[HBR] GC-HBR Project -- Was 'Navy guy ...'

[Walt Hutchens]

> > I said:
> > The plan is to take the 120 crystals of the BC-1335 WW-II portable
> > FM set, 5675-8650 kcs (25 kcs spacing) and multiply the
> > frequencies x10 to give 120 frequencies 56.75-86.50 Mcs.  

And Jim weighs in at last:
> whoa - hold it right there!

Jim, I, um, didn't REALLY think you snuck out the back way, but I 
was starting to wonder if maybe we should get up a search party.   
Here I'm throwing out the second or third long post about a paper 
receiver and I have yet to hear anything about why it won't work!    
LOL ...  

Of course the fact is that they don't all work -- the batting average 
is around 50% I think -- not even that good if you count the paper 
design.   So your comments are always appreciated.

> Multiplying by 10 requires a quintupler and a doubler. That quintupler isn't 
> going to be very efficient. Worse, there's going to be the challenge of 
> filtering out the 4th and 6th harmonics, which will not be very far away, 
> percentagewise. And it's percentagewise that counts!
> 
> For example, if the 6000 xtal is quintupled, we have harmonics at 24,000 and 
> 36,000 that we don't want, yet we want the 30,000 It's doable, but not easy, 
> and you'll need to switch the tuned circuits as well as the xtals.

This is of course the trade-off that one accepts in such a design.   
A five-tube ham receiver has no RF stage, an R-390 cost what 
would have been a couple years pay for most of us, the first few 
decades of solid-state receivers were indeed compact, cool-
running, and light-weight, but not too hot for hearing signals -- and 
multiplying few-Mcs crystals up to synthesize a first converter 
frequency will bring spurious signals that can minimized but 
probably not completely avoided.
> 
> > > Subtract 49.2 Mcs yielding 7.55-37.30 Mcs by 250 kcs steps.  
> 
> What bandswitch has 120 positions?

One with a whole lot of really teeny-tiny contacts?   But that's not 
what I'm contemplating.  Let alone 120 tuned circuits for each 
wafer.   The plan is a bandswitch with 9 positions, used in three 
groups.   Each group of three selects the same front end tuning 
range, roughly 1.75-5.5-15-30.5 Mcs.   The mixer input and 
synthesizer coils are switched in those ranges and must be 
manually tuned.   

For example, on the lowest band the 1st mixer is tuned 1.75-5.5 
Mcs.  That's one knob.  Using the plan of a quintupler-doubler 
might mean an untuned Pierce oscillator circuit (5.825-6.125 Mcs) 
followed by the quintupler tuning 29.125-30.625 and doubler with 
output at 58.250-61.250 Mcs.

The doubler drives a mixer that subtracts 50.7 Mcs (no longer 49.2) 
so the synthesizer mixer output (to the receiver 1st mixer) must be 
tuned 7.550-10.550.  

(Subtract the tunable IF of 5.550-5.800 Mcs from those frequencies 
to get the coverage of each 250 kcs band.)   

The synthesizer thus has three tuned circuits that on the lowest 
(1.75-5.50 Mcs) range cover 29.125-30.625, 58.250-61.250, and 
7.550-10.550 Mcs.   These can be ganged together, but NOT with 
the front end tuning because the latter must also be retuned 
WITHIN the 250 kcs band on the lowest few bands.   The 
synthesizer is thus a second knob that must be set up on the 
chosen 250 kcs band.  

(In a commercial grade version you'd use differential gearing and 
perhaps a cam to combine the setting of the LO (the tuning within 
each 250 kcs band) with the synthesizer tuning giving the mixer 
tuning as output.  Or you could add a gang or two to the LO cap 
and bandswitch the coupling to it to tweek a mixer that was mainly 
tuned by the synthesizer.   But none of that would meet home 
construction criteria.   I will use two knobs.)

This lowest range covers 14 bands of 250 kcs each.   (1.75-2.00, 
2.00-2.25 ... 5.25-5.50)  Selection of the individual band is by 
chosing the proper crystal.  The simplest way to do that is to put 
one socket on the front panel and you just stick in the right crystal. 
However since only three ranges are needed and a 9-position 
bandswitch is available, I plan to allow selecting of either of two 
internal crystals, plus a front panel socket, per range.   On the 
lowest range the internal crystals would likely be used for coverage 
1.75-2.00 and 3.75-4.00 Mcs.   When another band is wanted, that 
crystal goes in the external (front panel) socket.

Another wrinkle that affords some simplification is that the 
synthesizer covers a much smaller percentage tuning range than 
the receiver does -- just as a BC receiver front end tunes 550-1650 
(3:1) but for a 455 kcs IF the LO covers 1005-2105 or just over 2:1. 
I expect to combine the first two front end ranges 1.75-5.5-15 Mcs 
into one synthesizer range.  

