[HomeBrew] Direct conversion questions

[James Kearman]

R. Blackman wrote:
> Question 1) Does the 
> microvolt RF from the antenna actually make it through this mixer to the 
> audio amp 

Yes. In this type of receiver, all gain is provided at audio 
frequencies. As you need around 100-dB of gain, plus another 8 dB to 
make up for mixer loss, it's easy to see why DC receivers suffer from 
microphonic problems.

> Question 2) in DC designs which use the NE602 / SA612 mixer, which has gain, 
> I have not found a design yet which takes into account the 1.5K input and 
> output impedance of the SA612. Again, they show a direct connect to the 
> antenna / filter on the input and out to an audio preamp on the output.
>   

A filter can also serve as an impedance-matching network. The pi network 
in tube-type power amplifiers is an example. It matches the high 
impedance of the amplifier to a 50-ohm load, and also attenuates 
harmonics by virtue of being a low-pass filter. Some designs use a 
simple capacitive voltage divider to accomplish the transformation. I've 
seen this done by W7ZOI and K1SWL.

73,

Jim, KR1S
http://qrp.kearman.com/