General Coverage Receiver (GCRX)
Single Frequency Configuration
Temp out of PCBs, expect to resume in July 2011
When designing the PCB for my general
coverage receiver, I put places in to use a crystal instead of the
normal external VFO. The SA612 mixers will take an external VFO
signal, and have a provision for a crystal oscillator. I thought
that this would be in interesting option, if one wanted a special
receiver for a single frequency.
Applications that come to mind are WWV on 5, 10 or 15 MHz. 14070 (for 20m PSK), and 14100 and 18110, for DX beacons.
Of course one has to find the proper crystal. But since there is nothing fixed about the IF frequency, one has the option of changing that to.
I addition there is space for a trimmer capacitor which will allow about 5KHz of adjustment for the VFO crystal.
So for example a 9MHz if with a 5.068MHz VFO crystal can be tweaked to receive 14070 PSK signal.
This one operates with a 9.000-9.003 MHz IF filter, and a 9.003 MHz BFO thus receiving LSB. The BFO is at 5070, thus receiveing 14070 through 14073. PSK 31 is a symetrical mode, USB or LSB does not matter.
KIT $35.00
Similarly a 10MHz IF and a 4.096 MHz VFO will put you close to the 14100 DX beacon frequency.
When receiving AM, PSK31, or CW, it doesn't matter if the BFO is on the high side or low side of the crystal filter, since all those modes are not sensitive to USB vs. LSB. So the easiest thing to do is to simply buy 5 crystals of the same frequency, and your BFO will likely end up on the high side of the IF or LSB. However if you are designing one for a SSB signal then you must use some care. The oscillators in the SA612's operate in parallel resonant mode. Generally if you buy series resonant crystals for the IF, then a parallel resonant crystal of the same frequency will put the BFO low side for USB reception. Unless of course, if you are using a high side VFO then the spectrum is reversed.
And of course you don't need the relay or the parts used to drive it, just install the right filter to block out the image of the frequency you are receiving.
You can buy the experimenters kit version, which consists of the bare PCB plus two SA612's and some of the connectors.
$19.95
The schematic show below is a suggested starting point. All the crystal frequencies listed are inexpensive microprocessor crystals avaialbe from many sources.
In general you can use 68pF or 100pF for all the capacitors in the crystal filter. (C7-C11) use 100pF caps for the oscillators, C13, C14, C32, C33. And use 70pF trimmers for both VFO and BFO, this will give you lots of tuning range.
Back to the GCRX page
Applications that come to mind are WWV on 5, 10 or 15 MHz. 14070 (for 20m PSK), and 14100 and 18110, for DX beacons.
Of course one has to find the proper crystal. But since there is nothing fixed about the IF frequency, one has the option of changing that to.
I addition there is space for a trimmer capacitor which will allow about 5KHz of adjustment for the VFO crystal.
So for example a 9MHz if with a 5.068MHz VFO crystal can be tweaked to receive 14070 PSK signal.
This one operates with a 9.000-9.003 MHz IF filter, and a 9.003 MHz BFO thus receiving LSB. The BFO is at 5070, thus receiveing 14070 through 14073. PSK 31 is a symetrical mode, USB or LSB does not matter.
KIT $35.00
Similarly a 10MHz IF and a 4.096 MHz VFO will put you close to the 14100 DX beacon frequency.
When receiving AM, PSK31, or CW, it doesn't matter if the BFO is on the high side or low side of the crystal filter, since all those modes are not sensitive to USB vs. LSB. So the easiest thing to do is to simply buy 5 crystals of the same frequency, and your BFO will likely end up on the high side of the IF or LSB. However if you are designing one for a SSB signal then you must use some care. The oscillators in the SA612's operate in parallel resonant mode. Generally if you buy series resonant crystals for the IF, then a parallel resonant crystal of the same frequency will put the BFO low side for USB reception. Unless of course, if you are using a high side VFO then the spectrum is reversed.
And of course you don't need the relay or the parts used to drive it, just install the right filter to block out the image of the frequency you are receiving.
You can buy the experimenters kit version, which consists of the bare PCB plus two SA612's and some of the connectors.
$19.95
The schematic show below is a suggested starting point. All the crystal frequencies listed are inexpensive microprocessor crystals avaialbe from many sources.
In general you can use 68pF or 100pF for all the capacitors in the crystal filter. (C7-C11) use 100pF caps for the oscillators, C13, C14, C32, C33. And use 70pF trimmers for both VFO and BFO, this will give you lots of tuning range.
Back to the GCRX page