Wednesday, April 22, 2020

60m Region 1 Crystal Combination

Sorry for having been silent for so long. There was a lot going on over the last few years.

Anyway, the present situation brought me back to Ham Radio and thinking of building stuff. However, I have to mention that this post should be considered more as a note to myself or inspiration of fellow RF designers, rather than a fully worked out recipe.

The topic of one of my next projects will be building some QRP rig for 60m. Over here in the Netherlands and in Region 1, we got a very small slot from 5351.5kHz to 5366.5kHz assigned as secondary service with a maximum EIRP of 15W. This is a range which appears to me to be almost ideal for building some QRP gear.
In modern times, we could use DDS systems to get us anywhere with anything, I must admit. In the older days, using combinations of readily available inexpensive crystals was the #1 choice for QRP.
Whilst having CW in mind, some of my thoughts my actually be useful for a side-band superhet design.

Here is what I found:

  • 15.360 MHz crystals for use in a VXO (or superVXO)
  • 10.000 MHz crystals and oscillators for BFO and filter stages.
To be honest, I just cooked up the idea and have not thought is through entirely. But here is what my mind came up right away.
As a side remark, subtractively mixing of crystal oscillators is a well known method of eliminating temperature drifts. So, effectively, this could lead to very stable designs, when done properly.

Option A is a canned 10.000MHz oscillator and push the signal through a 10MHz crystal in order to produce a sine wave.
Option B would be a regular crystal controlled discrete beat oscillator.
Mix any of option A or B with a 15.360MHz VXO to generate a 5.360MHz output signal. Should the pull be insufficient (not very likely at 15MHz) one could still use a superVXO.

Here is where the concept presently struggles. 
Of course you would think building a crystal filter with inexpensive 10MHz crystals. However, this would probably exclude option A for the TX. Here is where the struggle lies, would it be possible to create a filter design with 10.000MHz crystals that pulls 600 or 800Hz away?
When using option B for the TX, of course, one would add a simple RIT design.

You have seen me writing about two different options. Why bother, you might ask yourself. Well, to me, it is all about simplicity. 
For a mere transmitter, in option A, I might just use a canned 10.000MHz high precision oscillator and key just said oscillator, rather than a buffer stage for CW operations.  Experimentation will have to show how that sounds. In terms of simplicity, this would be pretty neat. Such a TX could be combined with some sort of SDR receiver or maybe a Polyakov direct conversion RX based on a 2.68MHz VFO.
Option B would allow for a fairly regular QRP CW transceiver. Nothing to write home about, however, this will certainly be a more complex design.

Obviously, option A is no longer available here. However, seen that it is recommended to use USB on the Region 1 assigned 30m band, it would be an obvious measure to use a pulled 10MHz BFO for both RX and TX. However, mind the mirror, we are subtractively mixing here.
Having in mind the upper portion of the 60m band, i.e. the weak signal band from 5366.0 to 5366.5kHz, option A might just come into play again. Mind you, QRSS & Co, don't mind about the sideband too much, as long as the operator knows what she/he is doing.

Over the upcoming days, I will work on a concept/prototype and hopefully will be able to report on some progress soon.