[HBR] Yet Another HBR Concept

Sat, 11 Oct 2003 08:42:14 EDT

In a message dated 10/11/03 4:56:21 AM Eastern Daylight Time, 
[email protected] writes:

> A constraint is that one ingredient of the premix is a multiple of 1.75 
> Mcs because I want to use a Hahnel oscillator for the bands.   So 
> the series of solutions is n x 1.75 = IF +/-  VFO to get the low end of 
> each ham band.  Solutions that place the IF or a multiple of the VFO 
> too near a band are no good;  ditto those for which the VFO or a 
> harmonic goes through the IF; those with IF's <5 or >10 Mcs are 
> pretty much out; those with IFs in use by megastations are out; and, 
> I want to use clock crystals.  It's not that hard an exercise to go 
> through, making the table of all solutions and then Xing out the losers 
> -- but there's no assurance that they're not all losers.

Excel spreadsheet, anyone?

One thing to consider with the Hahnel oscillator: Locked oscillators of any 
kind can have significant phase noise. 

A trick attributed to ZL2 AMJ and copied by W1ICP may be of some use here:

One of the most expensive items in the classic "80 meter rx/xtal converter" 
scheme (a la HB-67) is the cost and procurement of the het xtals. ZL2AMJ  
solved this problem by using an inexpensive surplus xtal at about 3500 kc. on its 
third and fifth overtones to get output on about 10500 and 17500 without 
even-order harmonics or the fundamental making spurs in the front end. If the xtal 
was a little low, 40 could also be covered by using the fundamental. Or, a 
second xtal at 3600 would give 3rd overtone output at 10800 and permit tuning 40 
backwards from 3800. (20 could be tuned backwards from 4000 using the 5th 
overtone at 18000). 

Perhaps some version of that using a 1.75 xtal can work. 

> 
> I'm not stumped on this one yet -- just haven't had enough time to 
> think about it. 
> 

I went through a lot of paper and at least one failed premixer scheme 
developing the Type 7. I think the reason premixer designs were not popular in 
amateur gear is a direct result of the difficulty finding a spur-free premixer 
scheme. 

> > Anyone remember the W5OMX receiver? 
> 
> *That's* the one I was trying to think of ... it was described in an 
> article in either QST or CQ, maybe in the late 50's?  

QST, January, 1968, page 22-30. Nine full pages! Probably the last serious 
tube rx fully described in QST. (There were others later but not fully 
described).

 It was a 
> 
> premixed design -- i.e., just one mixer on the signal path -- I think the 
> only one of those I can remember.  The guy was an Air Force officer, 
> I think ... I think he alluded to problems with spurs but I don't 
> remember the details.
> 
> Can you remind us of the conversion scheme and tube lineup?
> 
> 

Sure:

6AZ8 P RF amp with separate gain control and AGC applied
6AZ8 T 3500 kc calibrator

7360 single-ended mixer with separate gain control

6AU6 unit VFO - high C Colpitts with differential temperature compensation 
and Polar tuning cap

6KE8 T Het osc on all bands 
6KE8 P Premixer on all bands  

9 Mc. McCoy SSB filter

3 6BA6 RF amps with separate gain control and AGC applied. 10.7 Mc IFTs 
padded? for 9 Mc. 3 stages permits excellent AGC action.

6BY6 product detector

12AX7 T1 infinite impedance AM detector
12AX7 T2 1st audio

12AT7 T1/T2 Unit xtal bfo for LSB/USB, xtals switched bymchoosing which 
cathode is grounded.

12AX7 T1 S meter amp
12AX7 T2 Audio AGC amp transformer coupled to SS diodes

6C4 2nd audio. Low gain, has 2200 ohm plate resistor. Good match to high Z 
phones, jack is between 2nd and 3rd audio

6AQ5 3rd audio

FWCT power supply built in, 2 section choke input filter, 200 volt B+, 0A2 
regulator, SS power supply diodes

Dial is Miller MD-7
Separate preselector using HFD variable and 4511 reduction drive (!)
Front end bandswitched - Miniductor for signal coils and slugtuneds for het 
osc and premixer
Premixer output filtered by a single tuned circuit peaked at band center.

Het scheme 

IF is 9 Mc. All bands use high side mixing

VFO tunes 5-5.5 Mc

80: 7.5 het xtal + 5 - 5.5 VFO = 12.5 - 13 
40: 11 het xtal + 5 - 5.5 VFO = 16 - 16.5
20: 18 het xtal + 5 - 5.5 VFO = 23 - 23.5 
15: 25 het xtal + 5 - 5.5 VFO = 30 - 30.5
10A: 32 het xtal + 5 - 5.5 VFO = 37 - 37.5 
10B: 32.5 het xtal + 5 - 5.5 VFO = 37.5 - 38
10C: 33 het xtal + 5 - 5.5 VFO = 38 - 38.5
10D: 33.5 het xtal + 5 - 5.5 VFO = 38.5 - 39

10A and 10B share same het osc and premixer coils, as do 10C and 10D

Note that the premixer is used on all bands even though 80 and 20 can be 
tuned directly by the 5-5.5 VFO output. Why this was done is a mystery, because it 
has to be a source of avoidable birdies and spurs. Premixing for 80 avoids 
having the band tune opposite the others, but 20 tunes the same way whether 
premixed or not. Perhaps having the LSB/USB come out the same way was a concern. 

If it were me, I'd:

- Use a different RF amp and at least first IF amp
- Balance the 7360, or use a Pullen, or use a 'DAF dualtriode
- Have selectable IF bandwidth
- IF derived AGC (if any)
- BC-221 dial (I'm spoiled)
- Slightly different het scheme (no premix on 80/20)
- Different premix (balanced 7360? Pullen? highlevel dualtriode as used in 
DAF xmtrs?
- Doubletuned bandpass premixer output circuit
- 100 kc. calibrator
- Cathode follower to feed premixer output to matching transmitter

.....so many ideas, so little time.....

73 de Jim, N2EY

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