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Receiver Myths; DC and Superhet (long)

from [James R. Duffey]

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To:	"Low Power Amateur Radio Discussion" <qrp-l@Lehigh.EDU>
Subject:	Receiver Myths; DC and Superhet (long)
From:	"James R. Duffey" <jamesd1@flash.net>
Date:	Sat, 22 Nov 97 12:04:26 -0700

The recent discussions on Direct Conversion Receivers vs. 
Superheterodynes prompted by the calls for more homebrewing evident in 
Milton's "Paradise Lost", and reinforced by Calvin's admonishment of kit 
building as an alternative to homebrewing (thereby putting ham radio 
building in the same class as other self indulgent sins such as drinking, 
card playing, gluttony and dancing :^)) have resulted in some 
misconceptions or misstatements (at least in my opinion) that I would 
like to clear up. Some of these have already been addressed so I won't 
repeat those here. I don't mean to flame anybody, so if you made a 
statement I criticize don't take it personally. Some of these same 
statements appear in the reputable literature as well.

In no particular order, some misconceptions that I would like to address;

1. "A direct conversion receiver is really just a simple superheterodyne."

I may be picking nits here, but statements like this show us how far 
removed we are from the beginnings of radio. Superheterodyne is a 
shortened form of Supersonicheterodyne. (You don't really need a radio 
history book to tell you this as it is in any good dictionary) The 
inventor of the superheterodyne receiver, Armstrong, coined this term for 
his newly invented reveiver having one or more intermediate conversion 
and gain stages between RF and AF. These conversion stages were above the 
audio or sonic range hence the use of the word supersonic. Of course, the 
direct conversion receiver as we know it today, and the regenerative 
receivers of Armstrong's day convert RF to AF directly at sonic (audio) 
frequiencies. Armstrong realized the advantages of distributing the gain 
and selectivity over several frequencies. This distribution was 
particularly important for the active devices of the day which were 
typically unstable at moderate to high gains and tended to oscillate with 
narrow bandwidths. As is evidenced by the good performance of receivers 
such as the R-2 this is less important today.

But another advantage is that it is the dual conversion stage that allows 
the opposite sideband to be eliminated easily with a narrow filter at a 
frequency above the frequency used for detection. Superhets don't lose 
the capability to listen on the opposite sideband, my old HQ-110A had 
this capability with a variable BFO, and my new TS850 has a switch to 
reverse sidebands.

The correct statement is of course "The direct conversion receiver and 
superheterodyne are both utilize the heterodyne principal." Most 
superheterodynes that use product detectors look like a direct ocnversion 
receiver form the product detector on  through the AF stage. To say that 
the direct conversion receiver is really just a simple superheterodyne is 
really a technical oxymoron (look it up, it is not offensive).

2. "DC receivers have superior audio." The quality of a receiver's audio 
is not a function of the type of receiver, but rather how the audio 
stages are implemented. Most DC receivers have no AGC, have active audio 
filtering to eliminate high frequency hiss and have good single ended 
class A audio amplifiers. Many superhets are missing one or more of these 
items. If you want to make your superhet sound better turn off the AGC, 
reduce the RF gain so you just barely hear the noise level, and use the 
audio level to adjust gain. You will find many of the advantages of the 
DC receiver's audio can be had in a superhet. Loud signals will be loud 
and stand out against a nearly quite band, weak signals are weak and fade 
in and out of the noise, and there will be no pumping of audio levels 
from nearby strong signals.

3. "DC receivers require lots of gain" The DC and superhet both need the 
same end to end gain to make a given level RF signal input appear at the 
same given level at the AF output. The configuration, DC or superhet, has 
little to do with this gain, at least on the HF Ham bands. (More on that 
later). That is if there is a 1 microvolt signal at the input and we want 
a 1 volt signal at the audio output, we need a gain of 1,000,000, or 120 
dB (volts). The difference is where the gain occurs. In a direct 
conversion receiver the gain must occur in the RF and AF. The superhet 
introduces a third stage, the intermediate frequency (IF) amplifier, 
where we can place the gain.  

