On Tue, Oct 10, 2023 at 11:10 AM, Ed Breya wrote:
You don't have to care about the diode reverse voltage ratings - when anti-parallel, each can only see forward voltages, unless the current rating is exceeded. What matters is that (indicates the RF power it can clamp at the input), and the zero-bias capacitance (for speed).
Of course.... That makes perfect sense.
The prescaler inputs are symmetric. You can treat it as a differential amplifier, and put the input signal to either one, while the other is bypassed - it will toggle one opposite edges if swapped. The ECL DC bias (Vbb) is usually developed by some sort of internal regulator circuitry, and each input connects to it through some medium resistance maybe in the hundreds of ohms range. You can put the DC bias into either one, just like the signal. What you want to do is upset the bias level a little bit to reduce the gain. HP used a selected R value for pulling the bias, and you can do the same if you can figure out how much is right. If you're designing for "production," it may be better to use a fixed resistor in series with a pot, to effectively select a value that's right for the particular part.
Yeah, this application doesn't care if the signal is inverted so we are free to do what we want.
This one will be just a small run to cover my initial costs so a selected resistor would be fine, and then the full design files will be released, so maybe I'll make provision in v2.0 for a selected resistor and a trimpot depending on the builders preference.
You can get some good ideas by studying the old ECL parts and documents. My favorite info source for this is the Motorola from the 1980s - I think the data book was called "MECL-III" family. Later generations like 10H and up started improving the performance, always tending faster, and with temperature-compensated Vbb,, lower operating voltage, and more function variety. A good proxy for a prescaler input section is the 116 or 216, for instance. These basic models extend from MC10216, to 10H216, to very high speed into the microwave 100E series and such. The 116 and 216 are triple line receivers, among the most highly used types, for interfacing small signals to ECL. Picture a single part with the three stages cascaded for high gain, so you can toggle the following flip-flops with mV input signals - if you have it biased right. The line receiver type parts almost always include access to the Vbb for setting up these scenarios with external resistors. The prescalers have the same setup with internal resistors, and don't provide access to Vbb. But, you can measure it - it's roughly the DC voltage present on the input pins.
Ed
I'll check that out, thanks!
On Tue, Oct 10, 2023 at 12:13 PM, Tom Lee wrote:
Hi Jared,
I assume that a decimal point went AWOL and that you meant 0.26pF. I
have never seen a 26pF diode with a 4V breakdown -- not because it's
impossible, but because there is no market for one.
Ah yup, a typo there, 0.26pF is what I meant. :D
A good guide for how much is too much is to calculate the capacitance
reactance at the highest frequency, and compare it to what shunts it in
that circuit (50 ohms, e.g.). At 1GHz, say, a 1pF cap presents about a
160 ohm reactance, which is larger, but not very much so, than 50 ohms.
At 10GHz, it's 16 ohms, which begins to look more like a short compared
to 50 ohms. You get the idea.
-- Cheers
Tom
It makes sense. I'm still learning this black magic high frequency stuff, so I may have a few more basic-level questions to come. Bear with me.... :)
Thanks!
Jared
Jared Cabot