Wednesday, November 10, 2010

The HC-8 CW QRP Transceiver

I got something on my mind, which is probably not too hard to design, and could be a nice trx to take along on journeys: a modern days clone of the HW-8.
The design will entirely be based on 74HC digital electronics, hence the name HC-8.
It will employ a 8.867MHz super-VXO. Reason for this being the HW-8 itself (find out more about this in an earlier post).

The CW portions of 160, 80, 40, 20 and 17 can easily be covered by such a super-VXO in combination with some cheap crystals. 30 required some more efforts. For good measures, I will include the already shown list:
  • 160m: 8.867 - 7.000 = 1.867
  • 80m: 8.867 - 12.406 = (-) 3.539
  • 40m: 8.867 - 1.843 = 7.024
  • 20m: 8.867 + 5.185 = 14.052
  • 17m: 8.867 + 9.216 = 18.083

As mentioned above, the plan is to use digital gates as much a possible.
This would be possible solutions:
  • 74HC00 - oscillators, drivers, AF-amp
  • 74HC04 - oscillators, drivers, AF-amp
  • 74HC86 - oscillators, drivers, mixers
  • 74HC4066 - oscillators, mixers, signal routing, class E "driver"
  • 74HC240 - driver, PA, AF-amp

Practical Considerations

With the 74HC240 running from 8V (upper limit for this particular chip) one could imagine to drive a class-E switch-mode PA (IRF510) that easily would produce 5W RF.

The HC-8 wont be providing any IF filtering. I intend to go for pure direct conversion, (super) VXO on 8.867Mc and r.i.t. on the respective other oscillators. I have not yet figured out how to do this the best
way.

Switching low pass filters in a multi-band transceiver can pose a problem, therefore I am envisaging to go for 2 bands only (switches are available, small and not too expensive), but build two rigs:
  • 80/40 for NVIS
  • 20/17 for DX
One could consider one trx for 40/20 to get the best of two worlds.


Added Value
 
You're missing 30m? OK, here is 30m, it's a tricky one however, in particular for filtering! Frequency division would be available using 74HC74 Flip-Flops. Signal source: a 5.0MHz xtal or a not so common 38.0MHz oscillator.
  • 8.867 + (5.000/4) = 8.867 + 1.25 = 10.117
  • 8.867 - (38.000/2) = 8.867 - 19.000 = (-) 10.133

Even long-wave would would be available (although such a rig would maybe not that practical for such low frequencies):

  • 8.867 - 9.000 = (-) 0.133


Note: For some bands the VXO is required to oscillate above 8.867MHz. Some thought should be invested when designing the VXO. Maybe for the reason of frequency coverage, the VXO should involve discrete transistors in place of digital gates.


As antenna to go along with the rig I see the RockLoop as best fitting. Very narrow-band antennae have the advantage of good harmonics suppression. Alternatively a high-Q transmatch should be used for the same reason.

2 comments:

  1. Hi Joachim,

    One comment I have is that with all the work I have been doing with receivers over the past year, the three most important criteria hae been (1) stability, (2) sensivity and (3) selectivity. Do you think that by using 74HC components that this will make a substantial improvement in performance? I would be most interested to hear your views.

    Best wishes,
    Andrew (hanging in there) G4CWX

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  2. Hi Andrew,

    nice to read from you!

    Well, as to selectivity, it is a direct-conversion design, and therefore, I would not expect too much in this respect.

    Sensitivity wont be an issue. There are quite some successful designs out there, which make use of switching mixers, e.g. Tayloe mixers.

    Stability is entirely up to the builder. I figure it may be clever to build some temperature control. However, this is not QRSS, we certainly can ignore a bit of drift.

    Proper voltage stabilization should avoid chirping when transmitting. In place of keying oscillators, I will key the transmit mixer.

    All the best!
    73, Joachim

    ReplyDelete