[HBR] Yet Another HBR Concept

[hbr@mailman.qth.net]

In a message dated 10/6/2003 1:00:52 PM Eastern Daylight Time, waltah@earthlink.net writes:
 
> Okay ... let's try to put it all together in an idea for Yet Another HBR.
> 
> 1. No RF stage.   Not needed for sensitivity below at least 10 meters, 
> useful as variable (AGC-controlled) attenuator but let's keep this 
> simple.

OK - and we can stick an attentuator outboard if desired.
> 
> I have done a no-RF-stage receiver before -- they work okay if you 
> have enough IF gain and use a 2nd detector with gain.
> 
> 2. Antenna tuned circuit is push-pull, drives grids of 6ES8 (or 6AQ8?) 
> mixer in push-pull.  That ought to give excellent IFDR ... like around 
> 100 db.  This idea is from the G2DAF Mk II receiver except that the 
> push-pull RF stage is omitted.

Right - but ya need at least two tuned circuits in the front end before the mixer. Not that complex - use two big air coils link coupled or some such. Build the whole thing as a subassembly.
> 
> 3.  IF is 6 Mcs.  VFO tunes 4.25-4.75 Mcs.   Premixed with multiple 
> of 1.75 Mcs to get f+6 Mcs, for example 4.25-4.75 + 5.25 = 9.5-10 
> Mcs to tune 3.5-4 Mcs.   I *believe* this conversion scheme has no 
> spurs in the low four bands -- does anyone see something I've 
> missed? 

You wanna run that through the spurs chart first. From my limited premixer experience, the VFO and xtal osc are way too close together. Listen to the birdies sing...
> 
> Hummm ... 2nd harmonic of the VFO comes very close to the output 
> frequency on this band.  Might be well to move the IF down a little 
> and reduce the VFO freqs -- maybe 5.75 Mcs and 4.0-4.5 Mcs or 
> something close to that.  

Keep going and ya wind up with the classic 5/9 combo.

> And/or might balance the premixer to 
> reject the VFO.   For example push-pull VFO premixer input, crystal 
> controlled signal injected on cathode, single ended premixer output.  
> (Unusual form of mixer -- might not work!  Need to check ...)

Ya just built a push-push doubler for the VFO and a grounded grid amp for the xtal osc.

> A push-
> pull VFO has very low 2nd harmonic output.  It would require a two-
> gang VFO capacitor but you get some help with stability because 
> tube capacitances are effectively halved.     But stability for a VFO at 
> < 5 Mcs is easy anyhow -- might not even bother with a 'unit VFO' 
> design.
> 
> The oscillator chain output is ganged with the mixer grid tuning -- 
> requires a three gang cap.   Three coils (center tapped mixer grid, 
> crystal osc output, oscillator chain output coil) must be 
> bandswitched -- counting the antenna winding on the grid coil, a total 
> of five contacts.   No two coils on the same frequency.

You do like ganged caps.
> 
> 4.  The multiple of 1.75 Mcs is produced by a Hahnel oscillator  
> which is basically an oscillator locked on multiples of a crystal 
> frequency.   They were investigated by the military in the 50's as a 
> possible way to do bunches of crystal controlled channels.   The 
> thing will do amazingly high multiples -- in the 100's -- with 30db or 
> so purity.  It isn't too complicated -- can be one tube with at most 
> two tuned circuits -- although the circuit parameters have to be 
> carefully adjusted.   And output must be link coupled from the 
> multiple tank circuit since RF voltage at both the crystal frequency 
> and the multiple is found throughout the circuit.

One word: Overtones. 
> 
> 5.  Drive to mixer is on cathode -- single ended.   Mixer output push-
> pull via transformer to 6 Mcs ladder filter.   The matching is not too 
> hard via a capacitive voltage divider.   I need to check what strong 
> SWBC stations are on or very near 6 Mcs -- with only one premixer 
> tuned circuit, one wouldn't want to be spot on one of those religious 
> megastations.
> 
> An IF trap will be needed in the antenna circuit in any case.

Oyez...
> 
> 6. Two stages 6EH7 IF @ 6 Mcs.   The high gain means great care 
> in shielding and bypassing.

Another subassembly.
> 
> 7.  BFO is 'rubber crystal.'   At 6 Mcs you can get plenty of swing to 
> receive both sidebands.
> 
> 8.  Usual 12AX7 AGC/audio plate detectors and 6U8->2x6AQ5 audio.
> 
> That's all.   I wish someone else would build this thing first.

How 'bout this:

Put the filter on 5.25 MHz or thereabouts. VFO tunes 8.75-9.25 or so for 80/20, no premix at all. Subtract from ~21.5 xtal for 40 meters (premix out 12.25 to 12.75) For 15,  subtract VFO from 27 MHz xtal. Use third overtones if needed (40 meter Novice rock at 7.135 and 9 mc cb rock...

Or if you're really brave, give up 15 meters and make the VFO bandswitching. Only two ranges needed (8.75-9.25 and 12.25-12.55) and they're not far apart. Use the same coil and switch a capacitor or two. Big heavy unit construction might not drift too bad. W2YM circuit will do.

I didn't invent this scheme - National did, for the NCX-3 and "National 200" transceivers. Worked.

Use a lot of subassemblies. Front end tuner is one, unit VFO another, IF, BFO, audio. No big chassis needed, just a panel and frame. 

73 de Jim, N2EY