[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [linrad] RE: Hardware

I send this to the group in case you find it useful information:

On 9 Jan 2003 at 16:10, Leif Asbrink wrote:

> Hi Jim and All,

>>   I do not fully understand why
> > so many conversion steps are recommended. I can see the difficultly
> > for spurs if frequencies are not carefully chosen.

> That is not the reason. If you want 144MHz, low end only, a good
> solution would be to use a couple of X-tal oscillators to convert
> directly to 2.5 MHz. If you start with 15.722MHz and add two triplers
> to get 141.5 you will easily be able to pull the crystal 50 or 100kHz
> on 144MHz. The IC202 uses this method to pull 200kHz, but at some
> stage there will be a loss of Q causing too much sideband noise. If
> you are happy with coverage from 144.000 to 144.400 this will give
> very good performance at low cost with easily made hardware.

My IC202 is grossly unstable. I think it would be a poor choice to use a 
VCXO that drifts and provides frequency uncertainty in an otherwise
top notch receiver. My plan would probably sacrific a bit of phase noise
performance for frequency agility. I can produce a signal by multipliying
a temperature compensated channel element from a Motorola Micor operating
at 12.945 Mhz. It would be multiplied by a modified Motorola Micor VHF exciter by
a factor of 9 to 116.505 Mhz. The 25 Mhz PLL signal is mixed with the 116.505 Mhz frequency
to produce the needed 141.5 Mhz LO. I do not own a decent RF signal generator.
A TUF1H would be used to convert 144 Mhz to 2.5 Mhz directly.  I own a total of
4 , 5 pole helical VHF filters robbed from Motorola Micors. I currently use two 
after my antenna mounted pre-amps, one for Vertical and one for Horizontal 
polarization. I could add the other two if needed for additional selectivity.
The intermod study for this set of frequencies suggest that some intermod
from the 146 - 147 Mhz range may be possible, but most combinations include
2.5 Mhz as an input, suggesting that a strong signal must exist inside my passband.
I will probably not have this during normal EME. My 5 pole helical filters provide
the following bandwiths:

-3db 1.2 Mhz
-30 db 4 Mhz
-40 db 6 Mhz
-50 db 8 Mhz

As you can see, my image at 139 Mhz will be 50 db down. Around here there
are nothing but airplanes in that frequency range. If need be I can add the second
5 pole filter and the image becomes -100db!
You can also see that the FM clutter in the 146 and 147 Mhz will be -30db and -40 db
respectively. This would also double if the second 5 pole filter is added.
The intermod study shows also that there are no additional spurs caused by using
25 and 116.505 Mhz to make the LO at 141.5. By the way, that should be 141.55 Mhz
L.O.  if the middle of the Linrad window is to be at 144.050 Mhz? That would cover the
random range nicely with 96KHZ bw.

> You are absolutely right in that the many conversions makes my
> receiver system rather complicated. I am not worried about the
> complexity because the individual boxes are so well isolated
> from each other and each unit performs only a single and well
> defined task. 

Each additional box adds cost and frequency drift in the best case and
additional IMD and noise in the worse case, but they can enhance 
image rejection and Spur rejection. I like KISS in this case. My ability
to mess up an otherwise excellent receiver is just too great.

> With simple mixers like dual gate MOSFETS these spurs would be a
> serious problem. When going directly from 144 MHz to 2.5 MHz
> even a dual gate MOSFET will produce good performance since
> the LO to RF tatio is so close to 1.00.

The TUF1H mixer should be a good choice for this then.
I am not aware of an optimal LO to RF ratio, but I am guessing
it relates to the possiblility of receiving spurs.

On single conversion to 2.5 Mhz.
> OK. But image rejection will probably not be quite as good as I 
> like to have it;) I try to make all false responses well below
> -140dBc/Hz for a single strong interference signal.

-50 db for a single 5 pole filter or -100 db if I cascade two of them.
An airplane here will never generate a signal equal to a fixed station
just down the street in this location.

> > Is an LNA at 2.5 Mhz helpful here to keep the mixers 
> > after my front end out of trouble? 

> Yes. You should then run the Delta44 at minimum gain.
> To not loose more than 1dB by an elevated noise floor,
> considering the input noise temp of RX2500, 16dBNF => 
> 12000K the output temp of your 2.5MHz LNA should be
> no more than 4000K so with a NF of 2dB the gain should
> be no more than 8dB with a 1dB saturation somewhere
> around +18dBm.

Is there a 2.5 Mhz LNA circuit I can copy? I am not sure that
you said how much front end gain is needed. I guess the TUF1H
adds 6db of noise figure, so if an LNA at 2.5 Mhz adds 2db
of noise figure then 8db of noise figure has to be overcome
at 144 Mhz. This 2.5 Mhz LNA is probably something I can
add later.

> > I suppose that selectivity between mixers is helpful too...?
> ?????
> If you go from 144 to 2.5 ?

No, I was talking about adding another conversion to an IF somewhere between
10 and 30 Mhz. This IF could have added selectivity within the limits set by the
need to QSY to a new window. It is looking now like single conversion will be 
sufficient, provided my 25 Mhz PLL does not add too much phase noise.
I wonder, since I am not changing the PLL to a new frequency to switch
from TX to RX, as it does in a CB, if I should experiment with loop filtering
time constants to reduce PLL phase noise at a later time? I guess Linrad
can be used as the tool to optimize (minimize) phase noise?

> > Per Conrad, it looks like my Pentium 475 Mhz computer will not be
> > fast enough for proper operation at 96 KHZ bandwidth. I will
> > continue my search for a capable Linrad PC.

> Do you have a cross yagi antenna?

Yes, my 4 X UWA12's are now an Xpol version. I currently have
45 degree steps for receive.

 For a single polarised antenna
> 475 MHz may well be enough. With an X-yagi you can start by
> using 48kHz sampling. You will then get alias spurs somewhere
> around 100dB down but the 40 to 45 kHz useful bandwidth is
> quite a lot for 144MHz EME.

Considering the relatively large cost of the Delta 44 card and the RX2500,
it does not seem that expensive to have a proper PC to do the full 96 KHZ
bandwidth. I do not wish to make an early investment obsolete. Better to
plan for the long run and not waste money in the short run. 

> 73
> Leif  /  SM5BSZ

73, Jim Shaffer, WB9UWA.