[linrad] SDR hardware - RE: W3SZ's files

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxx>]

Hi All,

Josh, KD7HGL wrote:
 
> Now if I could just figure out what hardware to build.  There are a lot of
> ideas running around, but none of them seem to have been created
> and described from start to finnish. 
> 
> Roger has a simple design on his page, but it does not have I and Q
> outputs.  Is this a problem?  I'm also not sure if I completly understand
> the LOs and XTAL Filter.  Leif has lots of ideas but he has a lot of
> cautions with all of them.  So I don't know if any of those are really
> ready for me.  The ESS Time Machine seems to be a starting point, but to
> get to higher frequences people are rapidly making huge changes.
> 
> How have people gotten started building homebrew stuff when they don't
> know how everything (or anything) works.

The easiest way to get started is to use Linrad with any old SSB radio
you may have. (One that allows you to switch off the AGC). The frequency
range will of course be limited to 2.5kHz or so but with an FT1000 you
can get up to 6kHz (I think)

Linrad is an SDR, a software defined radio. In the future we will see
conventional receivers disappear gradually with digital designs taking
over. I have a serious concern that digital technology will degrade the
performance of amateur radio equipment because cost will drive the digital
technology too close to the antenna. Radio amateurs may learn "the hard way" 
that the new technology is no good in crowded bands despite the fact that
IP3 numbers seem very good. 

An SDR will have nearly no third order intermodulation 1dB below the 
saturation point, so the IP3 numbers may be 40dB higher than the maximum
signal the SDR can tolerate. A conventional radio that has the same IP3
value will tolerate perhaps 25dB more signal than the SDR without any 
serious problem.

This is the background for the WSE RX series. Together with the WSE 
hardware a PC running Linrad will outperform any radio receiver you can
buy for money (as far as I know). I am doing it to make it an established
fact for the future that there are no problems with the digital technology 
as such. The hardware limitations are possible to overcome if the complexity
and cost is accepted.

> I'm good with computers and software, but just starting to try to get a
> handle on radio hardware projects and there is a lot to learn.  I need a
> mentor in the Seattle area.

If you want something to start with, I recommend you to get two +7dBm
schottky mixers from Minicircuits. Mount them in separate little boxes
with two RF coaxial connectors (BNC for example) and one audio connector
on each box. You may also buy mixers from Minicircuits that are already 
supplied with connectors.

Feed the antenna to a T-connector, then feed the two mixer RF ports with
cables of equal length from this T.

Find some way to acquire a signal generator capable of giving at least
+12dBm power at 50 ohms. Feed the signal generator to another T-connector,
then route two cables from it to the LO ports of the mixers, this time 
the cables should differ by 0.25 wavelength. On the 80 meter band you 
need 13 meters of RG58 which is not a big problem. The impedance that 
the signal generator will see is 25 ohms so you have to feed a little 
more power than the nominal 10dBm to the T-connector to make up for 
the mismatch.

Likewise the antenna will see a 25 ohm load so there is a small loss of 
sensitivity that there is no reason to worry about. 

The extremely simple setup will already allow serious work! There is a 
problem however and that is overtone responses. If you try to listen at
3.5 MHz you will also hear signals at 7MHz, 10.5MHz, 14Mhz ..... These 
false responses can be supressed with an antenna tuner, or better with 
a preamplifier for the desired band. 

A preamplifier is recommended anyway because your 10dBm LO power will
leak through the mixers and reach the antenna at a level of perhaps -20dBm
which is enough to cause problems for near neighbours. A preamplifier
will attenuate the QRM you could cause to others.

The two RF connectors go directly to the RF and to the LO ports of
the mixer. You may route the audio connector directly to the IF port
but you will get a somewhat better linearity if you route the IF port
of the mixer through a 10 uH inductor to the audio connector and also
connect the IF port to ground through a series RC link. 50 ohms in
series with 10nF. The three components make the IF port see 50 ohms 
at RF frequencies which is good for IP3. The audio frequencies should 
not be loaded, there should not be any 50 ohm load at the soundcard input.

If you just can make a simple RF amplifier for your favourite HF band
you will be surprised by the excellent performance (if the signal 
generator is good). 

At HF you would want a noise figure of say 20dB.

When referenced to 50 ohms the soundcard may have a noise figure
around 35dB. Without any amplifiers at all your noise figure might
be 45 dB or 50 dB. If you add an RF amplifier with 30dB gain you will
have a receiver with "normal" sensitivity for HF bands which will
be able to handle signals up to about -30dBm. (A good radio will 
handle at least -10dBm at a frequency separation of 100kHz)

The only difficult part is to make a good LO. (Avoid that to start with)

73

Leif  /  SM5BSZ
 

LINRADDARNIL