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

Hello Josh,
> I would like to build a receiver so I'm going to ask some more questions
> about how all of these pieces fit together.
> OK, so I want to order some mixers.  Minicircuits does not seem to accept
> orders, but DEM has a fair amount of mixers available.
> http://www.downeastmicrowave.com/Catalog.htm#filt  (scroll up a tiny bit)
> SBL-1X 1GHz max., 7dBm LO mixer 			8.50
> SYM-14H 100-1370MHz mixer, +17dBm LO, Surface Mount 	15.00
> TUF-1 500MHz mixer, small package 			5.50
> TUF-1H 500MHz, 17dBm LO mixer, small package 		12.00
> TUF-2HSM 1GHz, +17dBM LO Surface Mount Mixer 		14.00
> TUF-5SM 1.5 GHz +7dBm LO 				12.00
> LRFMS-4 Surface mount mixer, 1.9 Ghz, +7dBm LO 		12.00
> ADE-3G Surface mount mixer, 2.3-2.7GHz, +7dBM LO 	6.00
> Would you suggest a TUF-1, TUF-1H or SBL-1X as a starting point?
> Or a different vendor...
I suggest a pair of TUF-1

> > 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.
> So how does this look.
>       --- (RF)MIXER(IF) --- I --- Sound Card (I)
>       |	      (LO)	  |
>       |		|	  R
> RF ---|   LO----|	  C - Ground
>       |		|(1/4WL)
>       |	      (LO)	  
>       --- (RF)MIXER(IF) --- I --- Sound Card (Q)
> 			  |
> 			  R
> 			  C - Ground

With spacebar and no TAB:
(I was not sure I meant T-junction or "inductor")

       --- (RF)MIXER(IF) ----|---------L----- Sound Card (I)
       |       (LO)          |
       |         |	         R
 RF ---|   LO----|	         C - Ground
       |         |(1/4WL)
       |        (LO)	  
       --- (RF)MIXER(IF) ----|---------L------ Sound Card (Q)
                             C - Ground
> > 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.
> OK, when you say RF Amplifier here you are taling about a preamp again?

> Initially how critical is this?
Depends on the antenna and frequency band. As you see below, you will
get a noise figure around 45 dB so you will have a sensitivity
comparable to what a normal radio gives if you add a 30dB attenuator
in front of it. If you have no filter you will receive all overtones
so there may be a lot of QRM from other bands. Finally there is a risk
that the LO leakage will cause QRM to fellow hams.
> > 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)
> I think what you described is very similar to your webpage
> http://ham.te.hik.se/homepage/sm5bsz/linuxdsp/hware/sbl1.htm
> but you moved the 1/4 WL of coax to the LO 
Correct. It is actually a little better to put the 1/4wl at the LO
side, but then you have to worry about the input impedance at the RF
port. (The 100 ohm resistor on the delayed side) This is trivial
because Linrad will work as an oscilloscope and you can adjust the 
resistor for the same signal level in I and Q.

If the 0.25 wl delay is on the LO side, the different levels you
get in the LO drive levels will have a small effect only so there is
no need to worry about compensation for it.

You may easily move the cables around and investigate for yourself.

> and you changed the circuit ON the IF to the sound card a little.
Yes. Use either of the solutions - or just a straight connection to start with.

> > The only difficult part is to make a good LO. (Avoid that to start with)
> Have you look at or found any chips that give you both the LO and the 90
> Deg LO so you don't need ot use coax?
The easy solution is to use a 74AC74 to divide by four. Thus you will
have the desired LO frequenc?and it's fourth overtone. Use the overtone
to clock the LO through a second 74AC74 to get a 90 degree phase shift.

> What type of wave should the signal generator have?  Square, sine?  I'm
> not sure what to look for...
Does not matter much. The LO port is driven into saturation anyway
so a square wave is perfectly OK (might even be better, I do not know)
The only disadvantage is that the overtone responses will reach higher. 
You may get sensitivity for 88 to 108MHz signals if they are not stopped
by an RF amplifier or a filter.

> My current plan was to make a DDS VFO like
> http://www.qsl.net/yo5ofh/projects/dds_vfo_by_n2apb/dds_vfo.htm
> or 
> http://www.netppl.fi/~jonverro/ad9854.htm
> Which has Quadrature outputs so I should not need a 1/4 WL of coax
> for each frequency.  But there is a problem:  "The analog outputs are
> -80...-10 dBm. An amplifier stage is needed to drive a standard level
> diode mixer as an RX LO."  How do you amplify the signals?
I suggest you start by getting something that was intended for the lab bench.
Any signal generator that appears to have a good tone when you listen to it
on a conventional receiver. Try to borrow it somewhere or look 
for it at ham-fests. Something old with vacuum tubes is a good idea;-)

Use the 1/4wl lines to get your first experiences:-)

My own experience with DDS VFOs is not encouraging. There have been
too many, too strong spurs.

When there is no antenna connected you should notice no difference
when connecting the mixer to the soundcard. Try without LO signal
first, then with LO switched on. A poor LO will give spurs and an
increased noise floor.

To get a really good radio, use an X-tal oscillator for the LO. You
may also use any amateur transceiver that can transmit a CW carrier
at a low power level (anything that would fit a transverter). Use 
an attenuator to make the power level fit the mixers.

> Thanks again for everyone input on this.  I'm sure I'll eventually be able
> to answer these questions at this rate!  And I'll have my own pile of
> pieces to experment with.
> BTW, my email is josh@xxxxxxxxxxxx if the list program hides addresses.
OK. I think others might be interested so I sent this on the list.


Leif / SM5BSZ