**Comparison: Ramsey SR-1 vs. Homebrew SW**
Tony Zugec
Hello there.
I had earlier posted an article regarding the Ramsey SR-1
shortwave kit.
I had started compiling projects I was interested in using
active mixers and many different unique schemes for ham and
shortwave receivers.
Not having any prior experience with NE602/612 mixers, I
thought it would be interesting to see how the Ramsey kit
performed to give me an idea of the performance simple
direct conversion and superhet circuits I should expect.
After completing the SR-1, I found the sensivitity to
be good, selectivity to be lousy and the stability....
well, let's just say there is no stability. I thought
perhaps I had a bad component, but after much surgery
found the NE602 and the rest of the rf deck to be
fine. I got some great feedback from others who have
built this unit and this is the expected performance
for the SR-1. This unit uses Varactor tuning and I
think one person remarked that they may have stretched
the capabilities of the MVAM108 varactor a bit which
is contributing to the instability.
The SR-1 is a 'superhet' but does it in a way in which
the local oscillator sweeps across a front-end bandpass
of about 2.5 MHz. You can select any 2.5 MHz portion
of 4 to 10 MHz. The IF is 260 Khz, and you'll hear
the same station at the local osc +/-260 Khz which
is annoying. The local osc circuit which is part
of the NE602 appears to 'pull' when listening to
a strong broadcast station which is extrememly annoying,
and the stability really SUCKS! as evidenced by any
off it later.
What I did then was build a receiver using an NE612
and used a idea I found in an article in a magazine
from the early 90's that used two MV209 varactors in
a superhet configuration so that the local oscillator
tracks the front end with an IF of 455 Khz. I got
hold of a Toko ceramic filter to hang of the output
of the NE612 and changed some inductance values a bit
so I could lurk around about the 9 MHz shortwave band.
The usual LM386 completed the audio circuit. Used a
couple of 2N3904 BJT's in an RF amp config to get
the ceramic filter output to a germanium diode AM
detector. Found an AGC circuit and put that in,
I still want to add an S-meter and BFO but I haven't
gotten the BFO to oscillate yet. The difference
in performance in incredible. I have great sensitivity
with about 50 feet of wire, great selectivity between
the SW broadcast stations and it is VERY stable with
the addition of NP0 caps for the oscillator section.
I now have a nice little radio that is about 100 times
better than the Ramsey kit.
I guess the bottom line is, if you don't have the capability,
tools and instruments to design and build your own simple
superhet, the Ramsey will do....especially since beginners
don't ..so the kit does have some redeeming value. Maybe some
kid will decide this stuff is very interesting indeed and
go on to better things.
It certainly was interesting comparing the different
superhet schemes and the resulting difference in performance.
My next project is going to be another Shortwave receiver
using other types of active mixers, possibly mosfets etc..
and expanded band coverage... but first I have to get that
BFO working!!!!
See ya,
Your pal Tony