[Ed: This page is a bit rough at the moment. I'll try and get some nice simple pics of how to build one of these beasties. While the circuit is simple by such standards, the info here isn't quite join-the-dots.]
Thanks to the Parkes Simple team, most of the Simple rigs run a crystal locked oscilator. The disadvantage of this setup is that if the frequency you've set with the crystal gets clobbered by RFI, you can't escape. The big advantage is that the crystal holds the frequency very well. It also allows direct comparison of results between rigs using the same frequency as you (good science potential). Dave suspects that you're likely to get *less* RFI as the osc doesn't drift into sources of RFI.
This from an email Dave posted to the egroup:
The LO in the 1056 kits tends to drift around a fair bit, interestingly I've
never noticed this problem with the Jove kits, despite being a similar design.
Brett send me this link and a 20.120 MHz xtal and said he built the Parallel Resonant Circuit.
I followed his suggestion and made one up on a little breadboard. I ran it
off 12V instead of 9V. Brett built a 5-pole low-pass filter to follow the oscillator
in order to attenuate the 40MHz harmonic. I didn't bother with this because:
a) I couldn't see the harmonic on a CRO as you can see in the attached image (luckily, I don't have a spectrum analyser)..
b) The 20MHz filter at the RF front-end should knock the 40MHz signal from the antenna around a fair bit anyway.
c) Saved winding coils.
I used a hybrid junction splitter (since we've got two rxers) and then inject the LO into the SA602 mixer/oscillator chips. I don't know how important it is to use a proper splitter rather than a T piece in this case(?). A poke around with the CRO suggests the mod's worked well. The LO level on pin 6 of the SA602 is about 60mV p2p which is a bit lower than the (200mV?) specified in the data sheet, but from pin 7 it seems the on-chip oscillator is being driven okay. In fact the previous LO was driving the slave SA602 with only about 40mV!
I've attached a few snapshots in case any one is interested. Overall I'd say this circuit was very easy to build and very cheap. Simple through and through!
Shortly after... Eric added this regarding the possible harmonic distortion:
> a) I couldn't see the harmonic on a CRO as you can see in the attached
> image (luckily, I don't have a spectrum analyser)..
You don't need one, Look at the waveform and the assymetry between the +'ve and -'ve half cycles, it looks like you have a significant amount of 3rd harmonic distortion (60MHz) in the waveform.
This could mean the crystal is being overdriven and going into overtone mode. Might be worthwhile reducing the value of the feedback caps in the oscillator to cut the drive back a bit.
Dave's been toying with the idea of a digital tuner. Haven't had much luck getting the thing to work as yet.
Oh yeah, the name? This method is really like using a digital tuner... think digital tuna... BlueFin!