That's fine for testing, but not to
actually use in the circuit, both the cost and the power dissipation.

For about a watt dissipation, I can get similar gain and almost as
good TOI--but poorer NF--using a couple op amp packages, the first op
amp to get the gain and the second to buffer the output and drive 50
ohms. The NF in the mid-teens is still good enough for HF work,
especially when faced with huge signals. If you look at differential
op amps designed to drive high speed ADCs, you'll find some pretty
nice third order distortion numbers. Parts from Analog Devices,
Linear Technology and Texas Instruments all come to mind.

As Piero suggests, you need to pay attention to the mixers to get
similarly excellent performance there -- and even to the crystal
filters, to be sure that they are up to the task of keeping distortion
low.

Also, I've built quite a few preselection filters for HF, and after
trying lots of different types of inductors (ferrite and powdered iron
toroid core, and magnetic-cored solenoid coils), decided that the only
coils I could use and guarantee adequately low distortion were ones
with non-magnetic cores...nominally "air" core coils. Unfortunately,
they are not as nice about rejecting external fields as the toroids,
so I've had to be careful about shielding.

I also spent quite a bit of time looking for switches with adequately
low distortion. I needed to switch filters quickly and potentially
millions of times, so I looked at every electronic switch I could
find. None of them was particularly good at HF. I found some tiny
reed relays, and they are OK, but even they contribute significant
distortion--barely good enough for what I was doing. Thankfully, they
last pretty much forever when switching RF at less than perhaps
+10dBm. RF armature relays that I've tested are all pretty darned
good--but have finite life.

Summary: if you want to be serious about low distortion, you need to
look at pretty much every part of the circuit. That includes asking
questions like, "Do I really need that RF amplifier, or is the NF
going straight into the mixer good enough?" and, "Can I rearrange the
circuit blocks to get even better performance?"

Cheers,
Tom