At 11:56 13/08/2002 -0700, you wrote:
Perhaps someone who has already 'been there' can offer their advice.
I would like to start building the exciter stages for what will be my
136KHz tx (hopefully we will have access to the band within the year!).
The DJ1ZB exciter (modified by DF3LP) is the circuit that I am
questioning. It mixes two xtal controlled 4MHz oscillators, 136 KHz apart,
to extract the 136KHz signal. Can I build two VFO's, somehwere around 1MHz
and set them 136KHz apart, that will be stable enough to handle QRSS work?
Dear Steve, LF Group,
Mixing 2 VFOs or VXOs together to obtain a lower difference frequency does
nothing to improve the stability - in fact it makes it worse unless you can
guarantee that both oscillators have exactly the same drift, which they
won't. The crystals mixed together works quite well because the crystals
have quite good stability to start with, but the 136kHz difference
frequency will be much less stable than a single crystal oscillator
operating on, or divided down to, 136k. The same applies to VFOs - it would
be much better to make a single 136k VFO, and apply temperature
compensation to get near zero tempco than it would be to make 2 HF VFOs and
try and get identical tempcos.
I used a 13.6MHz VFO divided by 100 - this has the advantage that it is
easy to measure the VFO frequency to 1Hz resolution, and therefore the
output frequency to 0.01Hz resolution. It was fairly easy to get the output
frequency to stay within a few Hz, which is quite adequate for QRSS3
contacts, but you would not want to leave it running all night unattended,
or use it with more than 10s dot lengths. The crystal mixing scheme has
comparable stability, and is somewhat easier to make than a stable VFO. If
you could accept a narrow tuning range, perhaps 50Hz at 136k, replacing the
VFO with a VXO would be a viable solution. A simple crystal oscillator
seems to give adequate stability for QRSS30 provided the ambient
temperature is fairly constant. To improve on this, you need a TCXO or,
better, an OCXO - this is also true for PLL or DDS synthesisers, which are
only as stable as their reference oscillators, of course.
My own solution to a stable drive frequency was simply to buy an old
synthesised signal generator. These are fairly cheap now, especially if you
get one that does not cover VHF/UHF. I got a Racal 9084, which covers 10k -
104MHz - I divide the 13.6MHz output by 100 to give 0.1Hz resolution at
136k. The internal OCXO gives stability better than 1 part in 10^7, it
stays well within 0.01Hz at 136k. It has many other uses in the shack apart
from being a VFO of course. Bear in mind if you buy one of these old units,
you will probably have to repair it at some stage! Quite a few people have
used an HF rig with the output divided down in a similar way, but unless
the rig has a high stability reference, the stability will be that of the
"ordinary crystal" reference oscillator
The requirements for Jason are a bit different - you need digital selection
from 1 of 16 frequencies. The stability is somewhat relaxed by the
differential nature of Jason, but the frequency shifts need to be quite
accurate to preserve the coding correctly. This makes an analogue FSK
scheme quite tricky. You could use a DDS scheme like G4JNT and SM6LKM. In
my case, I take a 4.5kHz signal generated by the sound card output from
Jason, and mix it up to 136kHz with the Racal sig gen. You could generate
QRSS in a similar way if you wished - the drawback is the need to calibrate
the sound card frequency errors.
Cheers, Jim Moritz
73 de M0BMU
|
