EMRFD Message Archive 353

Message Date From Subject
353 2007-01-18 06:17:28 neomag_magneo Receiver noise figure for the top band and below
Hi all,

After studying the mixer chapter of EMRFD (and the concepts applied to
high performance receivers) I have understood, that the most crucial
part is the matching between the first mixer and the crystal filter.
Normal procedure seems to be a strong (high input/output intercept
amplifier) with the exception of the TRIAD receiver which utilizes a
special roofing filter with contant input impedance. With all these a
reasonably low noise figure together with good strong signal
performance can be achieved. When trying to simplify any design the
amplifier can be eliminated but according to W7ZOI, this would degrade
the noise performance excessively (again the TRIAD approach as an
exception). In my opinion, the amplifier may not be necessary for
frequencies below 3 MHz, where the atmospheric and other noise levels
are high and terminating the mixer with attenuator pad (6 - 10 dB)
would still allow a NF abt. 20 dB or so. Is this correct or have I
missed something ?

The background here is my intention to construct a high performance BC
receiver, mainly for the AM band (500 - 1500 kHz).


Heikki (OH2LZI)
355 2007-01-18 10:12:26 Wes Hayward Re: Receiver noise figure for the top band and below
Hi Heikki, and group,

Your thinking certainly seems to be correct from my viewpoint.
This is what folks do for spectrum analyzers as well as
instrumentation, or measurement receivers. A pad is a really
wonderful circuit, going well beyond what we would normally attribute
to three resistors.

The one place where you might want low noise figure for your BC band
receiver is when working with some special antennas. A lot of the
interesting antennas that are used on 160 M and
359 2007-01-19 07:08:37 w4zcb77 Re: Receiver noise figure for the top band and below
I must admit that I cannot now do any of these
> experiments. The only thing I have owned along these lines was a
> Yaesu FRG-100 receiver. But I sold back at about 2002 or so.
> Perhaps it is time to consider a purchase. (Did I type/say/think
> that?)
> >
> 73, Wes
> w7zoi

Good Grief! Did I just feel the Earth shudder in its rotation? NOT
allowed Wes.

Heikki, if you live anywhere within propagation distance of the Gulf
of Mexico, a receiver with a 30 dB noise figure is more than enough
for anything below 2 MHz and often 4 unless you're trying to get by
with a wet noodle for an antenna.

We have all those things mentioned, in the PicaStar transceiver.
Peter Rhodes interpretation of what a real radio should act like. A
switcheable post mixer feedback amplifier when in best IMD mode, and
electronically switched to a preamp when used in best noise figure
mode. Additionally, a DSP function known as "Condition Sensitivity"
where the gain is automatically reduced by digital AGC to whatever
the QRN demands. Makes a QSO in a summer thunderstorm environment
yield a whole new experience. Your BC receiver will really enjoy the
benefits available from DSP. Atmospheric noise reduction with either
a de-noiser algorithm or a noise blanker, (or both) auto notch of
heterodynes, a digital gain management system, tailorable bandwidth
on the fly, it's a whole new paradigm out there. Not so new either,
getting to be old hat although the best of DSP is in the programmer
not the hardware. I've used radios that have multiple processors
that don't control conditions anywhere near as well as a less
powerful processor progged by someone who understands what is needed
and how to provide it. DRM is just around the corner, you should
have the ability to decode that as well in your BC radio and DSP is
the only way you can do that.