On Jul 1, 8:19 am, wrote:
Noise figure and spurious responses are really determined by RX input
filters and the first stages of RF amplification. generally at VHF
and above you do not get an acceptable NF without an RF amp
for communications uses.

I was considering a Hittite HMC580ST89 InGaP HBT Gain Block
which has a NF of 2.8, covers DC - 1 GHz, and is easy to use
(i.e. can operate directly from +5V).

The mixer will produce spurs from several causes..
Dirty LO source, for that range I assume you using a carefully
designed PLL as DDS has many discrete spurs.

Currently the design calls for using a AD9952 DDS followed by
a 10.7 MHz BW=300 kHz crystal filter to provide the reference
for a ADF4002 PLL controlling a UMC VCO. ADIsimPLL claims
phase noise of -107 at 10 kHz.

-- John

On Jul 2, 8:05 am, wrote:
On Sun, 01 Jul 2007 11:48:15 -0700, wrote:
I was considering a Hittite HMC580ST89 InGaP HBT Gain Block
which has a NF of 2.8, covers DC - 1 GHz, and is easy to use
(i.e. can operate directly from +5V).

It's a nice package, I've used it. But, what are you putting before
that to prevent a Pager at 153mhz from completely overloading it?

A SAW filter.

Currently the design calls for using a AD9952 DDS followed by
a 10.7 MHz BW=300 kHz crystal filter to provide the reference
for a ADF4002 PLL controlling a UMC VCO. ADIsimPLL claims
phase noise of -107 at 10 kHz.

The LO sounds ok but the 300khz filter?? Thats FM broadcast
wide. The wider the IF the worse the system NF will be

The 300kHz filter is for the DDS output which is the reference for
the PLL so that I can vary the reference frequency enough to generate
a precise LO. It is not the filter for the IF.

-- John

On Jul 2, 9:27 pm, wrote:
A SAW filter.

Sorry I misspoke ... the SAW filter is between the LT5512 mixer and
the HMC580ST89 being used as the IF gain block.

I'm looking to use a selectable LC filter bank between the antenna
and the HMC580ST89 being used as the RF gain block.

On Jul 2, 10:35 pm, wrote:
That makes more sense. The selectable LC filters octave or fraction
of an octave?

I was looking at fraction of an octave, however I have yet to decide
anything more specific. Actually I started by playing with Ansoft
Designer to design the filters, keep being unhappy how they turned
out, and as a result keep having to increase the first IF to make it
easier to have good image rejection. I had naively started with 10.7
MHz as the first IF ... I may not have a lot accomplished, however
I'm certainly learning a thing or two. :-)

Generally it's antenna, a low loss selectivity, RFamp, Image stripping
filter then mixer.

What's the downside of doing the selectivity and image stripping as
one
filter prior to the RFamp?

I consider a RF amp at VHF/UHF of more than 1.5DB NF to be noisy.
This is because current technology for low noise amps are well under
1 DB to the 500mhz region maybe higher.

Care to mention a couple of model numbers?

-- John