> Some ideas:
> 1) The complex LO system can be big trouble. I learned the hard
> way why premixer designs are so rare in ham gear: the LO has to be
> *really* clean to avoid spurs. In a transmitter, if we have spurs
> 60 dB down, it's no big deal. But in a receiver, if we are trying
> to listen to a -130 dBm signal, and we have a spur that's 60 dB
> down in the LO, and the spur is in the wrong place, we may hear a
> -70 dBm signal that really isn't there. In the HF ham bands, -70
> dBm signals aren't rare when the band is open and you have a
> decent antenna. 

My thinking exactly.   That's why I went with double conversion.  
It's just a whole lot simpler to break the spurious signal problem 
into two chunks, producing first a crystal controlled signal that 
covers a wide range and has fairly widely separated spurs -- the 
strongest ones not closer than 5.825 Mcs -- and then separately a 
tunable LO that covers a much narrower range at a much lower 
frequency that (with care) can be kept out of the antenna circuit.

For a ham-band only design, I'd shoot for a premixer setup.  It has 
been done successfully by some good people -- Drake, right?   
One could do far worse than copying the Drake numbers with 
better mixers and tubes.   But for a general coverage setup ... no, I 
don't think so.   

> There's a circuit in a QST article about 1961 that allows
> fundamental-mode FT-243s to operate in overtone mode. (It's in the
> article by W1ICP that uses a single 6U8 and a 3500 kc xtal as a
> converter for  40, 20 and 15 to 80 meters, without having to get a
> costly xtal for 10.5 or 17.5 Mc. Being able to overtone those 
> xtals might have some uses. 

The reason I'm not considering overtone operation is that although 
I'm confident that at least most of these crystals could be operated 
on overtones but the oscillator would have to be tuned and I don't 
think that could be ganged with the multiplier/mixer tuning because 
the relative setting is likely to vary from crystal to crystal.  
Additionally, the 5th overtone might not be sufficiently stable on all 
of them.  And the difference between the 5th harmonic and 5th 
overtone will also vary from crystal to crystal, meaning a wider 
calibration tuning range.

Ever notice the little gray or shiny metal spots on the corners of a 
pressure-mounted crystal?  These crystals (FT-243's)  can't 
achieve the Q of the wire mounted ones because there's a frictional 
loss.  Add to that the reduction of Q by the fact of overtone 
operation ...  Overtone operation of non-overtone crystals does 
work but it's a per-crystal thing.   Plug and play requires a crystal 
that was ground for overtone operation. 

Real crystals have an assortment of resonant frequencies 
corresponding to different modes of vibration.   The Q and stability 
of operation at the desired frequency depends not just on the mode 
itself but also on energy coupling between the various modes -- all 
the unwanted modes are just losses to the desired one.   The 
design/production process aims to lower the Q of and coupling to, 
unwanted modes.   Of course the desired modes are dramatically 
different for overtone operation than for a fundamental.   

In the early WW-II time frame (BC-1335 crystals) this science was 
in its infancy:  The state of the crystal art then was pretty much 
that pressure mounted crystals in the few-Mcs range should be 
slabs with the edges very slightly thinner than the center.  Wire 
mounting was developed fairly early (the BC 603-604 tank FM set 
crystals in the half Mcs range, beloved of post-war hams for 
building crystal filters) but I believe overtone units did not enter 
service until after the end of the war.  

The U.S. Navy learned its lesson the hard way.   The TDZ-RDZ 
(transmitter/receiver) end of the war UHF sets used fundamental 
crystals multiplied from the 5 Mcs range to 225-390 Mcs -- 
catastrophic in task force operations due to all the spurious 
frequencies.   This equipment had nearly a negative service 
lifespan, something like 1947-49 which was a shame because the 
10-channel TDZ tx was a true thing of beauty -- six or eight Collins 
Autotune units all going at once, tuning up all those multipliers.  I 
was 9 when I first saw one, and the memory is with me still.

The successor TED-RED (single channel) used CR-24 'barrel' 
crystals which were overtone units in the 20-30 Mcs range as I 
recall.   These were far more practical radios and were still in 
service when I was in the Navy, early 60's.  

The aircraft radios had it better -- probably because of space/weight 
considerations, the first UHF sets used overtone crystals.  The 
ARC-12 used CR-24 crystals in a turret inside an oven to control 
the receiver and the receiver fed a discriminator driving a small 
motor that frequency locked a TX master oscillator in the low VHF. 

You still sometimes see CR-24 crystals -- 9/16" cylinders with an 
axial pin on each end -- at hamfests.  The most common frequency 
is 30.375 Mcs, 1/8th of 243.0, the military UHF emergency 
frequency.   All the early multichannel radios and many single 
channel emergency sets used one of these

Walt
KJ4KV