Before I get all OCD about voltage references I thought it would be a good time to document my 600MHz divide by 10 prescaler. This is to boost the capabliity of my 80MHz Philips PM6622 Counter timer - it may not go very high but the beauty of its Panaplex plasma display more than makes up for it, compared to the higher frequency LED models.
As I indicated in the title, I built this one from bits I had in the scrap box, hence it uses an old Plessey SP8630 divide by 10 ECL 600MHz prescaler. Unlike more modern prescalers, it does have the advantage of having no particular low frequency cutoff (aside from slew rate) and its input can also be biased to give zero output when there is no input frequency (rather than a spurious one). In this circuit it happily goes down to 45MHz (sine wave) which is a more than ample overlap with the counter.
I wanted a decent input level range, but not having any PIN diodes available I went for a clamp approach, both on the input and after the gain stage to ensure that the max input spec of the SP8630 isn't exceeded. Not having any MMICs available either, I went for a single salvaged BFS55 transistor, this has an FT of 3.3GHz so should be up to the job. This combination achieves an input range of -24 to +23dBM (14mV to over 3V) at 180MHz (the highest source I have at the moment). I'm sure sensitivity will drop off a bit at higher frequncies. The upper limit is set by power handling of the 47R input resistor. The clamp diodes are HP 5082-2835 schottkys (1pF). Output is approx. 0.5V p-p.
Construction is combination of good old Manhattan style and air wired using a mixture of t/h and smd caps (hard to see in the photo). The main aim being to keep the input capacitance as low as possible. The input impedance is fairly indeterminate, I decided to have the input on the BNC male connector since it's most likely that I will use it close up to the target board using a PCB mounted BNC socket, so accurate termination of a coax isn't really an issue. Impedance won't drop below 50R anyway. One valuable lesson is that the center pins of BNCs move, which isn't good for smd caps!, so some strain relief was added.
Not shown in the photo are the 5V LDO, supply protection, caps and ferrite beads etc. mounted on the back of the board to give a 6-12V input supply range.
As indicated at the beginning, the design choices were based on what I had lying around, so some compromises, but it works well, with a decent input range and extends the frequency capabilities of the PM6622 quite a bit.
[Edit: Yes, the OCXO in the Adret has drifted off again
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