RE:A simple front end

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

> I am keen to experiment with the "very easy way to get 40 kHz of 
> radio spectrum into a PC" that Leif described in Fig 2 of the Jan/Feb 
> 2003 QEX. My interest is only the cw end of 20 meters
> For an RF stage I plan to use a Mini-Circuits ERA-5 amplifier with a 
> band-pass filter ahead of it. Instead of the quarter wave of coax the 
> SPH-16 voltage controlled phase shifter looks like the way to go 
> unless I can find a cheaper solution.
Cheapest is to use an inductor and a capacitor to provide
plus 45 degrees and minus 45 degrees.

Like the RX2500:
http://antennspecialisten.se/~sm5bsz/linuxdsp/rxiq/mixer.htm
C111 and L22 in fig 1 serve this purpose. R141 and R133 serve the
purpose of setting the impedance to 100 ohms at the input of each 
mixer. When the frequency is increased, the C111,R133 phase shift 
decreases while the L22,R141 phase shift increases which keeps
the relative phase shift at 90 degrees. The amplitude ratio changes 
slightly but the circuit is reasonably wide in bandwidth.

> I looked for low noise audio 
> ICs that were not surface mount and the best I can find is the LM833.
I do not have data for it so I can not comment.

> Initially I will use a signal generator for the LO but a crystal osc 
> would make it a self-contained front end that could be packaged in 
> quite a small box.
A signal generator is probably a bit noisy. A crystal oscillator
will improve performance. My 14 MHz simple receiver has a local 
oscillator at 4 times higher frequency with a divide by 4 that provides
the 90 degree phase shift. Another easy way to go.

73

Leif / SM5BSZ



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