Topband: Comments on R4C from Sherwood Engineering

[Tom Rauch]

The R4C suffers two major problems:
 
> mods. To make a long story short, the main problem with the R4C was
> layout! I used a cheap and dirty method to test, a borrowed sig gen
> cranked up 1 kc from rx freq and "cut and pasted" piece of double sided
> copper circuit board inside the rx to lower the leakage. When I was done,
> the rx weighed a few lbs heavier, but was much improved. 

That's one.

The other is the silly injection scheme where a 50 kHz oscillator 
(an oscillator well below the first and second IF frequencies) is 
injected into a mixer.

That ill-conceived mixing scheme prevents driving the mixer into 
switching from the local 50 kHz oscillator, and greatly reduces 
mixer strong signal performance. If you inject the mixer properly, 
the harmonics generated from the 50 kHz signal cause multiples 
birdies as the passband tuning is changed!

What I did was convert the 50 kHz BFO up to a frequency above 
5.6 MHz, and re-inject the mixer with tons of local oscillator signal. 
Since the oscillator is above the signal frequency, all harmonics fall 
outside of the narrow IF response and no birdies appear.  

The simple Sherwood fix is to install very narrow filters in the first 
IF. That method cleans up both the filter blow-by and poor second 
mixer strong signal performance.

73, Tom W8JI
w8ji@contesting.com



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