With receiver IP3 of +15dBm this radio is a
serious instrument. It is designed for contesting, DX-ing,
experimenting. Low distortion audio, very quiet receiver with
huge dynamic range and silky smooth QSK.. With a half wave
wire and a location at the top of the hill you may have the
best receiving system at the moment.
Clean direct-conversion sound
After reading KK7B serial of articles about High-Performance
Direct-Conversion receivers I was intrigued. At a time when
the first receiver (later known as R-1) was published I was
developing a high quality transceiver for 2 meters. Long time
ago I have designed a small, qrp transceiver for 80 meters
band, with direct conversion receiver. Reading KK7B's article
about clean direct-conversion sound, "like a window on
the band" I instantly recognised the sound of my old
transceiver. I was hooked.
That feeling of "presence or transparency" was
something I was looking for, for a couple of reasons. First
- it is great listening experience, and after some time all
other radios sounds unclean, distorted. Other reason was that
I needed a low distortion receiver for a very weak signal
communications, like EME QSOs - it is very important there.
So I decided to build R-1 to check if it sounds clean like
I remembered. And it was. Very clean, beautiful. I decided
to check if it is a concept (direct-conversion) or just a
good design that sounds so great.
The best receiver
I have tried high level mixers, expensive crystal filters,
built a couple of strong audio amplifiers, played with low
noise audio preamplifiers. As a result I have made a lot of
different receivers, made a lot of measurements and spend
a great amount of time listening the bands in a quest for
a clean sounding receiver. Idea was to make a best receiver
possible, no matter how big the cost may be, just to verify
thoughts. One thing is sure - direct conversion receiver,
if well made, sounds great, but there are some problems that
cannot be solved in direct conversion concept. First of all
there is a lot of AM detection here in Europe on 7MHz in the
afternoon (I have measured 0.4 Vpp at my antenna - Radio Free
Europe, (that is -4 dBm, or S9 + 69dB, that is about 8 million
times stronger than S9 signal) and that goes to receiver.
Imagine if you had preamplifier in your receiver front end,
let's say that gain is 24 dB, at the output of the preamplifier
there will be 100 mW of power (if the preamp does not saturate
before, most preamps will). And that goes to first mixer.
And this signal is only about 200 kHz away from amateur band.
This is a real life situation, not just imaginary concept.
I do not know how to make direct conversion receiver that
can operate in such a conditions without AM detection. So,
it looks that superhet design must be used for best results.
What's wrong with superhet?
What's wrong with superhet concept? Why receiver made using
this concept does not sounds clean and nice like DC receiver?
I first thought that it was a problem with crystal filter.
Basically superhet receiver is nothing more than a DC receiver
with receive converter and a crystal filter in front of it.
If someone connect DC receiver after a high quality receive
converter this system will sounds as DC receiver alone. So
the problem must be in crystal filter. Delay on the edges
of a passband, or whatever. This is a great example how easy
is to make a mistake in a design process. The problem is not
in a crystal filter. It is possible to make superhet receiver
that will sounds exactly like DC receiver. With the feeling
of presence, like a window on the band, like a CD reproduction
from the band - all of that. The key concept here is a gain
distribution. Another important factor in a listening experience
is the AGC system. Also the quality of crystal filter. Building
superhet is easy. Easy to make it work. But to make it work
really good is another story. How to make a good superhet
is well known, and it is even explained in ARRL Handbooks
for years. The mystery remains why the good design principles
are so often overlooked, mostly by far east designers.
Carefully designed superhet (like carefully designed Direct
Conversion receiver, or carefully designed DSP receiver) will
all sounds great.
QRP fewer
There is always something mysterious in a small transmitters
for me. I have built many of them, but there is the same feeling
of mystery whenever I connect supply voltage to my TX. I connect
the dummy load, press the key and look the signal on my Tektronix
oscilloscope, but I need to touch resistors in dummy load,
to feel the heat, and mentally try to calculate output power.
And then mentally I compare it with the power of a candle.
Then I connect antenna, and make a QSO. It is always a thrill.
And then I reduce power under one watt, and call some DX.
It is not easy to describe the feeling after working some
DX (i.e. USA on 7MHz) with less than 1 W of power. After such
a QSO it is a good time to connect dummy load again, put the
full power to make TX endurance test and contemplate what
can be done better, simpler, what to do next, when is the
next contest..
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