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mc100el1648 vco , can't get it to work

I made a pcb for the mc100el1648, i used the schematic described in the datasheet to get a sine wave output (datasheet page 10 figure 13). All components are smd, decoupling of every dc voltage with 68nF caps, short tracks and opening in gnd plane around tank circuit to reduce parasitic capacitance to gnd.

Tank circuit exist of 56nH coil (smd) and varicaps are 3-40pF range. power supply betzeen 0 and 5V. Still i can't get it to work, the output had dc shift of 3.3V but the ac signal is crap, it is very noise, a lot of higher harmonics (ni nice sine wave) , pk-pk is 80mV and frequency is about 60Mhz independed of varicap voltage. I tried different loads on the output (from none to 50-100ohm termination) but without a significant difference.

I am wondering i perhaps blew up the device by accidently putting one of the varicaps in forward bias, is this possible or is the devide robust enough in this regard?

Anyone an idea of what could be wrong here? Thanks,

Yannick

Reply to
Yannick
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Double-check your connections.

What applied voltage do you have on the varicap? It must be above ~1.5V or the varicap will be forward-biased.

The MC1648 is one of my more popular designs ;-)

(MC100EL1648 just means it's on an Eclipse Lite Process.)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

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Ah you designed the original 1648, nice!!! So i actually double checked connection 3 times, all voltages are in good range, indeed varicap voltage is over 2V to be sure he is in reverse bias(i also tried a fixed capacitor but didnt work either). I maybve blew up the transistors of the diff pair from the tank input due too much current (in the beginning i had accidently forward biassed the varicap) as you are the expert and know how big these transistors W/L are , is it possible i blew up the device ?

thanks,

Yannick

Reply to
Yannick

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Jim is on a biz trip and I am not too familiar with this chip. But did you check the bias voltage at the inductor? AFAIR it should be a little over 1.5V in a PECL circuit. If it's far from that the chip may be blown.

BTW, it is always good policy to apply a varicap voltage via a resistor. Under normal operation there isn't any current.

--
Groetjes, Joerg

http://www.analogconsultants.com
Reply to
Joerg

The device emitter sizes were 25um X 37um in the original MC1648.

Now there probably 1.5um X 5um X M stripes... easy to blow.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

oorspronkelijk bericht niet weergeven -

Ok, thanks ... i'l try a new device, it's a pity they are so expensive in belgium !

Reply to
Yannick

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