Examples; If we consider the ubiquitous NE602 in a DC receiver, it has 
about 17 dB gain or so. In the above example we must generate another 103 
dB gain in the AF stage to get to our 120 dB. If we make a superhet out 
of an NE602 and a MC1350 IF amplifier, we get 17 dB in the RF stage, 
another 50 dB in the MC1350 IF stage, 17 dB in the NE602 product 
detector, which leaves 36 dB for the audio stage. It is possible to leave 
out gain at the IF as Dave Benson has done in his SWL receivers; then we 
get 17 dB for the NE602, another 17 dB for the NE602 product detector, 
and then the audio stage needs to generate 86 dB gain. These are all 
realizable with parts which are commonly available today. If we insist on 
good front end performance and go to a passive diode double balanced 
mixer, then instead of 17 dB gain in the NE602 mixers, we have a 7 dB or 
so loss. This needs to be made up by an additional RF gain stage in front 
of the mixer, or by more back end gain, either in the auido stage for the 
DC, or the audio or IF stage in the superhet. I have used these numbers 
as examples only, an audio signal of 0.1V is probably adequate, on the 
other hand we may wish to listen to signals stronger or weaker than 1 
microvolt. I have omitted the loss in the RF filtering stages ahead of 
the first mixer, these typically run a few dB or so. But you get the 
picture right?; its not the total gain it is where it is located.

One problem with too high gain in a single stage is the sensitivity of 
the reciever to mechanical vibrations, the "microphonic" problem. This is 
usually evidenced by a "boinng" when the receiver is tapped. Good layout 
will resolve most of the microphonic problems as will rigid mounting 
techniques. Glue down all the components with superglue or hot glue, and 
build the amplifier/receiver on brass or copper sheet rather than a PC 
board; some have built up walls around the amplifier and poured in 
beeswax to encapsulte the whole thing. (ICOM used to do this with their 2 
M synthesizers). Choice of components is also important, Ceramic 
capacitors are piezoelectric so they will generate a small signal when 
mechanically stressed. Use poly or tantulum capacitors instead. Today's 
ICs are less susceptible to microphonics since everything is rigid on a 
chip. This accounts for the revived popularity of the DC with the advent 
of the 602 and 386 chips.

A more correct statement here would be "DC receivers usually require the 
AF stage to have large gain".

Now to address my "more on that later" note above. If we use the receiver 
in a band where the noise is driven by the receiver rather than the 
external noise (probably above HF, say 2M), the DC will need slightly 
more gain to overcome the additional 3 dB noise floor caused by 
introduction of noise on both sidebands instead of just one. This gain 
should be provided in an RF stage.

None the less, as Laura states, properly designed receivers of either ilk 
will perform well. The design requirements are different for the two 
types, not more difficult for one than the other.  I personally find the 
biggest drawback of the DC to be the two signals one recieves for each 
signal on the band. This is not a problem on an uncrowded band, but on 40 
M during SS can be a nightmare. If a beginner wants to start homebrewing 
from scratch, I still think the DC receiver has a lot going for it. It is 
conceptually easy to understand. It is easy to trouble shoot with just a 
receiver on the same band;
1. Build up the audio stage.
2. Apply a signal from a receiver (or your thumb) to see if it works.
3. Build an oscillator.
4. Listen for its signal in a receiver.
5. Use a dbm passive mixer. No assembly or packaging needed.
6. Build an RF filter (2 pole).
7. Use a receiver to see if it works. Hook it all up.

Of coure if there are problems along the way a voltmeter is nice to have.

Having done this a beginner tackling a superhet will soon realize he is 
doing the same things he did when he built and trouble shoot (shot?) the 
DC receiver only several times, not just once.

That said, I think few people will really want to return to a homebrewing 
DC receiver after building one of Dave's (or NorCal, or EMtech or OHR) 
kits. But will they undertake to build another superhet from scratch?  I 
think that is what the original point made in the origianl post was, 
today's kits are so good few will bother to homebrew receivers.

Personally, I think that the greatest contribution that Dave made to 
simple receiver design is also one of the simplest. You can add it to any 
rig, DC or superhet. It is this' Controlling the volume (actually overall 
gain) with a pot at the antenna input. This insures that the receiver 
does not have an overall gain larger than necessary. This maximizes the 
strong signal handling capbilities of the receiver, and wrings out the 
most performance out of those 602s. 

Sorry that this has gone on for so long; I don't seem to be able to stop 
once I get going. I hope I have not offended anyone, but as they say in 
parliament; I now stand open for discussion. - Duffey KK6MC/5            

James R. Duffey KK6MC/5 DM65
30 Casa Loma Road
Cedar Crest NM 87008

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