Frequency counter insensitive

The "preamp" is an MC10116 ECL triple line receiver, and I found one section of the article amusing. "the use of a ... LCD ... has several advantages over LED displays, including much lower current consumption. This allows the unit to be operated from batteries if required." Sort of true, but the 10116 current draw will not be kind to battery operation.

Anyhow, back to your question. The 10116 is capable of providing a fair bit of gain, and the last stage should be producing a nice square wave output at all input signal levels above the threshold. Stick your CRO onto pin6 (= the base of Q2) and check the signal shape and swing. The input biasing of the first stage of the 10116 is fairly important to get it into the middle of its linear range, so tweak the bias while watching the result at the output of each line receiver in turn. Shift your sig-gen and watch the result - if it isn't showing a decent swing all the way up to 20 MHz there's something amiss in this area.

I am assuming that you have TRIPLE-checked all component values around the input and 10116.

The front-end configuration in the schematic has been around for thirty years with only slight changes. Even the 2N5485 was used in the 1976 one. Your device however adds a bipolar device AND a schmitt gate to get a hard logic level swing. The fact that the '76 design drove a logic gate direct from the

10216 (a faster version of the '116) indicates the sort of gain (i.e. sensitivity) that *should_be* delivered through the line receiver chain.

Rebel, thanks for your advice. I've been fiddling about along the lines you suggest and decded that I need to concoct a better test signal at a known level before I can look at expected levels of gain through each 10116 stage. However, I observe that a sine wave test signal does not end up as a particularly nice looking square wave at base of Q2 at end of chain. It has some overshoot and the sides aren't really very vertical. A square wave test signal starts off clean and ends up looking rather noisy (jagged on top) I rechecked all my component values and it's all okay. I have a spare 10116 coming. They seem to be gettng a bit scarce and expensive. When it gets here I'll breadboard both it and the original just to see if the original might be crook. While I'm waiting for it to come I'll be rigging up a 5 and 10 MHz crystal oscillator with an attenuator. Anyway, your suggestions were much appreciated because you assured me that there's nothing unproven or fundamentally wrong with the design and gave me a plan of attack.

Peter H

(snip)

On re-reading the published article I noted the low stage gain they quote/use/achieve. They also would get some effective gain from their common emitter "level shifter". But it's a bit "iffy" if the biasing of the level shifter isn't right for the ECL output signal - that would certainly degrade the sensitivity.

Testing with a square wave input would be interesting, but in normal service you would be hoping that RF input signals are low in harmonics so sine sources are the best for diagnosis. With an input signal at spec sensitivity, you should be able to view levels in each stage and work out what is happening. Keep us posted.

Also, if you have time for reading it, have a look at:

That was the inspiration for using the 10116/10216 in counter front-ends.

Budgie, Thanks for the link to the application note! I printed it out for bedtime reading which is where I am headed now.

Tonight I pieced together a little Colpitts osc/buffer/attenuator as signal source just for this exercise. It's not easy working with a toy two-inch CRO but a preliminary tryout of the signal source suggests that the 10116 is not amplifying. I wound the attenuator back while observing sig amplitude on CRO with one eye and counter display with other eye. The counter won't read anything less than half a volt at the input terminals. Less than that and the display starts showing random nonsense. When I get the counter display

*just* locked onto the frequency and follow signal through the stages it changes from sine wave to a dirty square wave but he signal never gets bigger. Until I get to collector of Q2 logic level shifter. Lots of voltage swing there. As you rightly say, the design seems to be getting some effective gain from that component. Q2 is a PN200 not a BF450 as in the original design. Kit suppliers say they can't get BF450 so they substituted.

However, unless there's something wrong with my methods I think I have a dud

10116. When you consider the half a volt sensitivity I measure versus the stage gains and 20mV sensitivity quoted by designer it all seems to point that way.

Heigh-ho for Aussie Post and the eventual arrival of another 10116 form Melbourne! BTW, bloke I deal with on Ebay has 10114's for about 95c each. According to spec sheet 10114 is pin-compatible with 10116 and about only diff I see is that it is just slightly *slower*. In a spirit of enquiry, I ordered two just to see what happens when I try them.

Peter H