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

I would like to build a receiver so I'm going to ask some more questions
about how all of these pieces fit together.

On Wed, 16 Oct 2002, Leif Asbrink wrote:

> 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.

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...

The mixers with connectors seem rather expensive at $43+ each...

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

OK, this makes since.
 
> 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.

Still following you.

> 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. 
> 
[SNIP info about preamplifiers]
> 
> 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

> 
> 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?

> 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 and you changed the circuit
ON the IF to the sound card a little.

> 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?

What type of wave should the signal generator have?  Square, sine?  I'm
not sure what to look for...

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?

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.

73, KD7HGL

							Later, JOSH
LINRADDARNIL
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