Sunday, June 16, 2013

A Single-Stage Heterodyne VFO

A heterodyne VFO is normally constructed using a minimum of three distinct circuit blocks, namely: a VFO, a crystal-controlled oscillator and a mixer. I've managed to pare it down to two stages in the past: a VFO followed by a self-oscillating, crystal-controlled "converter," but I always knew my "minimalist devil" would ultimately be satisfied with no more than one stage.

Past attempts were thwarted due to excessive oscillator interaction. But at last I seem to have stumbled over a reasonably well-behaved "all-in-one" heterodyne VFO circuit. The result employs a VFO configured as a common-collector Colpitts oscillator. Simultaneously, a crystal-controlled Colpitts oscillator works in the common-base mode. The passive oscillator component values selected reflect the need to roughly equalize the two oscillator energies.  

The crystal-controlled oscillator frequency was dictated by the contents of my junkbox; 16.877MHz, in this case. The VFO tunes from 2.877 to 2.777MHz, in order to cover the bottom 100kHz of the 20M band.  

I used a 2N3904 throughout most of the design process. Once the circuit worked to my liking I swapped this modern device for a 1950's-vintage, Germanium surface-barrier transistor. Aside from some component tweaks (R1, especially) the circuit behaved much the same. 

The frequency stability seems quite up to the task in my underground shack, however, I wouldn't think of operating this transmitter under the noonday sun. The Germanium transistors, compounded by the primitive DC biasing technique would likely spell disaster. The VFO isolation is commendable for such a simple circuit.

A tuned RF amplifier, primarily comprised of a second, vintage Germanium transistor, produces an RF output of slightly more than 2mW. The worst spurious frequency output power (at 16.877MHz) measures -26dBc (~5uW). While I intend to add a second RF amplifier stage, I was curious whether or not I could make a contact at 2mW. 

My answer came last evening when I returned K5TF's CQ. He replied straight-away with a 439 report from Atlanta; a distance of 937m/1508km. An avid QRP-op himself, Dick joined my happy-dance with, "WOW 2mW AMAZING." Indeed, it's the lowest power I've used to make a long-haul contact thus far. He missed one exchange from me due to QSB, otherwise, he appeared to enjoy a fairly good copy throughout most of the seven-minute QSO. Dick operates an Elecraft K2 at 5W to a Hexbeam. I had a steadfast - 589 - copy of his signal on my two-transistor regenerative receiver. Enjoying my new-found frequency agility, I subsequently roamed the CW band for several hours, calling the stronger DX stations but without success. 

Still, I was happy with my one QSO. Besides, the transmitter worked well enough to placate my minimalist devil...at least for now ;-)

20 June 13: Worked W4TZM in the NAQCC Milliwatt Sprint! (589/559, 1330km @ 2mW) 

    Parts List

R1:  2.2MegOhm
R2:  560 Ohms
R3:  3.3k Ohms
R4:  220 Ohms
R5: 470k Ohms
C1, C2:  1nF (s.m.)
C3: 1.67nF (s.m.)
C4:
C5: 120pF (s.m.)
C6: 560pF (s.m.)
C7, C13: 47nF
C8: 
C9: 0.47uF
C10: 678pF (s.m.)
C11, C12: 15-60pF 
L1:
L2:
T1: 11 turns / 3 turns, junkbox iron-powder toroidal core
T2: 15 turns / 1 turn, junkbox iron-powder toroidal core
X1: 16.877MHz (junkbox "uP" quartz xtal)
Q1: Philco T1657, Germanium surface-barrier transistor, circa 1959
Q2: Philco 2N502, ditto above    

    

5 comments:

  1. FB job Michael. I could tell you were running pretty low power but you were still easy copy, even on my doublet.
    -Tommy W4TZM

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  2. Thanks for the FB QSO, Tommy!

    73/72,
    Mike, AA1TJ

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  3. Nice work. The next question is, can you make the LC oscillator part into a regenerative detector? Then you could have a one-transistor superhet.

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  4. cannot reduce the static clearly

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  5. I've been looking at your germanium transistor receivers, and hope to get around to trying one of these myself when I can. Regarding the 'talking doll' circuit, I was looking at the 'high compression' regenerative receiver circuit found here: http://vintageradio.me.uk/radconnav/transtrf/highcomp3pw0964/circuit.jpg I like the idea of the 'thermal compensation' using a thermistor. Regenerative circuits using early germanium transistors often need readjusting when the transistor warms up or cools down. The thermal compensation circuit can help if the receiver is used outdoors, such as on field day. What do you think about it?

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