[ARC5] CW/MCW

[Kenneth G. Gordon]

On 11 Nov 2013 at 18:27, Fuqua, Bill L wrote:

> It really had not occurred to me before but one possible reason is
> that since radios of the day did not have product detectors, receiving
> CW required turning off the AVC, turning up the volume control and
> constantly changing the RF/IF gain for a suitable audio level.

Yes. That is one reason. Others here have suggested other reasons for 
MCW, one of the biggest being that DFing is much easier. Try DFing a CW 
signal sometime: it is very difficult. And automatic DFing is almost impossible 
under those conditions.

> Without AVC you could not just leave the radio unattended monitoring
> for a CW signal, especially for a beacon. 

Again, yes.

> I just don't understand why they did not begin using a product
> detector earlier except that possibly they were concerned that the BFO
> would leak into the earlier stages of IF and desense the receiver.

Well, no, not exactly. Injecting the BFO signal into the IF chain AFTER the 
AVC diode was common practice in some receivers (most notably, 
Hammarlunds). 

Even in the later model AN/ARC-5 receivers, this was common. Witness the 
12SF7 as the 2nd IF tube in the communications receivers of that block. The 
12SF7 contains a diode for that specific purpose: AVC not effected by the 
BFO. The AVC in those receivers is ALWAYS active, and works with the 
BFO on or off.

The problem with using AVC with the BFO on in MOST receivers isn't that 
the BFO itself desenses the IF chain: it is that the AVC diode "perceives" the 
BFO signal as just another very strong signal, rectifies that, and thus the 
resulting AVC voltage is much, much higher than that which would result 
from only the wanted signal, and THIS higher AVC voltage is what 
"desenses" the IF (and RF too). It simply reduces their gain as it normally 
would with a strong carrier present in AM...or in MCW which is simply 
another implementation of AM.

> I
> have a HQ129X that I got years ago. I have several. Actually my first
> Novice station had one. Anyway this one had an additional switch added
> to it for a solid state (dual gate mosfet) product detector. It was
> tied to the original BFO and works great as does the AGC. No leakage
> problems at all.

Yes.

One added factor concerning "product detectors"  is that the PDs of the early 
periods were very much more complex than the simple diode detectors 
which were most common for AM detection in most receivers at the time. 

Most of those early PDs required multiple tubes to implement. Like three 
triodes. Although I am not certain of this, it seems to me that Collins 
pioneered a MUCH simpler "product detector" using a single triode with their 
KWM-1.

Although this detector is not a true product detector (which is defined as one 
in which all audio output disappears if either the IF input or the BFO input is 
missing or removed) they work extremely well.

Heathkit used an enhanced version of this "Collins" single-triode PD in all 
their SSB rigs.

Although I have not examined it very closely from a circuit-analysis 
perspective, I have always suspected that that single-triode PD is actually an 
implementation of the so-called "infinite impedance" detector.

It does demodulate AM very well too when the BFO is turned off.

I might add that all a good SSB or CW detector is, is a mixer with its output 
at audio frequencies. Therefore, any really GOOD mixer circuit should work 
well in that service.

Ken W7EKB